Download "Harvard CS50 – Full Computer Science University Course"
Download this video with UDL Client
install
- Video mp4 HD+ with sound
- Mp3 in the best quality
- Any size files
%3Aformat(webp)%2Fi.ytimg.com%252Fvi%252F8mAITcNt710%252Fmaxresdefault.jpg&w=1139&q=75)
Please wait
Video tags
Subtitles
00:00:04
CS50 is considered by many to be one of the
best computer science courses in the world.00:00:09
This is a Harvard University course
taught by Dr. David Malan00:00:13
and we are proud to bring it to
the freeCodeCamp YouTube channel.00:00:17
Throughout a series of lectures, Dr. Malan will teach you
how to think algorithmically and solve problems efficiently.00:00:23
And make sure to check the description for a lot of
extra resources that go along with the course.00:00:27
[MUSIC PLAYING]00:01:45
DAVID MALAN: All right, this is CS50,
Harvard University's introduction00:01:50
to the intellectual
enterprises of computer science00:01:52
and the art of programming, back here
on campus in beautiful Sanders Theatre00:01:56
for the first time in quite a while.00:01:58
So welcome to the class.00:02:01
My name is David--00:02:02
OK.00:02:03
[CHEERING AND APPLAUSE]00:02:11
So my name is David Malan.00:02:13
And I took this class myself
some time ago, but almost didn't.00:02:16
It was sophomore fall and I
was sitting in on the class.00:02:20
And I was a little curious
but, eh, it didn't really00:02:22
feel like the field for me.00:02:24
I was definitely a computer
person, but computer science00:02:26
felt like something altogether.00:02:28
And I only got up the
nerve to take the class,00:02:30
ultimately, because the professor
at the time, Brian Kernighan,00:02:32
allowed me to take the
class pass/fail, initially.00:02:35
And that is what made
all the difference.00:02:37
I quickly found that
computer science is not just00:02:39
about programming and working
in isolation on your computer.00:02:42
It's really about problem
solving more generally.00:02:45
And there was something
about homework, frankly,00:02:48
that was, like, actually fun for perhaps
the first time in, what, 19 years.00:02:51
And there was something
about this ability00:02:53
that I discovered, along
with all of my classmates,00:02:56
to actually create something and bring
a computer to life to solve a problem,00:03:00
and sort of bring to bear something
that I'd been using every day00:03:03
but didn't really know how to harness,
that's been gratifying ever since,00:03:06
and definitely challenging
and frustrating.00:03:08
Like, to this day,
all these years later,00:03:10
you're going to run up against
mistakes, otherwise known as bugs,00:03:13
in programming, that
just drive you nuts.00:03:15
And you feel like you've hit a wall.00:03:16
But the trick really is
to give it enough time,00:03:18
to take a step back, take
a break when you need to.00:03:21
And there's nothing better, I daresay,
than that sense of gratification00:03:24
and pride, really,
when you get something00:03:26
to work, and in a class like
this, present, ultimately,00:03:28
at term's end, something like
your very own final project.00:03:32
Now, this isn't to say that
I took to it 100% perfectly.00:03:35
In fact, just this past week, I looked
in my old CS50 binder, which I still00:03:40
have from some 25 years
ago, and took a photo00:03:43
of what was apparently the very first
program that I wrote and submitted,00:03:47
and quickly received minus 2 points on.00:03:50
But this is a program that we'll
soon see in the coming days that00:03:53
does something quite simply like
print "Hello, CS50," in this case,00:03:57
to the screen.00:03:58
And to be fair, I
technically hadn't really00:04:00
followed the directions, which is
why I lost those couple of points.00:04:02
But if you just look at this, especially
if you've never programmed before,00:04:05
you might have heard
about programming language00:04:07
but you've never typed
something like this out,00:04:09
undoubtedly it's going to look cryptic.00:04:11
But unlike human
languages, frankly, which00:04:13
were a lot more sophisticated, a
lot more vocabulary, a lot more00:04:17
grammatical rules, programming, once
you start to wrap your mind around what00:04:21
it is and how it works and what these
various languages are, it's so easy,00:04:24
you'll see, after a few
months of a class like this,00:04:26
to start teaching
yourself, subsequently,00:04:29
other languages, as they may
come, in the coming years as well.00:04:32
So what ultimately matters
in this particular course00:04:36
is not so much where you end
up relative to your classmates00:04:38
but where you end up relative
to yourself when you began.00:04:41
And indeed, you'll begin today.00:04:43
And the only experience that matters
ultimately in this class is your own.00:04:46
And so, consider where you are today.00:04:49
Consider, perhaps, just how
cryptic something like that00:04:51
looked a few seconds ago.00:04:52
And take comfort in knowing just
some months from now all of that00:04:56
will be within your own grasp.00:04:58
And if you're thinking that, OK, surely
the person in front of me, to the left,00:05:01
to the right, behind me, knows more than
me, that's statistically not the case.00:05:05
2/3 of CS50 students have never taken
a CS course before, which is to say,00:05:10
you're in very good company
throughout this whole term.00:05:14
So then, what is computer science?00:05:16
I claim that it's problem solving.00:05:18
And the upside of that is
that problem solving is00:05:20
something we sort of do all the time.00:05:23
But a computer science
class, learning to program,00:05:25
I think kind of cleans up your thoughts.00:05:27
It helps you learn how to think more
methodically, more carefully, more00:05:31
correctly, more precisely.00:05:32
Because, honestly, the
computer is not going00:05:34
to do what you want unless you are
correct and precise and methodical.00:05:37
And so, as such, there's
these fringe benefits00:05:39
of just learning to think like a
computer scientist and a programmer.00:05:42
And it doesn't take all
that much to start doing so.00:05:45
This, for instance, is perhaps the
simplest picture of computer science,00:05:49
sure, but really problem
solving in general.00:05:51
Problems are all about taking input,
like the problem you want to solve.00:05:54
You want to get the solution, a.k.a.00:05:56
output.00:05:57
And so, something interesting
has got to be happening in here,00:05:59
in here, when you're trying to
get from those inputs to outputs.00:06:03
Now, in the world of
computers specifically,00:06:05
we need to decide in advance how we
represent these inputs and outputs.00:06:09
We all just need to decide, whether
it's Macs or PCs or phones or something00:06:13
else, that we're all going to speak
some common language, irrespective00:06:16
of our human languages as well.00:06:18
And you may very well know that
computers tend to speak only00:06:22
what language, so to speak?00:06:26
Assembly, one, but binary,
two, might be your go-to.00:06:29
And binary, by implying two,
means that the world of computers00:06:32
has just two digits at
its disposal, 0 and 1.00:06:35
And indeed, we humans have many more
than that, certainly not just zeros00:06:40
and ones alone. But a computer indeed
only has zeros and ones.00:06:43
And yet, somehow they can do so much.00:06:45
They can crunch numbers in
Excel, send text messages,00:06:47
create images and artwork
and movies and more.00:06:51
And so, how do you get from something
as simple as a few zeros, a few ones,00:06:54
to all of the stuff
that we're doing today00:06:56
in our pockets and laptops and desktops?00:06:58
Well, it turns out that
we can start quite simply.00:07:01
If a computer were to want to do
something as simple as count, well,00:07:05
what could it do?00:07:06
Well, in our human world,
we might count doing this,00:07:09
like 1, 2, 3, 4, 5, using so-called
unitary notation, literally the digits00:07:13
on your fingers where one finger
represents one person in the room,00:07:16
if I'm, for instance, taking attendance.00:07:18
Now, we humans would typically
actually count 1, 2, 3, 4, 5, 6.00:07:22
And we'd go past just those five
digits and count much higher,00:07:25
using zeros through nines.00:07:26
But computers, somehow, only
have these zeros and ones.00:07:29
So if a computer only somehow
speaks binary, zeros and ones,00:07:33
how does it even count
past the number 1?00:07:36
Well, here are 3 zeros, of course.00:07:38
And if you translate this
number in binary, 000,00:07:42
to a more familiar number in decimal,
we would just call this zero.00:07:46
Enough said.00:07:47
If we were to represent, with
a computer, the number 1,00:07:49
it would actually be 001,
which, not surprisingly,00:07:52
is exactly the same as we
might do in our human world,00:07:55
but we might not bother writing
out the two zeros at the beginning.00:07:59
But a computer, now, if it
wants to count as high as two,00:08:02
it doesn't have the digit 2.00:08:03
And so it has to use a different
pattern of zeros and ones.00:08:06
And that happens to be 010.00:08:08
So this is not 10 with
a zero in front of it.00:08:10
It's indeed zero one zero
in the context of binary.00:08:13
And if we want to count
higher now than two,00:08:15
we're going to have to tweak these
zeros and ones further to get 3.00:08:19
And then if we want 4
or 5 or 6 or 7, we're00:08:24
just kind of toggling these
zeros and ones, a.k.a.00:08:26
bits, for binary digits that represent,
via these different patterns,00:08:31
different numbers that you
and I, as humans, know,00:08:33
of course, as the so-called
decimal system, 0 through 9,00:08:36
dec implying 10, 10 digits,
those zeros through nine.00:08:40
So why that particular pattern?00:08:42
And why these particular zeros and ones?00:08:44
Well, it turns out that
representing one thing or the other00:08:48
is just really simple for a computer.00:08:50
Why? At the end of the day, they're
powered by electricity.00:08:53
And it's a really simple thing to
just either store some electricity00:08:56
or don't store some electricity.00:08:57
Like, that's as simple as
the world can get, on or off.00:09:00
1 or 0, so to speak.00:09:03
So, in fact, inside of a
computer, a phone, anything00:09:05
these days that's
electronic, pretty much,00:09:07
is some number of switches,
otherwise known as transistors.00:09:10
And they're tiny.00:09:11
You've got thousands, millions of them
in your Mac or PC or phone these days.00:09:14
And these are just tiny little switches
that can get turned on and off.00:09:17
And by turning those things
on and off in patterns,00:09:20
a computer can count from 0 on up
to 7, and even higher than that.00:09:24
And so these switches, really, you
can think of being as like switches00:09:27
like this. Let me just borrow one of
our little stage lights here.00:09:29
Here's a light bulb.00:09:30
It's currently off.00:09:32
And so, I could just think
of this as representing,00:09:34
in my laptop, a transistor,
a switch, representing 0.00:09:38
But if I allow some electricity
to flow, now I, in fact, have a 1.00:09:43
Well, how do I count higher than 1?00:09:44
I, of course, need another light bulb.00:09:46
So let me grab another one here.00:09:48
And if I put it in that same kind of
pattern, I don't want to just do this.00:09:53
That's sort of the old finger
counting way of unary, just 1, 2.00:09:57
I want to actually take
into account the pattern00:09:59
of these things being on and off.00:10:00
So if this was one a moment ago, what I
think I did earlier was I turned it off00:10:06
and let the next one over be on, a.k.a.00:10:10
010.00:10:12
And let me get us a
third bit, if you will.00:10:15
And that feels like enough.00:10:16
Here is that same pattern now,
starting at the beginning with 3.00:10:20
So here is 000.00:10:22
Here is 001.00:10:25
Here is 010, a.k.a., in our
human world of decimal, 2.00:10:32
And then we could, of course,
keep counting further.00:10:35
This now would be 3 and dot dot dot.00:10:37
If this other bulb now goes
on, and that switch is turned00:10:40
and all three stay on--
this, again, was what number?00:10:43
AUDIENCE: Seven.00:10:44
DAVID MALAN: OK, so, seven.00:10:45
So it's just as simple,
relatively, as that, if you will.00:10:49
But how is it that these
patterns came to be?00:10:53
Well, these patterns actually
follow something very familiar.00:10:56
You and I don't really
think about it at this level00:10:58
anymore because we've probably been
doing math and numbers since grade00:11:02
school or whatnot.00:11:04
But if we consider something in
decimal, like the number 123,00:11:09
I immediately jump to that.00:11:10
This looks like 123 in decimal.00:11:12
But why?00:11:13
It's really just three symbols,
a 1, a 2 with a bit of curve, a 300:11:17
with a couple of curves, that
you and I now instinctively00:11:20
just assign meaning to.00:11:22
But if we do rewind a few years,
that is one hundred twenty-three00:11:27
because you're assigning meaning
to each of these columns.00:11:30
The 3 is in the so-called ones place.00:11:33
The 2 is in the so-called tens place.00:11:36
And the 1 is in the
so-called hundreds place.00:11:39
And then the math ensues
quickly in your head.00:11:41
This is technically 100 times 1, plus
10 times 2, plus 1 times 3, a.k.a.00:11:47
100 plus 20 plus 3.00:11:48
And there we get the sort of
mathematical notion we know as 123.00:11:54
Well, nicely enough, in binary,
it's actually the same thing.00:11:58
It's just these columns mean
a little something different.00:12:01
If you use three digits in decimal,
and you have the ones place,00:12:05
the tens place, and the hundreds place,
well, why was that 1, 10, and 100?00:12:09
They're technically just powers of 10.00:12:11
So 10 to the 0, 10 to
the 1, 10 to the 2.00:12:13
Why 10?00:12:14
Decimal system, "dec" meaning 10.00:12:16
You have 8 and 10 digits, 0 through 9.00:12:18
In the binary system, if you're
going to use three digits,00:12:21
just change the bases if you're
using only zeros and ones.00:12:24
So now it's powers of 2, 2 to the
0, 2 to the 1, 2 to the 2, a.k.a.00:12:29
1 and 2 and 4, respectively.00:12:31
And if you keep going, it's going
to be 8s column, 16s column, 32, 64,00:12:36
and so forth.00:12:37
So, why did we get these
patterns that we did?00:12:40
Here's your 000 because it's 4 times
0, 2 times 0, 1 times 0, obviously 0.00:12:46
This is why we got the
decimal number 1 in binary.00:12:49
This is why we got the number 2
in binary, because it's 4 times00:12:53
0, plus 2 times 1, plus 1 times 0, and
now 3, and now 4, and now 5, and now 6,00:13:01
and now 7. And, of course, if you wanted to
count as high as 8, to be clear,00:13:05
what do you have to do?00:13:06
What does a computer need to
do to count even higher than 7?00:13:08
AUDIENCE: Add a bit.00:13:09
DAVID MALAN: Add a bit.00:13:10
Add another light bulb, another switch.00:13:12
And, indeed, computers
have standardized just how00:13:14
many zeros and ones,
or bits or switches,00:13:17
they throw at these kinds of problems.00:13:19
And, in fact, most computers would
typically use at least eight at a time.00:13:23
And even if you're only counting
as high as three or seven,00:13:25
you would still use eight and
have a whole bunch of zeros.00:13:28
But that's OK, because the
computers these days certainly00:13:31
have so many more, thousands,
millions of transistors and switches00:13:35
that that's quite OK.00:13:37
All right, so, with that said, if
we can now count as high as seven00:13:41
or, frankly, as high
as we want, that only00:13:44
seems to make computers
useful for things like Excel,00:13:46
like number crunching.00:13:48
But computers, of course,
let you send text messages,00:13:50
write documents, and so much more.00:13:52
So how would a computer represent
something like a letter,00:13:55
like the letter A of the English
alphabet, if, at the end of the day,00:13:59
all they have is switches?00:14:02
Any thoughts?00:14:03
Yeah. AUDIENCE: You can represent
letters in numbers.00:14:05
DAVID MALAN: OK, so we could
represent letters using numbers.00:14:08
OK, so what's a proposal?00:14:10
What number should represent what?00:14:11
AUDIENCE: Say if you were starting
at the beginning of the alphabet,00:14:15
you could say 1 is A, 2 is B, 3 is C.00:14:18
DAVID MALAN: Perfect.00:14:19
Yeah, we just all have to agree
somehow that one number is00:14:22
going to represent one letter.00:14:23
So 1 is A, 2 is B, 3 is
C, Z is 26, and so forth.00:14:28
Maybe we can even take into
account uppercase and lowercase.00:14:30
We just have to agree and sort of
write it down in some global standard.00:14:34
And humans, indeed, did just that.00:14:36
They didn't use 1, 2, 3.00:14:38
It turns out they started
a little higher up.00:14:40
Capital A has been
standardized as the number 65.00:14:44
And capital B has been
standardized as the number 66.00:14:47
And you can kind of imagine
how it goes up from there.00:14:50
And that's because whatever
you're representing,00:14:53
ultimately, can only be stored, at
the end of the day, as zeros and ones.00:14:57
And so, some humans in a room before,
decided that capital A shall be 65,00:15:01
or, really, this pattern of zeros
and ones inside of every computer00:15:05
in the world, 01000001.00:15:08
So if that pattern of zeros and
ones ever appears in a computer,00:15:12
it might be interpreted then as indeed
a capital letter A, eight of those bits00:15:17
at a time.00:15:18
But I worry, just to be clear, we
might have now created a problem.00:15:23
It might seem, if I play
this naively, that, OK,00:15:25
how do I now actually do
math with the number 65?00:15:28
If now Excel displays 65 is
an A, let alone Bs and Cs.00:15:33
So how might a computer
do as you've proposed,00:15:37
have this mapping from numbers to
letters, but still support numbers?00:15:41
It feels like we've given something up.00:15:43
Yeah?00:15:44
AUDIENCE: By having
a prefix for letters?00:15:46
DAVID MALAN: By having a prefix?00:15:47
AUDIENCE: You could have
prefixes and suffixes.00:15:48
DAVID MALAN: OK, so we could
perhaps have some kind of prefix,00:15:51
like some pattern of zeros and ones--00:15:53
I like this-- that
indicates to the computer00:15:56
here comes another pattern
that represents a letter.00:15:58
Here comes another pattern that
represents a number or a letter.00:16:02
So, not bad.00:16:03
I like that.00:16:04
Other thoughts?00:16:05
How might a computer
distinguish these two?00:16:08
Yeah. AUDIENCE: Have a
different file format, so,00:16:11
like, odd text or just
check the graphic or--00:16:16
DAVID MALAN: Indeed, and that's spot-on.00:16:17
Nothing wrong with what you suggested,
but the world generally does just that.00:16:20
The reason we have all of these
different file formats in the world,00:16:23
like JPEG and GIF and PNGs
and Word documents, .docx,00:16:28
and Excel files and so forth, is
because a bunch of humans got in a room00:16:32
and decided, well, in the context
of this type of file, or really,00:16:35
more specifically, in the
context of this type of program,00:16:38
Excel versus Photoshop versus
Google Docs or the like,00:16:42
we shall interpret any patterns of
zeros and ones as being maybe numbers00:16:46
for Excel, maybe letters in, like, a
text messaging program or Google Docs,00:16:51
or maybe even colors of the rainbow
in something like Photoshop and more.00:16:54
So it's context dependent.00:16:55
And we'll see, when we
ourselves start programming,00:16:58
you the programmer
will ultimately provide00:17:00
some hints to the computer that tells
the computer, interpret it as follows.00:17:05
So, similar in spirit to that, but
not quite a standardized with these00:17:08
prefixes.00:17:09
So this system here actually has a
name ASCII, the American Standard00:17:12
Code for Information Interchange.00:17:14
And indeed, it began here
in the US, and that's00:17:16
why it's actually a little
biased toward A's through Z's00:17:19
and a bit of punctuation as well.00:17:21
And that quickly became a problem.00:17:22
But if we start simply now,
in English, the mapping00:17:26
itself is fairly straightforward.00:17:28
So if A is 65, B it 66,
and dot dot dot, suppose00:17:33
that you received a text
message, an email, from a friend,00:17:36
and underneath the hood,
so to speak, if you kind of00:17:39
looked inside the computer, what you
technically received in this text00:17:42
or this email happened to
be the numbers 72, 73, 33,00:17:48
or, really, the underlying
pattern of zeros and ones.00:17:50
What might your friend have sent you
as a message, if it's 72, 73, 33?00:17:56
AUDIENCE: Hey.00:17:58
DAVID MALAN: Hey? Close.00:17:59
AUDIENCE: Hi. DAVID MALAN: Hi.00:18:00
It's, indeed, hi.00:18:01
Why?00:18:02
Well, apparently, according to this
little cheat sheet, H is 72, I is 73.00:18:06
It's not obvious from
this chart what the 33 is,00:18:09
but indeed, this
pattern represents "hi."00:18:11
And anyone want to guess,
or if you know, what 33 is?00:18:13
AUDIENCE: Exclamation point.00:18:14
DAVID MALAN: Exclamation point.00:18:16
And this is, frankly, not the
kind of thing most people know.00:18:18
But it's easily accessible by a
nice user-friendly chart like this.00:18:22
So this is an ASCII chart.00:18:23
When I said that we just need to
write down this mapping earlier,00:18:26
this is what people did.00:18:27
They wrote it down in
a book or in a chart.00:18:29
And, for instance, here is our
72 for H, here is our 73 for I,00:18:33
and here is our 33
for exclamation point.00:18:37
And computers, Macs, PCs,
iPhones, Android devices,00:18:41
just know this mapping
by heart, if you will.00:18:43
They've been designed to
understand those letters.00:18:46
So here, I might have received "hi."00:18:48
Technically, what I've received is
these patterns of zeros and ones.00:18:51
But it's important to note that when
you get these patterns of zeros and ones00:18:54
in any format, be it
email or text or a file,00:18:58
they do tend to come
in standard lengths,00:19:00
with a certain number of
zeros and ones altogether.00:19:04
And this happens to be 8 plus 8, plus 8.00:19:07
So just to get the message
"hi, exclamation point,"00:19:10
you would have received at least,
it would seem, some 24 bits.00:19:15
But frankly, bits are so tiny,
literally and mathematically,00:19:18
that we don't tend to think or
talk, generally, in terms of bits.00:19:21
You're probably more
familiar with bytes.00:19:23
B-Y-T-E-S is a byte,
is a byte, is a byte.00:19:27
A byte is just 8 bits.00:19:29
And even those, frankly, aren't
that useful if we do out the math.00:19:32
How high can you count
if you have eight bits?00:19:34
Anyone know?00:19:35
Say it again?00:19:38
Higher than that.00:19:39
Unless you want to go
negative, that's fine.00:19:42
256, technically 255.00:19:45
Long story short, if we actually got
into the weeds of all of these zeros00:19:48
and ones, and we figured out what
11111111 mathematically adds up00:19:54
to in decimal, it would
indeed be 255, or less00:19:57
if you want to represent
negative numbers as well.00:20:00
So this is useful because now we can
speak, not just in terms of bytes00:20:04
but, if the files are bigger,
kilobytes is thousands of bytes,00:20:06
megabytes is millions of bytes,
gigabytes is billions of bytes,00:20:10
terabytes are trillions
of bytes, and so forth.00:20:14
We have a vocabulary for these
increasingly large quantities of data.00:20:20
The problem is that, if you're using
ASCII and, therefore, eight bits or one00:20:24
byte per character, and
originally, only seven, you00:20:27
can only represent 255 characters.00:20:30
And that's actually 256 total
characters, including zero.00:20:33
And that's fine if you're using
literally English, in this case,00:20:37
plus a bunch of punctuation.00:20:39
But there's many human
languages in the world00:20:42
that need many more symbols
and, therefore, many more bits.00:20:45
So, thankfully, the world
decided that we'll indeed00:20:48
support not just the US
English keyboard, but all00:20:51
of the accented characters that
you might want for some languages.00:20:54
And heck, if we use enough
bits, zeros and ones,00:20:57
not only can we represent all
human languages in written form,00:21:01
as well as some emotions
along the way, we00:21:03
can capture the latter with
these things called emojis.00:21:06
And indeed, these are very
much in vogue these days.00:21:09
You probably send and/or receive
many of these things any given day.00:21:12
These are just characters, like
letters of an alphabet, patterns00:21:16
of zeros and ones that you're receiving,
that the world has also standardized.00:21:20
For instance, there
are certain emojis that00:21:22
are represented with
certain patterns of bits.00:21:24
And when you receive them, your
phone, your laptop, your desktop,00:21:28
displays them as such.00:21:30
And this newer standard
is called Unicode.00:21:32
So it's a superset of
what we called ASCII.00:21:35
And Unicode is just a mapping of many
more numbers to many more letters00:21:39
or characters, more
generally, that might00:21:42
use eight bits for
backwards compatibility00:21:45
with the old way of doing things with
ASCII, but they might also use 16 bits.00:21:49
And if you have 16
bits, you can actually00:21:51
represent more than
65,000 possible letters.00:21:54
And that's getting up there.00:21:55
And heck, Unicode might even use 32
bits to represent letters and numbers00:22:01
and punctuation symbols and emojis.00:22:03
And that would give you up
to 4 billion possibilities.00:22:06
And, I daresay, one of the reasons we
see so many emojis these days is we00:22:09
have so much room.00:22:11
I mean, we've got room for
billions more, literally.00:22:14
So, in fact, just as a
little bit of trivia,00:22:16
has anyone ever received this decimal
number, or if you prefer binary now,00:22:21
has anyone ever received this pattern
of zeros and ones on your phone,00:22:25
in a text or an email,
perhaps this past year?00:22:29
Well, if you actually look this up,
this esoteric sequence of zeros and ones00:22:33
happens to represent
face with medical mask.00:22:37
And notice that if you've got
an iPhone or an Android device,00:22:40
you might be seeing different things.00:22:43
In fact, this is the Android
version of this, most recently.00:22:46
This is the iOS version
of it, most recently.00:22:49
And there's bunches of other
interpretations by other companies00:22:51
as well.00:22:53
So Unicode, as a
consortium, if you will,00:22:55
has standardized the descriptions
of what these things are.00:22:58
But the companies themselves,
manufacturers out there,00:23:02
have generally interpreted
it as you see fit.00:23:05
And this can lead to some
human miscommunications.00:23:08
In fact, for like, literally,
embarrassingly, like a year or two,00:23:11
I started being in the habit of
using the emoji that kind of looks00:23:14
like this because I thought it was
like woo, happy face, or whatever.00:23:17
I didn't realize this
is the emoji for hug00:23:19
because whatever device I was using
sort of looks like this, not like this.00:23:24
And that's because of their
interpretation of the data.00:23:27
This has happened too when
what was a gun became a water00:23:31
pistol in some manufacturers' eyes.00:23:33
And so it's an interesting dichotomy
between what information we all00:23:37
want to represent and how we
choose, ultimately, to represent it.00:23:42
Questions, then, on these
representations of formats,00:23:45
be it numbers or letters, or soon more.00:23:49
Yeah? AUDIENCE: Why is decimal
popular for a computer00:23:52
if binary is the basis for everything?00:23:54
DAVID MALAN: Sorry,
why is what so popular?00:23:56
AUDIENCE: Why is the decimal popular
if binary is the fundamental--00:23:59
DAVID MALAN: Yeah, so we'll come
back to this in a few weeks, in fact.00:24:01
There are other ways
to represent numbers.00:24:03
Binary is one.00:24:04
Decimal is another.00:24:06
Unary is another.00:24:07
And hexadecimal is yet a fourth that
uses 16 total digits, literally 000:24:12
through 9 plus A, B, C,
D, E, F. And somehow,00:24:15
you can similarly count
even higher with those.00:24:18
We'll see in a few weeks
why this is compelling.00:24:21
But hexadecimal, long story
short, uses four bits per digit.00:24:24
And so, four bits, if you have two
digits in hex, that gives you eight.00:24:28
And it's just a very
convenient unit of measure.00:24:30
And it's also human convention in
the world of files and other things.00:24:34
But we'll come back to that soon.00:24:35
Other questions?00:24:36
AUDIENCE: Do the lights on the
stage supposedly say that--00:24:39
DAVID MALAN: Do the lights on the
stage supposedly say anything?00:24:42
Well, if we had thought in advance
to use maybe 64 light bulbs,00:24:46
that would seem to give us 8
total bytes on stage, 8 times 8,00:24:51
giving us just that.00:24:52
Maybe.00:24:54
Good question.00:24:55
Other questions on 0's and 1's?00:24:58
It's a little bright in here.00:25:01
No?00:25:02
Oh, yes?00:25:04
Where everyone's pointing
somewhere specific.00:25:08
There we go.00:25:09
Sorry.00:25:10
Very bright in this corner.00:25:11
AUDIENCE: I was just going
to ask about the 255 bits,00:25:14
like with the maximum characters.00:25:16
[INAUDIBLE] DAVID MALAN: Ah, sure, and we'll
come back to this, in some form,00:25:19
in the coming days too,
at a slower pace too,00:25:22
we have, with eight bits, two
possible values for the first00:25:26
and then two for the next, two
for the next, and so forth.00:25:28
So that's 2 times 2 times 2.00:25:30
That's 2 to the eighth
power total, which00:25:32
means you can have 256 total
possible patterns of zeros and ones.00:25:36
But as we'll see soon computer
scientists, programmers,00:25:40
software often starts counting at 0 by
convention and if you use one of those00:25:45
patterns, 00000000 to represent
the decimal number we know is zero,00:25:50
you only have 255 other patterns left
to count as high as therefore 255.00:25:56
That's all.00:25:57
Good question.00:25:59
All right, so what then might we
have besides these emojis and letters00:26:04
and numbers? Well, we of course have things
like colors and programs00:26:07
like Photoshop and pictures and photos.00:26:09
Well let me ask the question again.00:26:11
How might a computer, do you think,
knowing what you know now, represents00:26:14
something like a color?00:26:16
Like what are our options if all we've
got are zeros and ones and switches?00:26:19
Yeah?00:26:20
AUDIENCE: RGB00:26:21
DAVID MALAN: RGB.00:26:23
RGB indeed is this acronym that
represents some amount of red00:26:27
and some amount of green and
blue and indeed computers00:26:29
can represent colors by just doing that.00:26:32
Remembering, for instance, this dot.00:26:33
This yellow dot on the screen that
might be part of any of those emojis00:26:36
these days, well that's some amount
of red, some amount of green,00:26:39
some amount of blue.00:26:40
And if you sort of mix
those colors together,00:26:42
you can indeed get a very specific one.00:26:44
And we'll see you in
just a moment just that.00:26:46
So indeed earlier on, humans
only used seven bits total.00:26:51
And it was only once they decided,
well, let's add an eighth bit that they00:26:54
got extended ASCII and
that was initially in part00:26:57
a solution to the same problem of
not having enough room, if you will,00:27:00
in those patterns of zeros and ones
to represent all of the characters00:27:05
that you might want.00:27:06
But even that wasn't enough and that's
why we've now gone up to 16 and 3200:27:10
and long past 7.00:27:11
So if we come back now to
this one particular color.00:27:16
RGB was proposed as a scheme,
but how might this work?00:27:19
Well, consider for instance this.00:27:21
If we do indeed decide as a group to
represent any color of the rainbow00:27:25
with some mixture of some red,
some green, and some blue,00:27:28
we have to decide how to represent
the amount of red and green and blue.00:27:33
Well, it turns out if all we have
are zeros and ones, ergo numbers,00:27:37
let's do just that.00:27:38
For instance, suppose a computer we're
using, these three numbers 72, 73, 33,00:27:44
no longer in the context of
an email or a text message,00:27:47
but now in the context of something
like Photoshop, a program for editing00:27:51
and creating graphical files,
maybe this first number00:27:55
could be interpreted as representing
some amount of red, green, and blue,00:28:00
respectively. And that's exactly what happens.00:28:02
You can think of the first digit as
red, second as green, third as blue.00:28:05
And so ultimately when you combine that
amount of red, that amount of green,00:28:10
that amount of blue, it turns out it's
going to resemble the shade of yellow.00:28:14
And indeed, you can come up
with a numbers between 0 and 25500:28:18
for each of those colors to mix any
other color that you might want.00:28:21
And you can actually
see this in practice.00:28:23
Even though our screens,
admittedly, are getting really good00:28:26
on our phones and laptops such that you
barely see the dots, they are there.00:28:30
You might have heard
the term pixel before.00:28:32
Pixel's just a dot on
the screen and you've00:28:34
got thousands, millions of them these
days horizontally and vertically.00:28:38
If I take even this
emoji, which again happens00:28:41
to be one company's interpretation
of a face with medical mask00:28:46
and zoom in a bit, maybe
zoom in a bit more,00:28:48
you can actually start
to see these pixels.00:28:50
Things get pixelated
because what you're seeing00:28:53
is each of the individual dots
that compose this particular image.00:28:57
And apparently each of
these individual dots00:28:59
are probably using 24 bits, eight bits
for red, eight bits for green, eight00:29:04
bits for blue, in some pattern.00:29:07
This program or some other like
Photoshop is interpreting one pattern00:29:11
and it's white or yellow or
black or some brown in between.00:29:16
So if you look sort of awkwardly, but
up close to your phone or your laptop00:29:19
or maybe your TV, you can
see exactly this, too.00:29:23
All right, well, what
about things that we also00:29:25
watch every day on YouTube or the like?00:29:27
Things like videos.00:29:28
How would a computer,
knowing what we know now,00:29:30
represent something like a video?00:29:35
How might you represent a video
using only zeros and ones?00:29:37
Yeah?00:29:38
AUDIENCE: As we can see here,
they represent images, right?00:29:43
[INAUDIBLE] sounds of
the 0 and 1s as well.00:29:47
[INAUDIBLE]00:29:50
DAVID MALAN: Yeah, exactly.00:29:52
To summarize, what video really
adds is just some notion of time.00:29:55
It's not just one image, it's
not just one letter or a number,00:29:58
it's presumably some kind of
sequence because time is passing.00:30:01
So with a whole bunch of images,
maybe 24 maybe 30 per second,00:30:05
if you fly them by the
human's eyes, we can00:30:08
interpret them using our eyes
and brain that there is now00:30:11
movement and therefore video.00:30:13
Similarly with audio or music.00:30:16
If we just came up with some convention
for representing those same notes00:30:20
on a musical instrument, could we have
the computer synthesize them, too?00:30:23
And this might be
actually pretty familiar.00:30:25
Let me pull up a quick video here,
which happens to be an old school00:30:29
version of the same idea.00:30:31
You might remember from childhood.00:30:32
[MUSIC PLAYING]00:30:40
[CLICKING]00:30:54
So granted that particular
video is an actual video00:30:58
of a paper-based animation, but
indeed, that's really all you need,00:31:02
is some sequence of these images,
which themselves of course00:31:06
are just zeros and ones because they're
just this grid of these pixels or dots.00:31:10
Now something like musical notes like
these, those of you who are musicians00:31:14
might just naturally play
these on physical devices,00:31:16
but computers can certainly
represent those sounds, too.00:31:19
For instance, a popular
format for audio is00:31:22
called MIDI and MIDI
might just represent00:31:24
each note that you saw a moment ago
essentially as a sequence of numbers.00:31:29
But more generally, you might
think about music as having notes,00:31:32
for instance, A through G, maybe
some flats and some sharps,00:31:35
you might have the duration like how
long is the note being heard or played00:31:39
on a piano or some
other device, and then00:31:41
just the volume like how hard
does a human in the real world00:31:43
press down on that key and
therefore how loud is that sound?00:31:46
It would seem that just remembering
little details like that quantitatively00:31:51
we can then represent really all of
these otherwise analog human realities.00:31:57
So that then is really
a laundry list of ways00:32:00
that we can just represent information.00:32:02
Again, computers or digital have
all of these different formats,00:32:05
but at the end of the day and as
fancy as those devices in years00:32:07
are, it's just zeros and ones, tiny
little switches or light bulbs,00:32:11
if you will, represented in some
way and it's up to the software00:32:14
that you and I and others
write to use those zeros00:32:17
and ones in ways we want to get
the computers to do something00:32:21
more powerfully.00:32:22
Questions, then, on this representation
of information, which I daresay00:32:27
is ultimately what problem solving
is all about, taking in information00:32:30
and producing new via
some process in between.00:32:35
Any questions there?00:32:38
Yeah, in back.00:32:40
AUDIENCE: Yeah, so we talked about how
different file formats kind of allow00:32:43
you to interpret information.00:32:45
How does a file format like .mp4
discriminate between audio and video00:32:50
within itself as a value?00:32:52
DAVID MALAN: So a really good question.00:32:53
There are many other
file formats out there.00:32:55
You allude to MP4 for video
and more generally the use00:32:58
are these things called
codecs and containers.00:33:01
It's not quite as simple when
using larger files, for instance,00:33:04
in more modern formats that a
video is just a sequence of images,00:33:08
for instance. Why?00:33:09
If you stored that many images
for like a Hollywood movie,00:33:13
like 24 or 30 of them per second,
that's a huge number of images.00:33:17
And if you've ever taken
photos on your phone,00:33:19
you might know how many megabytes or
larger even individual photographs00:33:23
might be.00:33:24
So humans have developed over
the years a fancier software00:33:27
that uses much more math to represent
the same information more minimally00:33:32
just using somehow shorter
patterns of zeros and ones00:33:35
than are most simplistic
representation here.00:33:37
And they use what might
be called compression.00:33:40
If you've ever used a zip
file or something else,00:33:42
somehow your computer is
using fewer zeros and ones00:33:45
to represent the same
amount of information,00:33:47
ideally without losing any information.00:33:49
In the world of multimedia, which we'll
touch on a little bit in a few weeks,00:33:52
there are both lossy and
lossless formats out there.00:33:56
Lossless means you lose
no information whatsoever.00:33:59
But more commonly as you're alluding
to one is lossy compression, L-O-S-S-Y,00:34:05
where you're actually throwing
away some amount of quality.00:34:08
You're getting some amount
of pixelation that might not00:34:10
look perfect to the human, but heck
it's a lot cheaper and a lot easier00:34:13
to distribute.00:34:14
And in the world of multimedia,
you have containers like QuickTime00:34:17
and other MPEG containers
that can combine00:34:20
different formats of video, different
formats of audio in one file,00:34:24
but there, too, do
designers have discretion.00:34:26
So more in a few weeks, too.00:34:28
Other questions, then, on
information here as well?00:34:32
Yeah? AUDIENCE: So I know
computers used to be very big00:34:35
and taking up like a
whole room and stuff.00:34:37
Is the reason they've gotten
smaller because we can store00:34:41
this information piecemeal or what?00:34:43
DAVID MALAN: Exactly.00:34:44
I mean, back in the day you might
have heard of the expression a vacuum00:34:47
tube, which is like some
physically large device that00:34:50
might have only stored some 0 or 1.00:34:53
Yes, it is the miniaturization
of hardware these days00:34:55
that has allowed us to store as many
and many more zeros and ones much more00:35:00
closely together.00:35:01
And as we've built more
fancy machines that00:35:03
can sort of design this hardware
at an even smaller scale,00:35:06
we're just packing more and
more into these devices.00:35:08
But there, too, is a trade off.00:35:09
For instance, you might know by
using your phone or your laptop00:35:13
for quite a while, maybe on
your lap, starts to get warm.00:35:15
So there are these literal
physical side effects00:35:17
of this where now some
of our devices run hot.00:35:20
This is why like a data
center in the real world00:35:23
might need more air conditioning
than a typical place,00:35:25
because there are these
physical artifacts as well.00:35:28
In fact, if you'd like to see one of
the earliest computers from decades ago,00:35:31
across the river here in now Allston
in the new engineering building00:35:35
is the Harvard Mark 1 computer that
will give you a much better mental model00:35:40
of just that.00:35:41
Well if we come back now
to this first picture00:35:44
being computer science or
really problem solving,00:35:47
I daresay we have more
than enough ways now00:35:49
to represent information, input and
output, so long as we all just agree00:35:53
on something and thankfully all
of those before us have given us00:35:56
things like ASCII and Unicode.00:35:57
Not to mention MP4s, word
documents, and the like.00:36:01
But what's inside of this proverbial
black box into which these inputs are00:36:05
going in the outputs are coming?00:36:06
Well that's where we get this
term you might have heard, too.00:36:09
An algorithm, which is just step-by-step
instructions for solving some problem00:36:14
incarnated in the world
of computers by software.00:36:17
When you write software
aka programs, you00:36:20
are implementing one or more algorithms,
one or more sets of instructions00:36:26
for solving some problem, and maybe
you're using this language or that,00:36:29
but at the end of the day,
no matter the language00:36:31
you use the computer is going
to represent what you type00:36:34
using just zeros and ones.00:36:37
So what might be a
representative algorithm?00:36:40
Nowadays you might use
your phone quite a bit00:36:42
to make calls or send texts
or emails and therefore you00:36:45
have a whole bunch of
contacts in your address book.00:36:48
Nowadays, of course,
this is very digital,00:36:50
but whether on iOS or
Android or the like,00:36:53
you might have a whole
bunch of names, first name00:36:56
and/or last, as well as numbers
and emails and the like.00:36:58
You might be in the habit of like
scrolling through on your phone00:37:01
all of those names to find
the person you want to call.00:37:05
It's probably sorted alphabetically by
first name or last name, A through Z,00:37:09
or some other symbol.00:37:11
This is frankly quite the same as
we used to do back in my day, CS50,00:37:16
when we just used a physical book.00:37:18
In this physical book
might be a whole bunch00:37:20
of names alphabetically
sorted from left to right00:37:22
corresponding to a
whole bunch of numbers.00:37:24
So suppose that in this
old Harvard phone book00:37:27
we want to search for John Harvard.00:37:29
We might of course start
quite simply at the beginning00:37:31
here, looking at one page at a
time, and this is an algorithm.00:37:36
This is like literally step-by-step
looking for the solution00:37:40
to this problem.00:37:41
In that sense, if John
Harvard's in the phone book,00:37:43
is this algorithm page-by-page
correct, would you say?00:37:47
AUDIENCE: Yes.00:37:48
DAVID MALAN: Yes.00:37:49
Like if John Harvard's
in the phone book,00:37:51
obviously I'm eventually going to get to
him, so that's what we mean by correct.00:37:54
Is it efficient?00:37:56
Is it well designed, would you say?00:37:58
No. I mean this is going to take forever
even just to get to the Js or the Hs,00:38:01
depending how this thing's sorted.00:38:03
All right, well let
me go a little faster.00:38:04
I'll start like two pages at a time.00:38:06
2, 4, 6, 8, 10, 12, and so forth.00:38:11
Sounds faster, is faster, is it correct?00:38:13
AUDIENCE: No.00:38:14
DAVID MALAN: OK, why is it not correct?00:38:16
Yeah?00:38:17
AUDIENCE: So if you're
starting on page 1,00:38:19
you're only going odd number of pages,
so if it's on an even number page,00:38:22
you'll miss it. DAVID MALAN: Exactly.00:38:23
If I start on an odd number of
pages and I'm going two at a time00:38:26
I might miss pages in between.00:38:28
And if I therefore conclude
when I get to the back00:38:30
of the book there was no John
Harvard, I might have just errored.00:38:33
This would be again one of these bugs.00:38:35
But if I try a little harder,
I feel like there's a solution.00:38:39
We don't have to completely
throw out this algorithm.00:38:41
I think we can probably go
roughly twice as fast still.00:38:44
But what should we do
instead to fix this?00:38:47
Yeah, in back.00:38:48
AUDIENCE: [INAUDIBLE]00:38:58
DAVID MALAN: Nice.00:38:59
So I think what many of us, most of us,
if we even use this technology any more00:39:02
these days, we might go roughly
to the middle of the phone book00:39:05
just to kind of get us started.00:39:06
And now I'm looking down, I'm looking
for J, assuming first name, J Harvard,00:39:10
and it looks like I'm in the M section.00:39:13
So just to be clear,
what should I do next?00:39:15
AUDIENCE: [INAUDIBLE]00:39:21
DAVID MALAN: OK, and
presumably it is John Harvard00:39:23
would be to the left of this.00:39:24
So here's an opportunity to
figuratively and literally tear00:39:27
this particular problem in half,
throw half of the problem away.00:39:32
It's actually pretty easy
if you just do it that way.00:39:34
The hard way is this way.00:39:36
But I've now just decreased the
size of this problem really in half.00:39:41
So if I started with 1,000 pages
of phone numbers and names, now00:39:45
I'm down to 500.00:39:46
And already we haven't
found John Harvard,00:39:48
but that's a big bite
out of this problem.00:39:50
I do think it's correct because if
J is to the left of M, of course,00:39:54
he's definitely not
going to be over there.00:39:56
I think if I repeat this again
dividing and conquering, if you will,00:40:00
here I might have gone a little too far.00:40:02
Now I'm in like the E section.00:40:03
So let me tear the problem in half
again, throw another 250 pages away,00:40:08
and again repeat, dividing
and dividing and conquering00:40:11
until finally, presumably, I end
up with just one page of a phone00:40:14
book on which John Harvard's
name either is or is not,00:40:18
but because of the algorithm
you proposed, step by step,00:40:21
I know that he's not in
anything I discarded.00:40:24
So traumatic is that
might have been made out00:40:28
to be, it's actually just harnessing
pretty good human intuition.00:40:31
Indeed, this is what
programming is all about, too.00:40:33
It's not about learning
a completely new world,00:40:36
but really just how to harness intuition
and ideas that you might already00:40:40
have and take naturally
but learning how to express00:40:43
them now more succinctly,
more precisely,00:40:45
using things called
programming languages.00:40:49
Why is an algorithm like that if I found
John Harvard better than, ultimately,00:40:54
just doing the first
one or even the second00:40:56
and maybe doubling back
to check those even pages?00:40:59
Well let's just look
at little charts here.00:41:01
Again, we don't have to get
into the nuances of numbers,00:41:04
but if we've got like a chart
here, xy plot, on the x-axis00:41:07
here I claim as the size of the problem.00:41:09
So measured in the numbers
of pages in the phone book.00:41:12
So the farther you go out here, the
more pages are in the phone book.00:41:16
And here we have time
to solve on the y-axis.00:41:18
So the higher you go
up, the more time it's00:41:21
going to be taking to solve
that particular problem.00:41:24
So let's just arbitrarily say that
the first algorithm, involving00:41:27
like n pages, might be
represented graphically like this.00:41:31
No matter the slope,
it's a straight line00:41:33
because there's presumably
a one to one relationship00:41:36
between numbers of pages and number
of seconds or number of page turns.00:41:40
Why? If the phone company adds
another page next year00:41:43
because some new people
move to town, that's00:41:45
going to require one
additional page for me.00:41:47
One to one.00:41:49
If, though, we use the second
algorithm, flawed though it was,00:41:52
unless we double back a little bit
to fix someone being in between,00:41:56
that's too going to be a
straight line, but it's00:41:59
going to be a different slope because
now there's a 2 to 1 or a 1 to 200:42:02
relationship because I'm
going to pages at a time.00:42:06
So if the phone company adds
another page or another two pages,00:42:10
that still only just one more step.00:42:13
You can see the difference
if I kind of draw this.00:42:15
If this is the phone book in
question, this number of pages,00:42:18
it might take this many
seconds on the yellow line00:42:20
to represent or to find
someone like John Harvard.00:42:24
But of course on the first
algorithm, the red line,00:42:27
it's literally going to
take twice as many steps.00:42:29
And what do the n here mean?
n is the go-to variable00:42:32
for computer scientist or programmer
just generically representing a number.00:42:36
So if the number of pages
in the phone book is n,00:42:38
the number of steps the second
algorithm would have taken00:42:41
would be in the worst case n over 2.00:42:44
Half as many because
you're going twice as fast.00:42:46
But the third algorithm, actually
if you recall your logarithms,00:42:50
looks a little something like this.00:42:52
There's a fundamentally
different relationship00:42:54
between the size of the problem and
the amount of time required to solve it00:42:58
that technically is
log-based, too, again,00:43:00
but it's really the
shape that's different.00:43:02
The implication there is that if,
for instance, Cambridge and Allston,00:43:06
two different towns here in
Massachusetts, merge next year00:43:08
and there's just one phone
book that's twice as big,00:43:11
no big deal for that
third and final algorithm.00:43:14
Why?00:43:15
You just tear the problem
one more time in half,00:43:17
taking one more byte,
that's it, not another 1,00000:43:21
bytes just to get to the solution.00:43:23
Put another way, you
can walk it way, way,00:43:26
way out here to a much bigger
phone book and ultimately00:43:29
that green line is barely
going to have budged.00:43:32
So this then is just a way of
now formalizing and thinking00:43:35
about what the performance or
quality of these algorithms might be.00:43:42
Before we now make one more
formalization of the algorithm itself,00:43:46
any questions then on this notion of
efficiency or now performance of ideas?00:43:52
Yeah. AUDIENCE: How many phone
books have you got?00:43:56
DAVID MALAN: (LAUGHING) A lot
of phone books over the years00:43:58
and if you or your parents have any
more still somewhere we could definitely00:44:02
use them because they're hard to find.00:44:04
Other questions?00:44:05
But thanks.00:44:07
Other questions here, too?00:44:09
No.00:44:10
Oh, was that a murmur?00:44:11
Yes, over here.00:44:13
AUDIENCE: You could get Harry
Potter as a guest speaker.00:44:15
DAVID MALAN: Sorry, say again.00:44:16
AUDIENCE: You could get Harry
Potter as a guest speaker.00:44:18
DAVID MALAN: (LAUGHING) Oh, yeah.00:44:19
Hopefully.00:44:20
Then we'd have a little
something more to use here.00:44:23
So now if we want to formalize
further what it is we just did,00:44:28
we can go ahead and introduce this.00:44:30
A form of code aka pseudocode.00:44:33
Pseudocode is not a specific
language, it's not like something00:44:35
we're about to start coding in, it's
just a way of expressing yourself00:44:38
in English or any human
language succinctly00:44:41
correctly toward an end of getting
your idea for an algorithm across.00:44:45
So for instance, here
might be how we could00:44:47
formalize the code, the pseudocode
for that same algorithm.00:44:51
Step one was pick up the
phone book, as I did.00:44:54
Step two might be open to
the middle of the phone book,00:44:56
as you proposed that we do first.00:44:58
Step three was probably to
look down at the pages, I did.00:45:01
And step four gets a
little more interesting00:45:04
because I had to quickly make a
decision and ask myself a question.00:45:07
If person is on page, then
I should probably just00:45:11
go ahead and call that person.00:45:13
But that probably wasn't the
case at least for John Harvard,00:45:15
and I opened the M section.00:45:17
So there's this other
question I should now00:45:19
ask else if the person
is earlier in the book,00:45:22
then I should tear the problem
in half as I did but go left, so00:45:26
to speak, and then not just open to the
middle of the left half of the book,00:45:30
but really just go back to
step three, repeat myself.00:45:33
Why? Because I can just repeat what I
just did, but with a smaller problem00:45:37
having taken this big bite.00:45:39
But, if the person
was later in the book,00:45:41
as might have happened with a
different person than John Harvard,00:45:44
then I should open to the middle
of the right half of the book,00:45:47
again go back to line
three, but again, I'm00:45:49
not going to get sucked doing
something forever like this00:45:51
because I keep shrinking
the size of the problem.00:45:54
Lastly, the only
possible scenario that's00:45:56
left, if John Harvard is not on
the page and he's not to the left00:45:59
and he's not to the right,
what should our conclusion be?00:46:02
AUDIENCE: He's not there.00:46:03
DAVID MALAN: He's not there.00:46:04
He's not listed.00:46:05
So we need to quit in some other form.00:46:08
Now as an aside, it's kind of deliberate
that I buried that last question00:46:11
at the end because this is what
happens all too often in programming,00:46:16
whether you're new at
it or professional,00:46:17
just not considering all possible
cases, corner cases if you will,00:46:22
that might not happen that often,
but if you don't anticipate them00:46:25
in your own code,
pseudocode or otherwise,00:46:28
this is when and why
programs might crash00:46:31
or you might say stupid little
spinning beach balls or hourglasses00:46:34
or your computer might reboot.00:46:35
Why? It's doing something
sort of unpredictable00:46:38
if a human, maybe myself,
didn't anticipate this.00:46:42
Like what does this program do if
John Harvard is not in the phone book00:46:45
if I had omitted lines 12 and 13?00:46:48
I don't know. Maybe it would behave
differently on a Mac or PC00:46:50
because it's sort of undefined behavior.00:46:53
These are the kinds of omissions
that frankly you're invariably00:46:56
going to make, bugs
you're going to introduce,00:46:58
mistakes you're going to make early
on, and me, too, 25 years later.00:47:02
But you'll get better at
thinking about those corner cases00:47:06
and handling anything that can
possibly go wrong and as a result,00:47:09
your code will be all the better for it.00:47:11
Now the problem ultimately
with learning how to program,00:47:15
especially if you've
never had experience00:47:16
or even if you do but you
learned one language only,00:47:21
is that they all look a little
cryptic at first glance.00:47:25
But they do share certain commonalities.00:47:27
In fact, we'll use this
pseudocode to define those first.00:47:30
Highlighted in yellow
here are what henceforth00:47:32
we're going to start calling functions.00:47:34
Lots of different programming
languages exist, but most of them00:47:37
have what we might call
functions, which are actions00:47:40
or verbs that solve
some smaller problem.00:47:43
That is to say, you might use
a whole bunch of functions00:47:46
to solve a bigger problem
because each function tends to do00:47:50
something very specific or precise.00:47:52
These then in English might be
translated in code, actual computer00:47:57
code, to these things called functions.00:47:59
Highlighted in yellow now are
what we might call conditionals.00:48:02
Conditionals are things
that you do conditionally00:48:05
based on the answer to some question.00:48:07
You can think of them kind
of like forks in the road.00:48:09
Do you go left or go right
or some other direction00:48:12
based on the answer to some question?00:48:14
Well, what are those questions?00:48:16
Highlighted now in yellow or what we
would call Boolean expressions, named00:48:20
after a mathematician last name Bool,
that simply have yes no answers.00:48:25
Or, if you prefer, true or false answers
or, heck, if you prefer 1 or 0 answers.00:48:31
We just need to distinguish
one scenario from another.00:48:34
The last thing manifests
in this pseudocode00:48:37
is what I might highlight
now and call loops.00:48:39
Some kind of cycle, some kind of
directive that tells us to do something00:48:43
again and again so that I don't
need a 1,000-line program to search00:48:48
a 1,000-page phone book, I can get
away with a 13-line program but sort00:48:53
of repeat myself inherently in order to
solve some problem until I get to that00:48:58
last step.00:48:59
So this then is what we
might call pseudocode00:49:01
and indeed there are
other characteristics00:49:05
of programs that we'll touch on before
long, things like arguments and return00:49:08
values, variables, and more, but
unfortunately in most languages,00:49:13
including some we will very
deliberately use in this class00:49:16
and that everyone in the real
world these days still uses,00:49:19
its programs tend to look like this.00:49:22
This for instance, is a distillation
of that very first program00:49:24
I wrote in 1996 in CS50 itself just
to print something on the screen.00:49:29
In fact, this version here just tries
to print quote unquote, "Hello, world."00:49:34
Which is, dare say, the most
canonical first thing that most00:49:37
any programmer ever gets a
computer to say just because,00:49:41
but look at this mess.00:49:42
I mean, there's a hash symbol,
these angled brackets, parentheses,00:49:45
words like int, curly braces, quotes,
parentheses, semicolons, and back00:49:49
slashes.00:49:50
I mean there's more overhead
and more syntax and clutter00:49:54
than there is an actual idea.00:49:55
Now that's not to say that you won't
be able to understand this before long,00:50:00
because honestly there's not that many
patterns, indeed programming languages00:50:03
have typically a much smaller vocabulary
than any actual human language,00:50:07
but at first it might
indeed look quite cryptic.00:50:10
But you can perhaps infer I have no
idea what these other lines do yet,00:50:14
but "Hello, world." is
presumably quote unquote what00:50:18
will be printed on the screen.00:50:19
But what we'll do today,
after a short break,00:50:22
and set the stage for
next week is introduce00:50:24
these exact same ideas in
just a bit using Scratch,00:50:27
something that you
yourselves might have used00:50:29
when you're quite younger but
without the same vocabulary applied00:50:32
to those ideas.00:50:34
The upside of what we'll soon do using
Scratch, this graphical programming00:50:38
language from our friends down the
road at MIT, it'll let us today00:50:41
start to drag and drop things that
look like puzzle pieces that interlock00:50:46
together if it makes
logical sense to do so,00:50:48
but without the distraction
of hashes, parentheses,00:50:51
curly braces, angle brackets,
semicolons, and things00:50:54
that are quite beside the point.00:50:56
But for now, let's go ahead
and take a 10 minute break here00:50:58
and when we resume, we
will start programming.00:51:01
So this on the screen
is a language called00:51:04
C something that will dive
into next week and thankfully00:51:07
this now on the screen is
another language called Python00:51:10
that we'll also take a look at
in a few weeks before long along00:51:13
with other languages along the way.00:51:15
Today though, and for this first
week, week zero, so to speak,00:51:19
we use Scratch because
again it will allow00:51:21
us to explore some of those
programming fundamentals00:51:23
that will be in C and in Python and in
JavaScript and other languages, too,00:51:28
but in a way where we don't have to
worry about the distractions of syntax.00:51:32
So the world of Scratch looks like this.00:51:35
It's a web-based or downloadable
programming environment00:51:38
that has this layout here
by default. On the left here00:51:40
we'll soon see is a palette of puzzle
pieces, programming blocks that00:51:45
represent all of those
ideas we just discussed.00:51:47
And by dragging and
dropping these puzzle pieces00:51:50
or blocks over this big area
and connecting them together,00:51:54
if it makes logical
sense to do so, we'll00:51:56
start programming in this environment.00:51:58
The environment allows you to have
multiple sprites, so to speak.00:52:01
Multiple characters, things
like a cat or anything00:52:04
else, and those sprites exist
in this rectangular world00:52:08
up here that you can full screen to
make bigger and this here by default00:52:11
is Scratch, who can move up, down, left,
right and do many more things, too.00:52:16
Within its Scratch's
world you can think of it00:52:19
as perhaps a familiar
coordinate system with Xs and Ys00:52:23
which is helpful only when it comes
time to position things on the screen.00:52:27
Right now Scratch is at the default,
0,0, where x equals 0 and y equals 0.00:52:32
If you were to move the cat way
up to the top, x would stay zero,00:52:36
y would be positive 180.00:52:37
If you move the cat all the way
to the bottom, x would stay zero,00:52:40
but y would now be negative 180.00:52:42
And if you went left, x would become
negative 240 but y would stay 0,00:52:47
or to the right x would be
240 and y would stay zero.00:52:51
So those numbers generally don't
so much matter because you can just00:52:55
move relatively in this
world up, down, left, right,00:52:58
but when it comes time
to precisely position00:53:01
some of these sprites
or other imagery, it'll00:53:03
be helpful just to have that mental
model off up, down, left, and right.00:53:07
Well let's go ahead and make perhaps
the simplest of programs here.00:53:10
I'm going to switch over to the
same programming environment00:53:13
now for a tour of the left hand side.00:53:15
So by default selected here are
the category in blue motion,00:53:20
which has a whole bunch of puzzle
pieces or blocks that relate to motion.00:53:24
And whereas Scratch as
a graphical language00:53:27
categorizes things by the type
of things that these pieces do,00:53:30
we'll see that throughout
this whole palette00:53:32
we'll have functions and
variables and conditionals00:53:35
and Boolean expressions and more
each in a different color and shape.00:53:38
So for instance, moving 10 steps
or turning one way or the other00:53:42
would be functions categorized
here as things like motion.00:53:45
Under looks in purple, you
might have speech bubbles00:53:49
that you can create by
dragging and dropping00:53:51
these that might say "hello" or
whatever for some number of seconds.00:53:54
Or you could switch costumes, change
the cat to look like a dog or a bird00:53:58
or anything else in between.00:54:00
Sounds, too.00:54:01
You can play sounds like "meow" or
anything you might import or record,00:54:05
yourself.00:54:06
Then there's these things Scratch calls
events and the most important of these00:54:09
is the first, when green flag clicked.00:54:11
Because if we look over to the
right of Scratch's world here,00:54:14
this rectangular region has
this green flag and red stop00:54:17
sign up above, one of which is
for Play one of which is for Stop00:54:20
and so that's going to allow us to
start and stop our actual programs00:54:24
when that green flag
is initially clicked.00:54:27
But you can listen for other types of
events when the spacebar is pressed00:54:31
or something else, when this sprite
is clicked or something else.00:54:35
Here you already see like a
programmer's incarnation of things00:54:39
you and I take for granted like
every day now on our phones.00:54:42
Any time you tap an icon or drag your
finger or hit a button on the side.00:54:46
These are what a programmer
would call events,00:54:48
things that happen and
are often triggered by us00:54:51
humans and things that a program
be it in Scratch or Python00:54:55
or C or anything else can
listen for and respond to.00:54:59
Indeed, that's why when you tap
the phone icon on your phone,00:55:01
the phone application
starts up because someone00:55:04
wrote software that's listening for a
finger press on that particular icon.00:55:08
So Scratch has these same things, too.00:55:10
Under Control in orange,
you can see that we00:55:13
can wait for one second
or repeat something00:55:15
some number of times,
10 by default, but we00:55:17
can change anything in these
white circles to anything else.00:55:20
There's another puzzle
piece here forever,00:55:22
which implies some kind of loop where
we can do something again and again.00:55:25
Even though it seems a
little tight, there's00:55:27
not much room to fit
something there, Scratch00:55:29
is going to have these
things grow and shrink00:55:31
however we want to fill
similarly shaped pieces.00:55:33
Here are those conditionals.00:55:35
If something is true or false,
then do this next thing.00:55:40
And that's how we can put in
this little trapezoid-like shape.00:55:42
Some form of Boolean expression, a
question with a yes/no, true/false,00:55:46
or one/zero answer and decide
whether to do something or not.00:55:50
You can combine these things, too.00:55:52
If something is true, do this,
else do this other thing.00:55:55
And you can even tuck
one inside of the other00:55:57
if you want to ask three
or four or more questions.00:56:01
Sensing, too, is going to be a thing.00:56:03
You can ask questions aka Boolean
expressions like is the sprite touching00:56:07
the mouse pointer, the
arrow on the screen?00:56:10
So that you can start to
interact with these programs.00:56:12
What is the distance between
a sprite and a mouse pointer?00:56:15
You can do simple calculations
just to figure out maybe00:56:17
if the enemy is getting
close to the cat.00:56:20
Under Operator some lower level
stuff like math, but also the ability00:56:23
to pick random numbers,
which for a game is great00:56:25
because then you can kind
of vary the difficulty00:56:27
or what's happening in a game
without the same game playing00:56:30
the same way every time.00:56:31
And you can combine ideas.00:56:33
Something and something must be true
in order to make that kind of decision00:56:37
before.00:56:38
Or we can even join two words together.00:56:40
Says apple and banana by default,
but you can type in or drag and drop00:56:43
whatever you want there to
combine multiple words into full,00:56:46
larger sentences.00:56:48
Then lastly down here, there's in
orange things called variables.00:56:52
In math we've obviously
got x and y and whatnot.00:56:54
In programming we'll
have the same ability00:56:56
to store in these named symbols,
x or y, values that we care about.00:57:03
Numbers or letters or words or
colors or anything, ultimately.00:57:06
But in programming you'll see that
it's much more conventional not to just00:57:09
use simple letters like x
and y and z, but to actually00:57:13
give variables full singular or plural
words to describe what they are.00:57:20
Then lastly, if this isn't
enough color blocks for you,00:57:24
you can create your own blocks.00:57:25
Indeed, this is going to be a
programming principle we'll apply today00:57:29
and with the first problem set whereby
once you start to assemble these puzzle00:57:32
pieces and you realize, oh, would have
been nice if those several pieces could00:57:37
have just been replaced by one
had MIT thought to give me that00:57:40
one puzzle piece, you yourself
can make your own blocks00:57:44
by connecting these all together,
giving them a name, and boom,00:57:47
a new puzzle piece will exist.00:57:49
So let's do the simplest,
most canonical programs00:57:51
here, starting up with
control, and I'm going00:57:53
to click and drag and drop this
thing here when green flag clicked.00:57:57
Then I'm going to grab one
more, for instance under Looks,00:58:01
and under Looks I'm going
to go ahead and just say00:58:03
something like initially not
just Hello but the more canonical00:58:07
Hello comma world.00:58:09
Now you might guess that in
this programming environment,00:58:12
I can go over here now and
click the green flag and voila,00:58:15
Hello comma world.00:58:17
So that's my first
program and obviously much00:58:19
more user friendly than typing out
the much more cryptic text that we00:58:21
saw on the screen that you,
too, will type out next week.00:58:25
But for now, we'll just focus on
these ideas, in this case, a function.00:58:28
So what it is that just happened?00:58:29
This purple block here is
Say, that's the function,00:58:32
and it seems to take some form of input
in the white oval, specifically Hello00:58:37
comma world. Well this actually fits
the paradigm that we looked00:58:40
at earlier of just inputs and outputs.00:58:42
So if I may, if you consider
what this puzzle piece is doing,00:58:45
it actually fits this model.00:58:47
The input in this case is going
to be Hello comma world in white.00:58:51
The algorithm is going to be implemented
as a function by MIT called Say00:58:55
and the output of that is going
to be some kind of side effect,00:58:57
like the cat and the speech
bubble are saying Hello, world.00:59:01
So already even that
simple drag and drop00:59:03
mimics exactly this relatively
simple mental model.00:59:07
So let's take things further.00:59:08
Let's go ahead now and make the
program a little more interactive so00:59:11
that it says something like
Hello, David, or Hello, Carter,00:59:14
or Hello to you specifically.00:59:16
And for this, I'm going
to go under Sensing.00:59:18
And you might have to poke around
to find these things the first time00:59:21
around, but I've done this a few times
so I kind of know where things are00:59:24
and what color.00:59:25
There's this function here.00:59:26
Ask what's your name,
but that's in white,00:59:28
so we can change the
question to anything00:59:30
we want, and it's going to wait for
the human to type in their answer.00:59:34
This function called Ask
is a little different00:59:37
from the Say block, which just had
this side effect of printing a speech00:59:41
bubble to the screen.00:59:42
The ask function is even more powerful
in that after it asks the human to type00:59:47
something in. This function is going
to hand you back what00:59:50
they typed in in the form of
what's called a return value, which00:59:55
is stored ultimately and by
default this thing called Answer.00:59:57
This little blue oval here
called Answer is again01:00:00
one of these variables
that in math would01:00:02
be called just x or y but in
programming we're saying what it does.01:00:05
So I'm going to go ahead and do this.01:00:07
Let me go ahead and
drag and drop this block01:00:09
and I want to ask the question
before saying anything,01:00:11
but you'll notice that
Scratch is smart and it's01:00:13
going to realize I want to
insert something in between01:00:15
and it's just going to
move things up and down.01:00:17
I'm going to let go and ask the
default question, what's your name?01:00:20
And now if I want to go ahead
and say hello, David or Carter,01:00:23
let's just do Hello
comma, because I obviously01:00:26
don't know when I'm writing the
program who's going to use it.01:00:28
So let me now grab another looks block
up here, say something again, and now01:00:35
let me go back to Sensing and now
grab the return value, represented01:00:39
by this other puzzle piece, and
let me just drag and drop it here.01:00:42
Notice it's the same shape, even
if it's not quite the same size.01:00:45
Things will grow or shrink as needed.01:00:47
All right, so let's now zoom out.01:00:49
Let me go and stop the old version
because I don't want to say Hello,01:00:52
world anymore.01:00:53
Let me hit the green
flag and what's my name?01:00:55
All right, David.01:00:56
Enter.01:00:58
Huh.01:00:59
All right, maybe I just wasn't
paying close enough attention.01:01:01
Let me try it again.01:01:03
Green flag, D-A-V-I-D, Enter.01:01:06
This seems like a bug.01:01:09
What's the bug or
mistake might you think?01:01:13
Yeah?01:01:14
AUDIENCE: Do you need to somehow add
them together in the same text box?01:01:17
DAVID MALAN: Yeah, we kind of want
to combine them in the same text box.01:01:20
And it's technically a bug because
this just looks kind of stupid.01:01:23
It's just saying David
after I asked for my name.01:01:26
I'd like it to say
maybe Hello then David,01:01:29
but it's just blowing past
the Hello and printing David.01:01:32
But let's put our finger
on why this is happening.01:01:34
You're right for the solution, but
what's the actual fundamental problem?01:01:38
In back.01:01:39
AUDIENCE: So it says hello,
but it gets to that last step01:01:42
so quickly you can't see it.01:01:43
DAVID MALAN: Perfect.01:01:44
I mean, computers are
really darn fast these days.01:01:47
It is saying Hello, all of us
are just too slow in this room01:01:50
to even see it because it's then saying
David on the screen so fast as well.01:01:54
So there's a couple of solutions
here, and yours is spot on,01:01:57
but just to poke around,
you'll see the first example01:01:59
of how many ways in programming be
it Scratch or C or Python or anything01:02:03
else, that there are going
to be to solve problems?01:02:05
We'll teach you over the
course of these weeks,01:02:07
sometimes some ways are
better relatively than others,01:02:10
but rarely is there a
best way necessarily,01:02:13
because again reasonable
people will disagree.01:02:15
And what we'll try to teach
you over the coming weeks01:02:17
is how to kind of think
through those nuances.01:02:20
And it's not going to be
obvious at first glance,01:02:22
but the more programs you
write, the more feedback01:02:24
you get, the more bugs
that you introduce,01:02:26
the more you'll get your footing with
exactly this kind of problem solving.01:02:30
So let me try this in a couple of ways.01:02:33
Up here would be one
solution to the problem.01:02:35
MIT anticipated this kind of issue,
especially with first-time programmers,01:02:40
and I could just use a
puzzle piece that says01:02:42
say the following for
two seconds or one second01:02:44
or whatever, then do the
same with the next word01:02:47
and it might be kind
of a bit of a pause,01:02:50
Hello, one second, two seconds, David,
one second, two seconds, but at least01:02:54
it would look a little
more grammatically correct.01:02:56
But I can do it a little more
elegantly, as you've proposed.01:02:59
Let me go ahead and throw
away one of these blocks,01:03:01
and you can just drag and let
go and it'll delete itself.01:03:04
Let me go down to Operators because
this Join block here is the right shape.01:03:10
So even if you're not sure what goes
where, just focus on the shapes first.01:03:13
Let me drag this over here.01:03:15
It grew to fill that.01:03:17
Let me go ahead and
say hello comma space.01:03:20
Now it could just say by
default Hello, banana,01:03:22
but let me go back to
Sensing, Drag answer,01:03:27
and that's going to drag and drop there.01:03:29
So now notice we're sort of stacking
or nesting one block on another01:03:34
so that the output of one becomes the
input to another, but that's OK here.01:03:38
Let me go ahead and zoom
out, hit Stop, and hit Play.01:03:42
All right, what's your name?01:03:43
D-A-V-I-D, Enter, and voila.01:03:45
Now it's presumably
as we first intended.01:03:48
[APPLAUSE]01:03:49
(LAUGHING) Oh, thank you.01:03:55
Thank you.01:03:56
No minus 2 this time.01:03:57
So consider that even with
this additional example,01:04:02
it still fits the same mental model,
but in a little more interesting way.01:04:05
Here's that new function
Ask something and wait.01:04:08
And notice that in this case too
there's an input, otherwise known01:04:12
henceforth as an argument
or a parameter, programming01:04:15
speak for just an input in
the context of a function.01:04:18
If we use our drawing as before
to represent this thing here,01:04:21
we'll see that the input now is going
to be quote unquote "What's your name?"01:04:26
The algorithm is going to be implemented
by way of this new puzzle piece,01:04:30
the function called Ask, and the
output of that thing this time01:04:33
is not going to be the
cat saying anything yet,01:04:36
but rather it's going
to be the actual answer.01:04:39
So instead of the visual side effect
of the speech bubble appearing,01:04:43
now nothing visible is happening yet.01:04:45
Thanks to this function it's sort of
handing me back like a scrap of paper01:04:49
with whatever I typed in written on it
so I can reuse D-A-V-I-D one or more01:04:55
times even like I did.01:04:57
Now what did I then do with that value?01:05:00
Well consider that with
the subsequent function01:05:04
we had this Say block,
too, combined with a join.01:05:08
So we have this variable
called Answer, we're joining it01:05:11
with that first argument, Hello.01:05:14
So already we see that
some functions like Join01:05:16
can take not one but two arguments,
or inputs, and that's fine.01:05:20
The output of Join is presumably going
to be Hello, David or Hello, Carter01:05:24
or whatever the human typed in.01:05:27
That output notice is essentially
becoming the input to another function,01:05:31
Say, just because we've
kind of stacked things01:05:33
or nested them on top of one another.01:05:35
But methodically, it's
really the same idea.01:05:40
The input now are two things,
Hello comma and the return value01:05:44
from the previous Ask function.01:05:47
The function now is going to be Join,
the output is going to be Hello, David.01:05:51
But that Hello, David
output is now going01:05:53
to become the input to another function,
namely that first block called Say,01:05:57
and that's then going to have the side
effect of printing out Hello, David01:06:01
on the screen.01:06:02
So again as sort of sophisticated
as ours as yours as others programs01:06:06
are going to get, they really do
fit this very simple mental model01:06:09
of inputs and outputs and you just have
to learn to recognize the vocabulary01:06:12
and to know what kinds of puzzle
pieces or concepts ultimately to apply.01:06:17
But you can ultimately really
kind of spice these things up.01:06:19
Let me go back to my
program here that just is01:06:21
using the speech bubble at the moment.01:06:22
Scratch's inside has some pretty
fancy interactive features, too.01:06:26
I click the Extensions button
in the bottom left corner.01:06:28
And let me go ahead and choose
the Text to Speech extension.01:06:33
This is using a Cloud service, so
if you have an internet connection01:06:36
it can actually talk to the
Cloud or a third party service,01:06:39
and this one is going to give me a
few new green puzzle pieces, namely01:06:42
the ability to speak
something from my speakers01:06:45
instead of just saying it textually.01:06:47
So let me go ahead and drag this.01:06:48
Now notice I don't have to interlock
them if I'm just kind of playing around01:06:51
and I want to move some things around.01:06:52
I just want to use this as
like a canvas temporarily.01:06:55
Let me go ahead and
steal the Join from here,01:06:58
put it there, let me throw away
the Say block by just moving it01:07:01
left and letting go, and
now let me join this in01:07:04
so I've now changed my program
to be a little more interesting.01:07:08
So now let me stop the old version.01:07:10
Let me start the new.01:07:11
What's your name?01:07:12
Type in David.01:07:13
And voila:01:07:14
PROGRAM: Hello, banana.01:07:18
DAVID MALAN: (LAUGHING)
OK, minus 2 for real.01:07:20
All right, so what I accidentally
threw away there, intentionally01:07:27
for instructional purposes,
was the actual answer01:07:30
that came back from the ask block.01:07:32
That's embarrassing.01:07:33
So now if I play this again,
let's click the green icon.01:07:37
What's your name?01:07:38
David. And now:01:07:40
PROGRAM: Hello, David.01:07:41
DAVID MALAN: There we go.01:07:42
Hello, David.01:07:43
All right, thank you.01:07:45
[APPLAUSE]01:07:50
OK, so we have these functions then
in place, but what more can we do?01:07:54
Well what about those conditionals
and loops and other constructs?01:07:57
How can we bring these programs
to life so it's not just01:07:59
clicking a button and voila,
something's happening?01:08:01
Let's go ahead and make this
now even more interactive.01:08:04
Let me go ahead and throw
away most of these pieces01:08:06
and let me just spice things up
with some more audio under Sound.01:08:09
I'm going to go to Play
Sound Meow until done.01:08:12
Here we go, green flag.01:08:13
[MEOW]01:08:15
OK, it's a little loud, but it
did exactly do what it said.01:08:18
Let's hear it again.01:08:19
[QUIETER MEOW]01:08:21
OK. It's kind of an underwhelming
program eventually01:08:23
since you'd like to think that the
cat would just meow on its own, but.01:08:26
[MEOW] I have to keep hitting the button.01:08:28
Well this seems like an opportunity
for doing something again and again.01:08:31
So all right, well if I
wanted to meow, meow, meow,01:08:33
let me just grab a few of these, or you
can even right click or Control click01:08:37
and you can Copy Paste
even in code here.01:08:39
Let me play this now.01:08:41
[THREE MEOWS]01:08:43
All right, so now like
it's not really emoting01:08:45
happiness in quite the same way.01:08:46
It might be hungry or upset.01:08:47
So let's slow it down.01:08:49
Let me go to Control, wait
one second in between,01:08:52
which might be a little less worrisome.01:08:55
Here we go, Play.01:08:56
[THREE SLOWER MEOWS]01:09:01
OK, so if my goal was to make
the cat meow three times,01:09:06
I dare say this code or
algorithm is correct.01:09:10
But let's now critique its design.01:09:12
Is this well-designed?01:09:14
And if not, why not?01:09:17
What are your thoughts here?01:09:19
Yeah?01:09:21
AUDIENCE: You could use the forever
or a repeat to make it more--01:09:27
DAVID MALAN: Yeah, so yeah, agreed.01:09:28
I could use forever or repeat,
but let me push a little harder.01:09:31
But why? Like this works, I'm kind of done with
the assignments, what's bad about it?01:09:36
AUDIENCE: There's too much repetition.01:09:37
DAVID MALAN: Yeah, there's
too much repetition, right?01:09:40
If I wanted to change the
sound that the cat is making01:09:42
to a different variant of meow or
have it bark instead like a dog,01:09:46
I could change it from the
dropdown here apparently,01:09:48
but then I'd have to change it here
and then I'd have to change it here,01:09:51
and God, if this were even longer
that just gets tedious quickly01:09:54
and you're probably
increasing the probability01:09:56
that you're going to
screw up and you're going01:09:57
to miss one of the dropdowns or
something stupid and introduce a bug.01:10:00
Or, if you wanted to change the
number of seconds you're waiting,01:10:02
you've got to change it in
two, maybe even more places.01:10:05
Again, you're just
creating risk for yourself01:10:07
and potential bugs in the program.01:10:09
So I do like the repeat or the forever
idea so that I don't repeat myself.01:10:13
And indeed, what I
alluded to being possible,01:10:15
copy pasting earlier, doesn't
mean it's a good thing.01:10:18
And in code, generally
speaking, when you01:10:20
start to copy and paste puzzle
pieces or text next week,01:10:23
you're probably not doing
something quite well.01:10:26
So let me go ahead and throw away most
of these to get rid of the duplication,01:10:30
keeping just two of the
blocks that I care about.01:10:33
Let me grab the Repeat block for now,
let me move this inside of the Repeat01:10:37
block, it's going to grow to fit
it, let me reconnect all this01:10:40
and change the 10 just
to a 3, and now, Play.01:10:44
[THREE SLOW MEOWS]01:10:49
So, better.01:10:50
It's the same thing.01:10:51
It's still correct, but
now I've set the stage01:10:53
to let the cat meow, for instance,
four times by changing one thing,01:10:57
40 times by changing one thing, or
it could just use the Forever block01:11:00
and just walk away and it
will meow forever instead.01:11:03
If that's your goal,
that would be better.01:11:05
A better design but still correct.01:11:07
But you know what?01:11:08
Now that I have a
program that's designed01:11:10
to have a cat meow, wow like why?01:11:13
I mean, MIT invented
Scratch, Scratch as a cat,01:11:16
why is there no puzzle
piece called Meow?01:11:18
This feels like a missed opportunity.01:11:20
Now to be fair, they gave
us all the building blocks01:11:22
with which we could implement that
idea, but a principle of programming01:11:26
and really computer science
is to leverage what we're01:11:28
going to now start calling Abstraction.01:11:30
We have step-by-step instructions
here, the Repeat, the Play,01:11:34
and the Wait that collectively
implements this idea01:11:37
that we humans would call meowing.01:11:38
Wouldn't it be nice to abstract
away those several puzzle01:11:41
pieces into just one that literally
just says what it does, meow?01:11:45
Well here's where we
can make our own blocks.01:11:48
Let me go over here to Scratch
under the pink block category01:11:52
here and let me click Make a Block.01:11:55
Here I see a slightly
different interface01:11:57
where I can choose a name for it
and I'm going to call it Meow.01:12:00
I'm going to keep it simple.01:12:01
That's it. No inputs to meow yet.01:12:03
I'm just going to click OK.01:12:05
Now I'm just going to
clean this up a bit here.01:12:08
Let me drag and drop Play
Sound and Wait over here.01:12:12
And you know what?01:12:13
I'm just going to drag this
way down here, way down01:12:15
here because now that I'm
done implementing Meow,01:12:18
I'm going to literally abstract
it away, sort of out of sight,01:12:21
out of mind, because now notice at
top left there is a new pink puzzle01:12:25
piece called Meow.01:12:26
So at this point, I'd argue it doesn't
really matter how Meow is implemented.01:12:31
Frankly, I don't know how Ask
or Say was implemented by MIT.01:12:35
They abstracted those
things away for us.01:12:37
Now I have a brand new puzzle
piece that just says what it is.01:12:40
And this is now still correct,
but arguably better design.01:12:44
Why?01:12:45
Because it's just more
readable to me, to you,01:12:47
it's more maintainable
when you look at your code01:12:49
a year from now for the first time
because you're sort of finally looking01:12:52
back at the very first
program you wrote.01:12:53
It says what it does.01:12:55
The function itself has semantics,
which conveys what's going on.01:12:59
If you really care about
how Meow is implemented,01:13:01
you could scroll down and start
to tinker with the underlying01:13:04
implementation details, but otherwise
you don't need to care anymore.01:13:09
Now I feel like there's an
even additional opportunity01:13:13
here for abstraction and to factor
out some of this functionality.01:13:17
It's kind of lame that I
have this Repeat block that01:13:21
lets me call the Meow function,
so to speak, use the Meow function01:13:24
three times.01:13:25
Wouldn't it be nice if I could
just call them Meow function,01:13:28
aka use the Meow function, and pass
it in input that tells the puzzle01:13:32
piece how many times I want it to meow?01:13:35
Well let me go ahead and
zoom out and scroll down.01:13:37
Let me right click or Control click on
the pink piece here and choose Edit,01:13:41
or I could just start from scratch,
no pun intended, with a new one.01:13:44
Now here, rather than just give this
thing a name Meow, let me go ahead01:13:48
and add an input here.01:13:50
I'm going to go ahead and
type in, for instance, n,01:13:53
for number of times to
meow, and just to make01:13:56
this even more user friendly
and self descriptive,01:13:58
I'm going to add a label,
which has no functional impact,01:14:00
it's just an aesthetic,
and I'm just going01:14:03
to say Times, just to make
it read more like English01:14:05
in this case that tells me
what the puzzle piece does.01:14:08
Now I'm going to click OK.01:14:09
And now I need to refine
this a little bit.01:14:12
Let me go ahead and grab
under Control a repeat block,01:14:18
let me move the Play, Sound,
and Wait, into the repeat block.01:14:22
I don't want 10 and I
also don't want 3 here.01:14:24
What I want now is this n that is
my actual variable that Scratch01:14:29
is creating for me that represents
whatever input the human programmer01:14:33
provides.01:14:34
Notice that snaps right in place.01:14:35
Let me connect this and now voila, I
have an even fancier version of Meow01:14:39
that is parameterized.01:14:40
It takes input that affects
its behavior accordingly.01:14:44
Now I'm going to scroll back up,
because out of sight, out of mind,01:14:47
I just care that Meow exists.01:14:49
Now I can tighten up my code, so
to speak, use even fewer lines01:14:52
to do the same thing by
throwing away the Repeat block,01:14:55
reconnecting this new puzzle piece here
that takes an input like 3 and voila,01:15:00
now we're really programming, right?01:15:02
We've not made any forward
progress functionally.01:15:04
The thing just mouse three times.01:15:06
But it's a better design.01:15:09
As you program more and
more, these are the kinds01:15:11
of instincts still start
to acquire so that one,01:15:13
you can start to take a big assignment,
a big problem set, something01:15:16
for homework even, that feels kind of
overwhelming at first, like, oh my God01:15:20
where do I even begin?01:15:21
But if you start to identify what are
the subproblems of a bigger problem?01:15:25
Then you can start making progress.01:15:27
I do this to this day where if I have to
tackle some programming-related project01:15:32
it's so easy to drag my feet and ugh,
it's going to take forever to start,01:15:36
until I just start writing
down like a to do list01:15:38
and I start to modularize the
program and say, all right, well01:15:41
what do I want this thing to do?01:15:42
Meowing.01:15:43
What's that mean? I've got to have it say
something on the screen.01:15:45
All right, I need to have it
say something on the screen01:15:48
some number of times.01:15:49
Like literally a mental or written
checklist, or pseudocode code,01:15:52
if you will, in English on a
piece of paper or text file,01:15:55
and then you can decide,
OK, the first thing I01:15:57
need to do for homework to
solve this real world problem,01:16:00
I just need a Meow function.01:16:02
I need to use a bunch
of other code, too,01:16:04
but I need to create a
Meow function and boom,01:16:06
now you have a piece of the problem
solved not unlike we did with the phone01:16:10
book there, but in this case, we'll
have presumably other problems to solve.01:16:14
All right, so what more can we do?01:16:16
Let's add a few more
pieces to the puzzle here.01:16:18
Let's actually interact
with the cat now.01:16:20
Let me go ahead and now when the
green flag is clicked, let me go ahead01:16:24
and ask a question using an event here.01:16:26
Let me go ahead and
say, let's see, I want01:16:31
to do something like implement
the notion of petting the cat.01:16:34
So if the cursor is touching the
cat like here, something like this,01:16:39
it'd be cute if the cat meows
like you're petting a cat.01:16:42
So I'm going to ask the question,
when the green flag is clicked,01:16:45
if let's see I think I need Sensing.01:16:48
So if touching mouse
pointer, this is way too big01:16:50
but again the shape is
fine, so there goes.01:16:52
Grew to fill.01:16:53
And then if it's touching
the mouse pointer,01:16:56
that is if the cat to whom
this script or this program,01:16:59
any time I attach puzzle
pieces MIT calls them a script01:17:03
or like a program, if you will, let
me go ahead then and choose a sound01:17:07
and say play sound meow until done.01:17:10
All right, so here it is to be clear.01:17:11
When the green flag is
clicked, ask the question,01:17:13
if the cat is touching the mouse
pointer then place sound meow.01:17:18
Here we go. Play.01:17:20
[SILENCE]01:17:23
All right, let's try again.01:17:25
Play.01:17:26
[SILENCE]01:17:28
Huh.01:17:29
I'm worried it's not Scratch's
fault. Feels like mine.01:17:34
What's the bug here?01:17:37
Why doesn't this work?01:17:39
Yeah, in back, who just turned.01:17:42
AUDIENCE: [INAUDIBLE]01:17:47
DAVID MALAN: Yeah, the problem is
the moment I click that green flag,01:17:50
Scratch asks the question, is the
cat touching the mouse pointer?01:17:54
And obviously it's not because the
cursor was like up there a moment ago01:17:57
and it's not down there.01:17:58
It's fine if I move the cursor
down there, but too late.01:18:01
The program already asked the question.01:18:03
The answer was no or false or zero,
however you want to think about it,01:18:06
so no sound was played.01:18:08
So what might be the solution here be?01:18:10
I could move my cursor
quickly, but that feels01:18:12
like never going to work out right.01:18:14
Other solutions here?01:18:16
Yeah, in way back?01:18:18
Could you use the forever loop?01:18:20
The Forever loop.01:18:21
So I could indeed use this Forever
loop because if I want my program01:18:24
to just constantly listen to me, well
let's literally do something forever,01:18:28
or at least forever as
long as the program is01:18:30
running until I explicitly hit Stop.01:18:32
So let me grab that.01:18:33
Let me go to Control, let
me grab the Forever block,01:18:36
let me move the If inside of this
Forever block, reconnect this,01:18:40
go back up here, click the green
flag, and now nothing's happened yet,01:18:43
but let me try moving my cursor now.01:18:45
[MEOW]01:18:47
Oh. So now.01:18:48
[MEOW]01:18:49
That's kind of cute.01:18:50
So now the cat is actually
responding and it's01:18:52
going to keep doing
this again and again.01:18:54
So now we have this idea of taking these
different ideas, these different puzzle01:18:58
pieces, assembling them into
something more complicated.01:19:01
I could definitely put a name to this.01:19:03
I could create a custom
block, but for now01:19:05
let's just consider what kind
of more interactivity we can do.01:19:08
Let me go ahead and do this.01:19:09
By again grabbing a,
when green flag clicked,01:19:12
let me go ahead and
click the video sensing,01:19:16
and I'm going to rotate the
laptop because otherwise we're01:19:18
going to get a little inception thing
here where the camera is picking up01:19:21
the camera is up there.01:19:22
So I'm going to go reveal to
you what's inside the lectern01:19:25
here while we rotate this.01:19:29
Now that we have a non video
backdrop, I'm going to say this.01:19:34
Instead of the green flag
clicked, actually, I'm01:19:37
going to say when the video motion
is greater than some arbitrary01:19:40
measurement of motion, I'm going to go
ahead and play sound meow until done.01:19:47
And then I'm going to
get out of the way.01:19:48
So here's the cat.01:19:50
We'll put them on top of there.01:19:53
[MEOW]01:19:54
OK.01:19:55
All right, and here we go.01:19:57
[MEOW]01:20:00
So my hand is moving faster
than 50 something or other,01:20:03
whatever the unit of measure is.01:20:05
[MEOW]01:20:06
AUDIENCE: Aw. DAVID MALAN: (LAUGHING) Thank you.01:20:08
So now we have an even
more interactive version.01:20:10
[MEOW]01:20:12
But I think if I sort of slowly.01:20:15
[LAUGHING]01:20:17
(LAUGHING) Right?01:20:18
It's completely creepy, but I'm
not like exceeding the threshold--01:20:23
[MEOW]01:20:24
Until finally my hand
moves as fast as that.01:20:27
And so here actually is
an opportunity to show you01:20:29
something a former student did.01:20:31
Let me go ahead here and--01:20:32
[MEOW TWICE]01:20:33
OK, got to stop this.01:20:35
Let me go ahead and zoom out
of this in just a moment.01:20:38
[MEOW] If someone would be--01:20:39
[LAUGHING]01:20:40
(LAUGHING) If someone
would be comfortable01:20:42
coming up not only masked but
also on camera on the internet01:20:44
I thought we'd play one of your former
classmate's projects here up on stage.01:20:48
Would anyone like to volunteer
here and be up on stage?01:20:51
Who's that? Yeah.01:20:52
Come on down. What's your name?01:20:53
AUDIENCE: Sahar.01:20:54
DAVID MALAN: Sahar.01:20:55
All right, come on down.01:20:57
Let me get it set up for you here.01:20:58
[MEOW]01:20:59
[APPLAUSE]01:21:05
[MEOW]01:21:10
All right, let me go ahead
and full screen this here.01:21:13
So this is whack-a-mole by one
of your firmer predecessors.01:21:17
It's going to use the camera focusing
on your head, which will have01:21:20
to position inside of this rectangle.01:21:22
Have you ever played the
whack-a-mole game at an arcade?01:21:24
AUDIENCE: Yeah.01:21:25
DAVID MALAN: OK. So for those who haven't,
these little moles pop up01:21:27
and with a very fuzzy
hammer you sort of hit down.01:21:29
You though, if you
don't mind, you're going01:21:31
to use your head to do this virtually.01:21:34
So let's line up your head with
this red rectangle, if you could,01:21:39
we'll do beginner.01:21:40
[MUSIC PLAYING]01:21:41
All right, here we go.01:21:42
Sahar.01:21:44
Give it a moment.01:21:45
OK, come a little closer.01:21:47
[DINGING]01:21:48
And now hit the moles with your head.01:21:50
[DING]01:21:51
There we go, one point.01:21:53
[DING] One point.01:21:56
[DINGING]01:21:58
Nice.01:22:00
15 seconds to go.01:22:01
There we go. Oh yeah.01:22:02
One point.01:22:04
[LAUGHING]01:22:06
[DINGING]01:22:07
Six seconds.01:22:09
AUDIENCE: Oh no.01:22:10
DAVID MALAN: There we go.01:22:11
Quick!01:22:12
[DINGING]01:22:14
All right, a round of
applause for Sahar.01:22:16
Thank you.01:22:18
[APPLAUSE]01:22:24
So beyond having a
little bit of fun here,01:22:26
the goal was to
demonstrate that by using01:22:28
some fairly simple, primitive,
some basic building blocks01:22:31
but assembling them in a fun
way with some music, maybe01:22:34
some new costumes or artwork, you
can really bring programs to life.01:22:37
But at the end of the day, the
only puzzle pieces really involved01:22:40
were ones like the ones I just
dragged and dropped and a few more,01:22:43
because there were
clearly lots of moles.01:22:45
So the student probably created a few
different sprites, not a single cap,01:22:49
but at least four different moles.01:22:50
They had like some kind of graphic
on the screen that showed Sahar01:22:53
where to position her head.01:22:54
There were some kind of
timer, maybe a variable01:22:57
that every second was counting down.01:22:59
So you can imagine taking what looks
like a pretty impressive project01:23:02
at first glance, and
perhaps overwhelming01:23:04
to solve yourself, but just think about
what are the basic building blocks?01:23:07
And pluck off one piece of the
puzzle, so to speak, at a time.01:23:12
So indeed if we rewind a little bit.01:23:15
Let me go ahead here
and introduce a program01:23:17
that I myself made
back in graduate school01:23:20
when Scratch was first
being developed by MIT.01:23:23
Let me go ahead and open
here, give me just one second,01:23:26
something that I called back
in the day Oscar Time that01:23:30
looks a little something like this.01:23:32
If I fullscreen it and hit Play.01:23:34
[MUSIC - SESAME STREET, "I LOVE TRASH"]01:23:38
OSCAR THE GROUCH:
(SINGING) Oh, I love trash.01:23:40
DAVID MALAN: So you'll notice
a piece of trash is falling.01:23:42
I can click on it and drag and as I get
close and close to the trash can notice01:23:45
OSCAR THE GROUCH: (SINGING)
Anything ragged or--01:23:47
DAVID MALAN: It wants
to go in, it seems.01:23:49
And if I let go--01:23:50
OSCAR THE GROUCH: (SINGING) Yes, I--01:23:52
DAVID MALAN: One point.01:23:54
Here comes another. OSCAR THE GROUCH: (SINGING) If you
really want to see something trashy--01:23:56
DAVID MALAN: I'll do
the same, two points.01:23:57
OSCAR THE GROUCH: (SINGING) I have here
a sneaker that's tattered and worn--01:24:00
DAVID MALAN: There's a
sneaker falling from the sky,01:24:01
so another sprite of some sort.01:24:03
OSCAR THE GROUCH: (SINGING)
The laces are torn.01:24:05
A gift from my mother--01:24:07
DAVID MALAN: I can also
get just a little lazy01:24:09
and just let them fall into the
trash themself if I want to.01:24:13
So you can see it doesn't have
to do with my mouse cursor,01:24:15
it has to do apparently
with the distance here.01:24:18
Let's listen a little further.01:24:19
I think some additional trash
is about to make its appearance.01:24:23
Presumably there's some kind of variable
that's keeping track of this score.01:24:26
OSCAR THE GROUCH: (SINGING) I love--01:24:28
DAVID MALAN: OK, let's see
what the last chorus here is.01:24:30
OSCAR THE GROUCH:
(SINGING) Rotten stuff.01:24:32
I have here some newspaper, crusty and01:24:35
DAVID MALAN: OK, and thus he continues.01:24:37
And the song actually
goes on and on and on01:24:40
and I do not have fond memories
of implementing this and hearing01:24:43
this song for like 10
straight hours, but it's01:24:46
a good example to just consider
how was this program composed?01:24:50
How did I go about implementing
it the first time around?01:24:52
And let me go ahead and
open up some programs now01:24:54
that I wrote in advance
just so that we could01:24:56
see how these things are assembled.01:24:58
Honestly, the first thing
I probably did was probably01:25:02
to do something a little like this.01:25:04
Here is just a version
of the program where01:25:07
I set out to solve
just one problem first01:25:09
of planting a lamp post in the program.01:25:12
Right? I kind of had a vision of what I wanted.01:25:14
You know, it evolved
over time, certainly,01:25:15
but I knew I wanted
trash to fall, I wanted01:25:17
a cute little Oscar
the Grouch to pop out01:25:19
of the trashcan, and some other
stuff, but wow that's a lot01:25:22
to just tackle all at once.01:25:23
I'm going to start easy, download
a picture of a lamp post,01:25:26
and then drag and drop it into the
stage as a costume and boom, that's01:25:30
version one.01:25:31
It doesn't functionally do anything.01:25:33
I mean, literally that's the
code that I wrote to do this.01:25:36
All I did was use like
the Backdrops feature01:25:38
and drag and drop and
move things around,01:25:41
but it got me to version
one of my program.01:25:44
Then what might version two be?01:25:46
Well I considered what
piece of functionality01:25:48
frankly might be the easiest to
pluck off next and the trash can.01:25:52
That seems like a pretty
core piece of functionality.01:25:54
It just needs to sit
there most of the time.01:25:56
So the next thing I
probably did was to open up,01:25:59
for instance, the trash can version
here that looks a little something now01:26:05
like this.01:26:06
So this time I'll show
you what's inside here.01:26:08
There is some code, but not much.01:26:10
Notice at bottom right I change the
default cat to a picture of a trashcan,01:26:14
instead, but it's the same
principle that I can control.01:26:17
And then over here I added this code.01:26:20
When the green flag is
clicked, switch the costume01:26:23
to something I arbitrarily
called Oscar 1.01:26:25
So I found a couple
of different pictures01:26:26
of a trash can, one that looks
closed, one that looks partly open,01:26:29
and eventually one that
has Oscar coming out,01:26:32
and I just gave them different names.01:26:33
So I said Switch to Oscar 1, which
is the closed one by default,01:26:36
then forever do the following:
if touching the mouse pointer,01:26:40
then switch the costume to
Oscar 2, else switch to Oscar 1.01:26:45
That is to say, I just wanted to
implement this idea of the can opening01:26:49
and closing, even if it's not
exactly what I wanted ultimately,01:26:52
I just wanted to make
some forward progress.01:26:54
So here, when I run this program by
clicking Play, notice what happens.01:26:59
Nothing yet, but if I get
closer to the trash can,01:27:03
it indeed pops open because
it's forever listening01:27:07
for whether the sprite,
the trash can in this case,01:27:10
is touching the mouse pointer.01:27:11
And that's it.01:27:12
That was version 2, if you will.01:27:15
If I went in now and added the lamp
post and compose the program together,01:27:18
now we're starting to make progress.01:27:20
Right? Now it would look a little
something more like the program01:27:22
I intended ultimately to create.01:27:25
What piece did I probably
bite off after that?01:27:27
Well, I think what I did
is I probably decided01:27:30
let me implement one of the pieces of
trash, not the shoe in the newspaper01:27:33
all at once.01:27:34
Let's just get one piece of
trash working correctly first.01:27:37
So let me go ahead and open this one.01:27:40
And again, all of these examples will
be available on the course's website01:27:43
so you can see all of
these examples, too.01:27:46
It's not terribly long, I
just implement it in advance01:27:48
so we could flip
through kind of quickly.01:27:50
Here's what I did here.01:27:52
On the right hand side, I turned
my sprite into a piece of trash01:27:56
this time instead of a cat,
instead of a trash can,01:27:58
and I also created, with Carter's help,
a second sprite, this one a floor.01:28:04
It's literally just a black line
because I just wanted initially01:28:07
to have some notion of a
floor so I could detect01:28:09
if the trash is touching the floor.01:28:12
Now without seeing the code yet,
just hearing that description,01:28:15
why might I have wanted the second
sprite and this black line for a floor01:28:20
with the trash intending
to fall from the sky?01:28:22
What might I have been thinking?01:28:24
Like what problem might
I be trying to solve?01:28:26
Yeah? AUDIENCE: You don't want the
first sprite to go through it.01:28:28
DAVID MALAN: Yeah, you don't want
the first sprite to start at the top,01:28:31
go through, and then boom,
you completely lose it.01:28:34
That would not be a very useful thing.01:28:36
Or it would seem to maybe eat up more
and more of the computer's memory01:28:39
if the trash is just endlessly
falling and I can't grab it.01:28:42
It might be a little traumatic
if you tried to get it01:28:44
and you can't pull it back out
and you can't fix the program.01:28:46
So I just wanted the thing to stop.01:28:48
So how might I have implemented this?01:28:50
Let's look at the code at left.01:28:51
Here I have a bit of randomness,
like I proposed earlier exists.01:28:56
There's this blue
function called Go To x,01:28:59
y that lets me move a
sprite to any position,01:29:03
up, down, left, right, I picked a random
x location, either here or over here,01:29:07
negative 240 to positive 240, and then
a y value of 180, which is the top.01:29:12
This just makes the
game more interesting.01:29:14
It's kind of lame pretty quickly if the
trash always falls from the same spot.01:29:18
Here's this a little bit of randomness,
like most any game would have,01:29:21
that spices things up.01:29:23
So now if I click the green flag,
you'll see that it just falls,01:29:26
nothing interesting is
going to happen, but it01:29:28
does stop when it touches the black
line because notice what we did here.01:29:33
I'm forever asking the question if
the distance of the sprite, the trash,01:29:37
is to the floor is greater
than zero, that's fine.01:29:41
Change the y location by negative 3.01:29:44
So move it down 3 pixels, down 3
pixels, until the distance to the floor01:29:48
is not greater than zero, it is zero
or even negative, at which point01:29:52
it should just stop moving altogether.01:29:54
There's other ways we could
have implemented this,01:29:56
but this felt like a nice,
clean way that logically, just01:29:58
made it make sense.01:29:59
OK, now I got some trash falling, I
got a trash can that opens and closes,01:30:03
I have a lamp post, now I'm a
good three steps into the program.01:30:08
We're making progress.01:30:09
If we consider one or two
final pieces, something01:30:12
like the dragging of the trash, let me
go ahead and open up this version 2.01:30:17
Dragging the trash requires
a different type of question.01:30:21
Let me zoom in here.01:30:22
Here's the piece of trash.01:30:24
I only need one sprite, no
floor here because I just01:30:26
want the human to move it up,
down, left, right and the human's01:30:29
not going to physically be able
to move it outside of the world.01:30:32
If we zoom in on this code, the way
we've solved this is as follows.01:30:36
We're using that And conjunction
that we glimpsed earlier because when01:30:40
the green flag is clicked, we're
forever asking this question or really01:30:44
these questions, plural,
if the mouse is down01:30:47
and the trash is touching the
mouse pointer, that's equivalent01:30:53
logically to clicking on the trash.01:30:55
Go ahead and move the
trash to the mouse pointer.01:30:58
So again it takes this
very familiar idea01:31:00
that you and I take for granted
every day on Macs and PCs of clicking01:31:04
and dragging and dropping.01:31:05
How is that implemented?01:31:06
Well Mac OS or Windows are
probably asking a question.01:31:10
For every icon, is the mouse down
and is the icon touching the mouse?01:31:15
If so, go to the location
of the mouse forever01:31:19
while the mouse button is clicked down.01:31:21
So how does this work in reality now?01:31:23
Let me go ahead and click on the Play.01:31:25
Nothing happens at first, but if
I click on it, I can move it up,01:31:30
down, left, right.01:31:31
It doesn't move thereafter.01:31:33
So I now need to kind of combine
this idea of dragging with falling,01:31:36
but I bet I could just start
to use just one single program.01:31:39
Right now I'm using separate
ones to show different ideas,01:31:42
but now that's another
bite out of the problem.01:31:45
If we do one last one,
something like the scorekeeping01:31:48
is interesting, because recall that
every time we dragged a piece of trash01:31:51
into the can, Oscar popped out
and told us the current score.01:31:54
So let me go ahead and find
this one, Oscar variables,01:31:59
and let me zoom in on this one.01:32:01
This one is longer because we
combined all of these elements.01:32:04
So this is the kind of thing that
if you looked at first glance, like,01:32:07
I have no idea how I would
have implemented this01:32:09
from nothing, from scratch literally.01:32:12
But again, if you take your
vision and componenitize it01:32:16
into these smaller,
bite-sized problems, you01:32:18
could take these baby
steps, so to speak, and then01:32:21
solve everything collectively.01:32:22
So what's new here is this bottom one.01:32:25
Forever do the following:
if the trash is touching01:32:30
Oscar, the other sprite that
we've now added to the program,01:32:33
change the score by 1.01:32:35
This is an orange and
indeed if we poke around01:32:37
we'll see that orange is a variable,
like an x or y but with a better name,01:32:42
changing it means to add 1 or
if it's negative subtract 1.01:32:46
Then go ahead and have the
trash go to pick random.01:32:51
What is this all about?01:32:53
Well, let me show you what it's doing
and then we can infer backwards.01:32:56
Let me go ahead and hit Play.01:32:58
All right, it's falling, I'm clicking
and dragging it, I'm moving it over,01:33:01
and I'm letting go.01:33:02
All right, let me do it once more.01:33:04
Letting go, let me stop.01:33:06
Why do I have this function at the
end called Go To x and y randomly?01:33:13
Like what problem is this solving here?01:33:16
Yeah, in way back.01:33:18
AUDIENCE: Just the same
track teleported to the top01:33:21
after you put it in the trash can.01:33:23
DAVID MALAN: Yeah, exactly.01:33:24
Even though the human perceives
this as like a lot of trash falling01:33:27
from the sky, it's
actually the same piece01:33:29
of trash, just kind of being
magically moved back to the top01:33:32
as though it's a new one.01:33:33
There, too, you have this
idea of reusable code.01:33:36
If you were constantly copying
and pasting your pieces of trash01:33:40
and creating 20 pieces of trash, 30
pieces of trash, just because you01:33:43
want the game to have that many
levels, probably doing something wrong.01:33:46
Reuse the code that you wrote,
reuse the sprites that you wrote,01:33:49
and that would give you not just
correctness, but also a better design.01:33:54
Well let's take a look at one
final set of building blocks01:33:57
that we can compose
ultimately into something01:33:59
particularly interactive as follows.01:34:02
Let me go ahead and
zoom out here and let01:34:03
me propose that we implement something
like some kind of maze-based game.01:34:08
Let me go ahead here.01:34:09
So I want to implement
some maze-based game that01:34:13
looks at first glance like this.01:34:15
Let me hit Play. It's not a very fun game yet,
but here's a little Harvard01:34:18
shield, a couple of black lines, this
time vertical instead of horizontal,01:34:22
but notice you can't
quite see my hand here,01:34:24
but I'm using my arrow keys to go down,
to go up, to go left, to go right,01:34:28
but if I keep going right, right,
right, right, right, right,01:34:31
right it's not going anywhere.01:34:32
And left, left, left, left, left, left,
left, left, left, left, left, left,01:34:35
left it eventually stops.01:34:36
So before we look at the code,
how might this be working?01:34:41
What kinds of scripts,
collections of puzzle pieces,01:34:44
might collectively
help us implement this?01:34:47
What do you think?01:34:48
AUDIENCE: [INAUDIBLE]01:34:56
DAVID MALAN: Perfect, yeah.01:34:57
There's probably some question being
asked, if touching the black line,01:35:00
and it happens to be a couple
of sprites, each of which01:35:03
is just literally a vertical black line
we're probably asking a question like,01:35:06
are you touching it?01:35:07
Is the distance to it
zero or close to zero?01:35:10
And if so, we just ignore the left
or the right arrow at that point.01:35:16
So that works. But otherwise, if we're
not touching a wall,01:35:18
what are we probably doing
instead forever here?01:35:22
How is the movement working presumably?01:35:24
Yeah and back.01:35:26
Oh are you scratching?01:35:28
OK, sure.01:35:29
Let's go on.01:35:30
AUDIENCE: [INAUDIBLE]01:35:33
DAVID MALAN: Sorry, say a little louder.01:35:35
AUDIENCE: Presumably it's continually
looking for you to hit the arrow keys01:35:38
and then moving when you do.01:35:40
DAVID MALAN: Exactly.01:35:41
It's continually, forever listening for
the arrow keys up, down, left, right,01:35:44
and if the up arrow is
pressed, we're probably01:35:47
changing the y by a positive value.01:35:49
If the down arrow is pressed,
we're going down by y,01:35:52
and left and right accordingly.01:35:54
So let's actually take a quick look.01:35:55
If I zoom out here and take a look
at the code that implements this,01:35:59
there's a lot going on at
first glance, but let's see.01:36:01
First of all, let me drag
some stuff out of the way01:36:03
because it's kind of
overwhelming at first glance,01:36:05
especially if you, for instance, were
poking around online as for problem set01:36:09
0 just to get inspiration,
most projects out there01:36:11
are going to look
overwhelming at first glance01:36:13
until you start to wrap your
mind around what's going on.01:36:16
But in this case, we've
implemented some abstractions01:36:19
from the get go to explain to
ourselves and to anyone else looking01:36:22
at the code what's going on.01:36:24
This is that program with the two black
lines and the Harvard shield going up,01:36:29
down, left, and right.01:36:30
It initially puts the shield
in the middle, 0,0, then01:36:33
forever listens for keyboard,
as I think you were describing,01:36:37
and it feels for the walls, as
I think you were describing.01:36:40
Now how is that implemented?01:36:42
Don't know yet. These are custom blocks we created
as abstractions to kind of hide01:36:46
those implementation details
because honestly that's01:36:48
all I need to know right now.01:36:50
But, as aspiring programmers,
if we're curious now,01:36:52
let's scroll down to the
actual implementation01:36:55
of listening for keyboard.01:36:57
This is the one on the left
and it is a little long,01:36:59
but it's a lot of similar structure.01:37:02
We're doing the following, if the up
arrow is pressed, then change y by 1.01:37:07
Go up.01:37:08
If the down arrow is pressed,
then change y by negative 1.01:37:11
Go down.01:37:12
Right arrow, left arrow, and that's it.01:37:15
So it just assembles all
of those ideas, combines it01:37:17
into one new block just because
it's kind of overwhelming,01:37:20
let's just implement it
once and tuck it away.01:37:22
And if we scroll now over to
the Feel for Walls function,01:37:26
this now is asking the
question as hypothesized,01:37:30
if I'm touching the left wall, change my
x value by 1, sort of move away from it01:37:35
a little bit. If I'm touching the right
wall, then move x by negative 101:37:38
to move a little bit away from it.01:37:40
So it kind of bounces off the wall.01:37:42
Just in case it slightly went over, we
keep the crest within those two walls.01:37:47
All right, then a couple of
more pieces here to introduce.01:37:51
What if we want to actually add
some kind of adversary or opponent01:37:54
to this game?01:37:55
Well, let me go ahead to maybe this one
here where the adversary in this game01:38:02
might, for instance, be designed to
be bouncing to stand in your way.01:38:05
This is like a maze and you're trying to
get the Harvard shield from the bottom01:38:08
to the top or vice versa.01:38:09
Uh oh, Yale is in the way and it
seems to be automatically bouncing01:38:14
back and forth here.01:38:15
Well, let me ask someone else.01:38:16
Hypothesize.01:38:17
How is this working?01:38:19
This is an idea you have,
this as an idea you see.01:38:21
Let's reverse engineer in
your head how it works.01:38:26
How might this be working?01:38:27
Yeah, in back.01:38:28
AUDIENCE: If the Yale symbol is
touching a right wall or left wall,01:38:32
then have it bounce.01:38:34
DAVID MALAN: Yeah, so
if the Yale symbol is01:38:36
touching the left wall or the right
wall, we somehow have it bounce.01:38:39
And indeed we'll see there's a
puzzle piece that can do exactly01:38:41
that technically off
the edge, as we'll see,01:38:44
but there's another way we can do this.01:38:45
Let's look at the code.01:38:46
The way we ourselves
can implement exactly01:38:49
that idea bounce is just
with a little bit of logic.01:38:52
So here's what this version
of the program is doing.01:38:54
It's moving Yale by default to 0,0
just to arbitrarily put it somewhere,01:38:58
pointing it direction 90 degrees, which
means just horizontally, essentially,01:39:03
and then it's forever doing
this: if touching the left wall01:39:06
or touching the right wall,
here's our translation of bounce.01:39:10
We're just turning 180 degrees.01:39:11
And the nice thing
about that is we don't01:39:13
have to worry if we're going from
right to left or left to right.01:39:16
180 degrees is going to
work on both of the walls.01:39:19
And that's it.01:39:21
After we do that, we just move
one step, one pixel, at a time01:39:24
but we're doing it forever so
something is happening continually01:39:28
and the Yale icon is
bouncing back and forth.01:39:30
Well one final piece
here, what if now we01:39:33
want another adversary, a more advanced
adversary down the road for instance,01:39:39
to go and follow us wherever
we are such that this time01:39:44
we want the other sprite to
not just bounce back and forth,01:39:51
but literally follow us
no matter where we go.01:39:55
How might this be
implemented on the screen?01:39:59
I bet it's another forever
block, but what's inside?01:40:01
AUDIENCE: So forever get the
location of the of the Harvard shield01:40:05
and move one step towards it.01:40:06
DAVID MALAN: Yeah, forever point at
the location of the Harvard shield01:40:09
and go one step toward it.01:40:11
This is just going to go on forever if I
just give up, at least in this version.01:40:15
Notice it's sort of twitching
back and forth because it goes one01:40:18
pixel then one pixel then one pixel.01:40:20
It's sort of in a frantic state here.01:40:21
We haven't finished the game yet, but if
we see inside, we'll see exactly that.01:40:25
It didn't take much to
implement this simple idea.01:40:28
Go to a random position just
to make it kind of fair,01:40:30
initially, then forever
point towards Harvard,01:40:33
which is what we called the Harvard
crest sprite, move one step.01:40:37
Suppose we now wanted to
make a more advanced level.01:40:39
What's a minor change I could
logically make to this code just01:40:42
to make MIT even better at this?01:40:45
AUDIENCE: Change the
number of steps to two.01:40:46
DAVID MALAN: All right, change
the number of steps to two.01:40:48
So let's try that.01:40:49
So now they got twice as fast.01:40:51
Let me go ahead and just
get this out of the way.01:40:53
Oops, let me make it a fair fight.01:40:56
Green flag.01:40:58
All right, I unfortunately am
still moving one pixel at a time,01:41:01
so this isn't going to end well.01:41:03
It caught up to me.01:41:04
And if we're really aggressive and
do something like 20 steps at a time,01:41:10
click the green flag.01:41:11
Jesus, OK, so that's how you might
then make your levels progressively01:41:16
harder and harder.01:41:17
So it's not an accident that we
chose these particular examples01:41:20
here involving these particular
schools because we have one more01:41:23
demonstration we thought
we'd introduce today01:41:25
if we could get one other
volunteer to come up and play01:41:30
what was called by one of your
predecessors Ivy's Hardest Game.01:41:34
Let's see, you in the middle.01:41:35
Do you want to come on up?01:41:36
What's your name?01:41:37
AUDIENCE: Celeste.01:41:38
DAVID MALAN: Say again?01:41:39
AUDIENCE: Celeste.01:41:40
DAVID MALAN: Come a
little closer, actually.01:41:41
Sorry, hard to hear here.01:41:44
All right, round of applause
here if we could, too.01:41:47
[APPLAUSE]01:41:53
OK, sorry, what was your name?01:41:54
AUDIENCE: Celeste.01:41:55
DAVID MALAN: Ceweste AUDIENCE: Celeste.01:41:56
DAVID MALAN: Celeste. AUDIENCE: Yes.01:41:57
DAVID MALAN: Come on over. Nice to meet you, too.01:41:59
So here we have on this other
screen Ivy's Hardest Game01:42:02
written by a former CS50 student.01:42:04
I think you'll see that it
combines these same principles.01:42:07
The maze is clearly a
little more advanced.01:42:09
The goal at hand is to initially move
the Harvard crest to the sprite all01:42:14
the way on the right so that you
catch up to him in this case,01:42:17
but you'll see that
there's different levels01:42:18
and different levels of sophistication.01:42:21
So if you're up for it, you can use just
these arrow keys up, down, left, right.01:42:24
You'll be controlling the
Harvard sprite and if we01:42:27
could raise the volume just a little
bit, we'll make this our final example.01:42:32
Here we go, clicking the green flag.01:42:34
[MUSIC PLAYING]01:42:38
Feeling ready?01:42:39
AUDIENCE: Yep.01:42:40
DAVID MALAN: Spacebar.01:42:41
[MUSIC - MC HAMMER, "U CAN'T TOUCH
THIS"]01:42:43
MC HAMMER: (SINGING) Can't touch this.01:42:45
You can't touch this.01:42:48
You can't touch this.01:42:52
Can't touch this.01:42:55
My, my, my, my music--01:42:56
DAVID MALAN: Excellent.01:42:57
MC HAMMER: (SINGING) so hard.01:42:58
Makes me want to say, oh my Lord.01:43:00
Thank you for blessing me--01:43:02
DAVID MALAN: Two Yales now.01:43:03
MC HAMMER: (SINGING) Feels
good when you know you're down.01:43:06
A super dope homeboy--01:43:07
AUDIENCE: Oh! DAVID MALAN: Oh!01:43:09
Keep going.01:43:10
MC HAMMER: (SINGING)
You can't touch this.01:43:12
I told you, homeboy.01:43:13
Can't touch this.01:43:15
Yeah, that's how living--01:43:16
DAVID MALAN: All right.01:43:17
MC HAMMER: (SINGING) Can't touch this.01:43:19
Look at my eyes, man.01:43:20
You can't touch this.01:43:23
You let me bust the funky lyrics.01:43:25
You can't touch this.01:43:26
Fresh new kicks and pants.01:43:27
You got it like that and
you know you want to dance.01:43:29
So move out of your seat and get
a fly girl and catch this beat.01:43:33
[LAUGHING] Hold on.01:43:34
Pump a little bit and let them know
what's going on like that, like that.01:43:38
Cold on a mission, so fall on back.01:43:39
Let them know that you're too--01:43:41
DAVID MALAN: There you go.01:43:42
There you go.01:43:43
[APPLAUSE]01:43:46
MC HAMMER: (SINGING) Can't touch this.01:43:48
Why you standing there, man?01:43:50
You can't touch this.01:43:51
Yo, sound the bell.01:43:52
School's in, sucker.01:43:54
Can't touch this. Give me a song or rhythm, making
them sweat that's what give them.01:43:58
[CHEERING]01:43:59
They know.01:44:00
You talking the Hammer when
you're talking about a show.01:44:02
That's hyped and tight.01:44:03
Singers are sweating so them
a wipe or a tame to learn.01:44:06
DAVID MALAN: Second to last level.01:44:08
Oh! MC HAMMER: (SINGING) That chart's legit.01:44:10
Either work hard or
you might as well quit.01:44:13
That word because you know--01:44:14
DAVID MALAN: Oh!01:44:15
Keep going, keep going!01:44:17
Yes!01:44:18
MC HAMMER: (SINGING)
You can't touch this.01:44:20
DAVID MALAN: You're almost there.01:44:22
MC HAMMER: (SINGING) Break it down.01:44:23
DAVID MALAN: There you go.01:44:24
Go, go, go!01:44:26
Oh.01:44:27
One more.01:44:30
Yes!01:44:31
[CHEERING]01:44:32
There you go.01:44:36
MC HAMMER: (SINGING) Stop, Hammer time.01:44:38
"Go with the flow," it is said.01:44:39
If you can't groove to this,
then you're probably dead.01:44:42
So wave your hands in the
air, bust a few moves,01:44:44
run your fingers through your hair.01:44:45
This is it. For a winner.01:44:46
Dance to this and you're
going to get thinner.01:44:49
Now move, slide your rump.01:44:50
Just for a minute let's all do the bump.01:44:53
[CHEERING]01:44:54
DAVID MALAN: Yes!01:44:55
[APPLAUSE]01:44:56
Congratulations.01:45:03
All right, that's it for CS50.01:45:05
Welcome to the class.01:45:06
We'll see you next time.01:45:08
[MUSIC PLAYING]01:46:26
DAVID J. MALAN: All right.01:46:27
So this is CS50.01:46:29
And this is week 1, the one in which
you learn a new language, which01:46:33
is something we technically said
last week, at least if you had never01:46:36
played with this graphical language
known as Scratch before, which itself01:46:39
was a programming language.01:46:41
But today, as promised,
we transition to something01:46:43
a little more traditional,
a little more text-based,01:46:45
not puzzle piece- or
block-based, known as C.01:46:48
This is an older language.01:46:49
It's been around for decades.01:46:50
But it's a language that underlies so
many of today's more modern languages,01:46:54
among them something called
Python that we'll also01:46:57
come to in a few weeks' time.01:46:58
Indeed, at the end of
the semester, the goal01:47:00
is for you to feel that
you've not learned Scratch,01:47:02
you've not learned C, or
even Python, for that matter,01:47:04
but fundamentally that you've
learned how to program.01:47:07
Unfortunately, when you
learn how to program01:47:09
with a more traditional language like
this, there's just so much distraction.01:47:13
Last week I described all of the
syntax, all of the weird punctuation01:47:17
that you see in this, like the
hash symbol, these angled brackets,01:47:19
parentheses, curly braces,
backslash n, and more.01:47:22
Well, today we're not going to reveal
what all of those little particulars01:47:26
mean.01:47:27
But by next week, will this no
longer look like the proverbial Greek01:47:31
to you, a language that, presumably,
you've never actually seen or typed01:47:35
before.01:47:36
But to do that, we'll explore some
of the very same topics as last week.01:47:40
So recall that, via Scratch-- and
presumably via problem set 1--01:47:43
we took a look at things called
functions that are actions or verbs.01:47:46
And related to functions
were arguments like inputs.01:47:49
And related to some functions
were returned values like outputs.01:47:52
Then we talked a bit about conditionals,
forks in the road, so to speak,01:47:56
Boolean expressions, which are
just yes/no questions or true/false01:47:59
questions, loops, which let you do
things again and again, variables,01:48:03
like in math, that let you
store values temporarily,01:48:05
and then even other topics still.01:48:07
So if you were comfortable on the
heels of problem set 0 and last week,01:48:11
realize that all of these topics
are going to remain with us.01:48:14
So really, today is just about acquiring
all the more of a mental model for how01:48:18
you translate those ideas into,
presumably, a very cryptic new syntax--01:48:22
a new syntax, frankly,
that's actually more01:48:25
simple in some ways than
your own human language,01:48:27
be it English or something else, because
there's far fewer vocabulary words.01:48:31
There's actually far less
syntax that you might01:48:33
have in, say, a typical human language.01:48:35
But you need to be with these computer
languages all the more precise01:48:39
so that you're most,
ultimately, correct,01:48:41
and ultimately will see to your code
is successful along a few other lines01:48:46
as well. So if you think about the last time
you kind of wandered around not really01:48:49
knowing what you were doing
or encountered something new--01:48:51
might not have been that long ago,
entering Harvard Yard for the very01:48:54
first time, or Old Campus or the like,
be it in Cambridge or New Haven--01:48:58
you didn't really need to know how
to do everything as a first year.01:49:01
You didn't need to
know who everyone was,01:49:03
where everything was, how Harvard or
Yale, or anything else for that matter,01:49:07
worked. You sort of got by day to day by just
focusing on those things that matter.01:49:10
And anything you didn't
really understand,01:49:12
you sort of turned a blind
eye to until it's important.01:49:14
And that's, indeed, what
we're going to do today.01:49:16
And really, for the next
several weeks, we'll01:49:18
focus on details that
are initially important01:49:21
and try to wave our hands, so to speak,
at details that, yeah, eventually01:49:24
we'll get to, might be interesting.01:49:25
But for now, they might be distractions.01:49:27
And by distractions, I really
mean some of that syntax01:49:29
to which I alluded earlier.01:49:31
So by the end of today-- and
really, by the end of problem set 1,01:49:34
your first foray, presumably,
into this language called C--01:49:37
you'll have written some code.01:49:39
And you'll be asking yourself--
we'll be asking yourselves--01:49:41
just how good is that code?01:49:43
Well, first and foremost, per
last week, be it in Scratch01:49:46
or phone book form, code ultimately
needs to be correct, to be well done.01:49:51
You want the problem
to be solved correctly.01:49:53
So that one sort of goes without saying.01:49:55
And along the way this term, we'll
provide you with tools and techniques01:49:59
so you don't have to just sit there
sort of endlessly trying an input,01:50:02
checking the output, trying
another input, checking the output.01:50:05
There's a lot of automation
tools in the real world--01:50:07
and in this class and
others like it-- that01:50:09
will help facilitate you answering
that question for yourself, is my code01:50:13
correct, according to our
specifications or the like.01:50:15
But then something that's
going to take more time01:50:18
and you're probably not going to feel
100% comfortable with the first week,01:50:21
the first weeks, is just how
well designed your code is.01:50:24
It's one thing to speak
English or write English,01:50:27
but it's another thing-- or
any language, for that matter.01:50:30
But it's another thing to
speak it or write it well.01:50:32
And we spend all these years in middle
school, high school, presumably,01:50:35
writing papers and other documents,
getting grades and feedback on them01:50:38
as to how well formulated your
arguments were, how well structured01:50:41
your paper was, and the like.01:50:43
And there's that same
idea in programming.01:50:44
It doesn't matter necessarily that
you've just solved a problem correctly.01:50:49
If your code is a complete visual
mess, or if it's crazy long,01:50:53
it's going to be really
hard for someone else01:50:55
to wrap their mind around what your code
is doing and, indeed, to be confident01:50:59
if it is correct.01:51:00
And honestly, you-- the
next morning, the next year,01:51:04
the next time you look at
that code-- might have no idea01:51:07
what you yourself were even thinking.01:51:09
But you will if you focus,
too, on designing good code,01:51:13
getting your algorithms efficient,
getting your code nice and clean,01:51:16
and even making sure your
code looks pretty, which01:51:19
we'd describe as a matter of style.01:51:20
So in the written human world, having
punctuation in the right place,01:51:24
capitalization and the like-- the
sort of way you write an essay01:51:27
but not necessarily
send a text message--01:51:29
relates to style, for instance.01:51:31
And so good style in
code is going to have01:51:33
a few of these characteristics that are
pretty easily taught and remembered.01:51:36
But you just have to start to get in the
habit of writing code in a certain way.01:51:41
So these three axes, so to speak,
correctness, design, and style,01:51:44
are really the overarching
goals when writing code that01:51:48
ultimately is going to look like this.01:51:50
So this program we
conjectured last week does01:51:52
what if you run it on a Mac or
PC or somewhere else, presumably?01:51:56
What does it do?01:51:57
Yeah?01:51:59
AUDIENCE: [INAUDIBLE].01:52:00
DAVID J. MALAN: It just
prints, Hello, world.01:52:02
And honestly, that's kind
of atrocious that you01:52:04
need to hit your keyboard keys this
many times with this cryptic syntax just01:52:08
to get a program to say, Hello, world.01:52:09
So a spoiler-- in a
few weeks' time when we01:52:11
introduce other, more modern
languages, like Python,01:52:14
you can distill this same logic
into literally one line of code.01:52:18
And so we're getting there, ultimately.01:52:20
But it's helpful to understand
what it is that's going on here,01:52:23
because even though this
is a pretty cryptic syntax,01:52:25
there's nothing after this week and,
really, next week that you shouldn't01:52:28
be able to understand even about
something that right now looks01:52:30
a little something like this.01:52:32
So how do you write code?01:52:33
Well, I've given us sort
of the answer to a problem.01:52:35
How do you print, Hello,
world, on the screen?01:52:37
So what do I do with this code?01:52:39
Well, we're in the habit of typically
writing things with, like, Microsoft01:52:42
Word or Google documents.01:52:43
And yeah, I could open up Word or
Google Docs or Pages or the like01:52:48
and just literally transcribe
that character for character,01:52:50
save it, and boom, I've got a program.01:52:53
But the problem, per last week, is
that computers only understand or speak01:52:56
what other language, so to speak?01:52:59
AUDIENCE: [INAUDIBLE].01:53:00
DAVID J. MALAN: Yeah, so
binary, zeros and ones.01:53:02
And so this, obviously,
is not zeros and ones.01:53:04
So it doesn't matter if I put it in
a Word doc, Google Doc, Pages file,01:53:07
or the like. The computer is not going to
understand it until I somehow01:53:10
translate it to zeros and ones.01:53:11
And honestly, none of those
tools that I rattled off01:53:14
are really appropriate for programming.01:53:15
Why?01:53:16
Well, they come with features
like bold facing and italics01:53:19
and sort of fluffy, aesthetic stuff
that has no functional impact on what01:53:22
you're trying to do with your code.01:53:24
And they don't have
the ability, it would01:53:26
seem, to convert that code
ultimately to zeros and ones.01:53:29
But tools that do have
this capability might01:53:32
be called Integrated Development
Environments, or IDEs,01:53:36
or, more simply, text editors.01:53:38
A text editor is a tool that a
programmer uses perhaps every day01:53:42
to write their code.01:53:44
And it's a simple program-- here,
for instance, a very popular one01:53:46
called Visual Studio Code, or VS Code.01:53:49
And at the top here, you
see that I've actually01:53:51
created in advance before class a very
simple empty file called "hello.c."01:53:56
Why?01:53:57
Well, .c indicates by convention that
this is going to be a file in which01:54:00
there is C code.01:54:01
It's not .docx, which would mean in
this file is a Microsoft Word document,01:54:06
or .pages is a Pages file.01:54:08
This is .c, which means in this file is
going to be text in the language called01:54:12
C. This number 1 here is just an
automatic line number that's going help01:54:16
me keep track of how long
or short this program is.01:54:18
And the cursor is just
blinking there, waiting01:54:21
for me to start typing some code.01:54:23
Well, let me go ahead and type
out exactly the same code.01:54:26
For me, it comes pretty
comfortably from memory.01:54:28
So I'm going to go ahead and include
something called standardio.h--01:54:31
more on that later.01:54:32
I'm going to magically type int
main(void), whatever that means--01:54:37
we'll come back to that later--01:54:38
one of these curly braces and then a
sibling there that closes the same.01:54:43
Then I'm going to hit Tab
to indent a few spaces.01:54:46
And then I'm going to type not print,
but printf, then "Hello, world," /n,01:54:52
close quote, close
parenthesis, semicolon.01:54:55
And I dare say this was essentially
the very first program I wrote some01:55:00
25 years ago.01:55:01
I wrote it to say, "Hi, CS50."01:55:03
Now it just says the more canonical,
conventional, "Hello, world."01:55:06
But that's it.01:55:07
That's my very first program.01:55:08
And all I need to now do is
maybe hit Command-S or Control-S01:55:11
to save the file.01:55:12
And voila, I am a programmer.01:55:15
The catch though, is,
OK, how do I run this?01:55:17
Like, on your Mac or PC,
how do you run a program?01:55:19
Well, usually double-click an icon.01:55:21
On your phone, you tap an icon.01:55:23
In this environment that we're using
and that many programmers-- dare01:55:26
say most programmers-- use, you don't
have immediately a nice, pretty icon01:55:31
to double-click on.01:55:32
That's very user friendly,
but it's not very necessary.01:55:35
Especially when you get more
comfortable with programming,01:55:38
you're going to want to type commands
because it's just faster than pointing01:55:41
and clicking a mouse.01:55:42
And you're going to
want to automate things,01:55:43
which is a lot easier if it's all
command or text-based, as opposed01:55:46
to mouse and muscular movements.01:55:49
And so here I have my program.01:55:52
It lives in this file called "hello.c."01:55:54
I need to now convert it,
though, to zeros and ones.01:55:58
Well, how do I go about doing this,
and how am I going to get from this01:56:02
so-called code--01:56:03
or source code, as it's
conventionally called--01:56:06
to this, these zeros and ones that
we'll now start calling machine code.01:56:11
The zeros and ones from last
week can be used not only01:56:13
to represent numbers and letters,
colors, audio, video, and more.01:56:18
It can also represent instructions to a
computer, like print, or play a sound,01:56:24
or delete a file, or save a file.01:56:26
All the sort of basics
of a computer somehow01:56:28
can be represented by other
patterns of zeros and ones.01:56:32
And just like last week,
it depends on the context01:56:34
in which these numbers are stored.01:56:36
Sometimes they're interpreted as
numbers, like in a spreadsheet.01:56:39
Sometimes they're interpreted as colors.01:56:41
Sometimes they're interpreted as
instructions, commands to your computer01:56:46
to do very low-level operations,
like print something on the screen.01:56:50
So fortunately, last week's definition
of computer science of problem solving01:56:55
is a nice mental model for
exactly the goal at hand.01:56:58
I have some input, AKA source code.01:57:00
I want to output ultimately
machine code, those zeros and ones.01:57:05
I certainly don't want to do
this kind of process by hand.01:57:07
So hopefully there's an algorithm
implemented by some special program01:57:11
that does exactly that.01:57:12
And those of you who do
have some prior experience,01:57:14
this program might be called a?01:57:16
A compiler.01:57:18
So a few of you have,
indeed, programmed before.01:57:20
Not all languages use compilers.01:57:21
C, in fact, is a language
that does use a compiler.01:57:24
And so I just need to find myself--01:57:27
on my computer somewhere,
presumably-- a so-called compiler,01:57:31
a program whose purpose in life is
to convert one language to another.01:57:35
And source code written textually
in C, like we saw a moment ago,01:57:40
is source code.01:57:41
The machine code is the
corresponding zeros and ones.01:57:44
So let me go back to the same
programming environment called01:57:48
Visual Studio Code or VS Code.01:57:50
This is typically a program you
or any programmer on the internet01:57:53
can download onto their own Mac or
PC and be on their way with whatever01:57:57
computer you own writing some code.01:57:59
A downside, though, of that
approach is that all of us01:58:02
have slightly different
versions of Macs or PCs.01:58:04
We have slightly different
versions of operating systems.01:58:07
They may or may not be up to date.01:58:08
It's just a technical support nightmare
to create a uniform environment,01:58:13
especially for an introductory
class, where everyone should ideally01:58:15
be on the same page so we can
get you up and running quickly.01:58:19
And so I'm actually using a cloud-based
version of VS Code, something01:58:23
that you only need a browser to access.01:58:25
And then you can be on any
computer, today or tomorrow.01:58:28
By the end of the semester, we're
going to get you out of the cloud,01:58:32
so to speak, as best we can and
get you onto your own Mac or PC,01:58:36
so that after this class, especially if
it's the only CS class you ever take,01:58:39
you feel like you can continue
programming in any number of languages,01:58:43
even with CS50 behind you.01:58:45
But for now, wonderfully, the
browser version of VS Code01:58:47
should pretty much be identical
to what the eventual downloadable01:58:51
version of the same would be.01:58:53
And you'll see in problem
set 1 how to access this01:58:55
and how to get going yourself
with your first programs.01:58:58
But I haven't mentioned this
bottom part of the screen,01:59:01
this bottom part of the screen.01:59:03
And this is an area where we have
what's called a terminal window.01:59:06
So this is sort of old-school technology
that allows you, with a keyboard,01:59:10
to interact with a computer, wherever
it may be-- on your lap, in your pocket,01:59:15
or even, in this case, in the cloud.01:59:17
So on the top-hand
portion of this screen01:59:20
is my text editor, like tabbed
windows, like in many programs, where01:59:24
I can just create files and write code.01:59:26
The bottom of the screen here,
my so-called terminal window,01:59:30
gives me the ability to
run commands on a server01:59:33
that currently I have
exclusive access to.01:59:35
So because I logged into VS
Code with my account online,01:59:39
I have my own sort of virtual
server, if you will, in the cloud--01:59:44
otherwise known as, in
this context, a container.01:59:46
This has its own operating system
for me, its own hard drive,01:59:49
if you will, where I can save
and create files of my own,01:59:52
separate from yours and vice versa.01:59:54
And it's at this very
simple prompt, which01:59:57
is conventionally-- but not always--
abbreviated by a dollar sign,02:00:00
has nothing to do with currency.02:00:01
It just means, type your commands here.02:00:03
This is where I'm going to
be able to type commands,02:00:05
like compile my source
code into machine code.02:00:09
So it's a Command Line Interface, or
CLI, on top of an operating system02:00:15
that you might not have ever used
or seen, but it's very popular,02:00:18
called Linux.02:00:19
Odds are almost all of us in this room
are using Mac OS or Windows right now,02:00:23
but we're all going to start using an
operating system called Linux, which02:00:26
is in a family of operating systems
that offer not only this command02:00:30
line interface, but are used not
just for programming, but for serving02:00:33
websites and developing
applications and the like.02:00:35
And it's, indeed, a familiar and very
powerful interface, as we'll see.02:00:40
So how do I go about making this
file, hello.c, into a program?02:00:45
There's no icon to double-click,
but there is a command.02:00:48
I can type, make hello, at this dollar
sign prompt, go ahead and hit Enter,02:00:54
and nothing appears to happen.02:00:56
But that's a good thing.02:00:57
And as we'll see in
programming, almost always,02:00:59
if you don't see anything go wrong,
that means everything went right.02:01:02
So this is going to
be a rarity at first,02:01:04
but this is a good thing that
it just seems to do nothing.02:01:07
But now there is in the
folder in my accounts02:01:11
in this on the cloud
a file called "hello."02:01:15
And it's a bit of a weird command,
but you'll get familiar with it02:01:18
before long.02:01:19
. just means go into my current folder.02:01:22
/hello means run the program called
"hello" in this current folder.02:01:28
So ./hello, and then Enter, and voila,
now I'm actually not just programming,02:01:32
but running my actual code.02:01:35
So what have I just done?02:01:38
Let me go ahead and do this.02:01:40
I'm going to go ahead and open
up the sidebar of this program,02:01:42
and you'll see in problem
set 1 how to do this.02:01:44
And this might look a little different
based on your own configuration.02:01:48
Even the color scheme I'm using might
ultimately look different from yours,02:01:51
because it supports a
nice colorful theme.02:01:52
So you can have different colors
and brightnesses depending02:01:56
on your mood or the time of day.02:01:58
What I've opened here, though, is
what is called in VS Code Explorer,02:02:01
and this is just all of the
files in my cloud account.02:02:04
And there's not many right now.02:02:05
There's only two.02:02:06
One is the file called
hello.c, and it's highlighted02:02:09
because I've got it open right there.02:02:10
And the other is a file called
"hello," which is brand new02:02:14
and was created when I ran that command.02:02:17
And what's now worth noting is that
now things are getting a little more02:02:21
like Mac OS and Windows.02:02:22
Like on the left-hand side, you have
a GUI, a Graphical User Interface.02:02:26
But on the bottom here, again, you
have a CLI, Command Line Interface.02:02:30
These are just different ways
to interact with computers,02:02:32
and you'll get comfortable with both.02:02:34
And honestly, you're certainly familiar
and comfortable with GUIs already,02:02:37
so it's the command line one
with which we'll spend some time.02:02:40
Now suppose that I just
wanted to do something02:02:43
more than compile this program.02:02:45
Suppose I wanted to go
ahead and remove it.02:02:47
Like, uh-uh, no, I made a mistake.02:02:48
I want to say, "Hello,
CS50," not "Hello, world."02:02:51
I could just hover up here, like in
any software, and I could right-click,02:02:55
and I could poke around, and
there, delete permanently.02:02:57
So most of us might have
that instinct on a Mac or PC.02:03:00
You right-click or Control-click,
and you poke around.02:03:02
But in a command line interface,
let me do this instead.02:03:05
The command for removing
or deleting a file02:03:08
in the world of Linux, this
other operating system,02:03:12
is just a type rm for remove,
and then "hello," Enter.02:03:16
It's a somewhat cryptic confirmation
message, but this just means,02:03:19
are you sure? I'm going to go ahead
and type Y for Yes.02:03:21
And now when I hit
Enter, watch what happens02:03:24
at top left in the Explorer, the
GUI, the graphical interface.02:03:28
Voila, it disappears.02:03:30
Not terribly exciting,
but this just means02:03:32
this is a graphical version
of what we're seeing here.02:03:35
And in fact, if you want to
never use the GUI again--02:03:38
I'll go ahead and close it
with a keyboard shortcut here--02:03:41
you can forever just type
ls for list and hit Enter.02:03:45
And you will see in the
command line interface02:03:48
all of the files in your current folder.02:03:50
So anything you can do
with a mouse, you can02:03:52
do with this command line interface.02:03:54
And indeed, we'll see many more
things that you can do as well.02:03:57
But the inventors of this, this
operating system and its predecessors,02:04:01
were very succinct.02:04:02
Like, the command is rm for remove.02:04:04
The command is ls for list.02:04:06
It's very terse. Why?02:04:07
Because it's just faster to type.02:04:10
So before we forge ahead with making
something more interesting than02:04:14
just "Hello, world,"
let me pause here to see02:04:16
if there's questions on
source code or machine02:04:19
code or compiler or this
command line interface.02:04:24
Yeah?02:04:25
AUDIENCE: [INAUDIBLE].02:04:26
DAVID J. MALAN: Really good
question, and let me recap.02:04:28
If I were to make
changes to the program,02:04:30
run it, and then maybe make other
changes and try to rerun it,02:04:33
would those changes be reflected,
even though I've reworded slightly.02:04:37
Well, let's do this. I already removed the old version.02:04:39
So let me go ahead and point
out that if I do ./hello now,02:04:43
I'm going to see some kind of error
because I just deleted the file.02:04:47
No such file or directory, so
it's not terribly user friendly,02:04:50
but it's saying what the problem is.02:04:51
Let me go ahead and remake
it by typing make hello.02:04:55
Now if I type ls, I'll see not one
but two files again, and one of them02:04:59
is even green with a little asterisk
to indicate that it's executable.02:05:03
It's sort of the textual
version of something02:05:05
you could double-click
in our human world.02:05:07
So now, of course, if I run hello, we're
back where I started, "Hello, world."02:05:10
But now suppose I change it to
"Hello, CS50," like I did years ago.02:05:15
Let me go ahead and save the file with
Command-S or Control-S. Down here now,02:05:21
let me run ./hello again, and voila.02:05:23
Huh.02:05:25
So let me ask someone else
to answer that question.02:05:27
What's the missing step?02:05:29
Why did it not say, "Hello, CS50."02:05:31
Yeah?02:05:32
AUDIENCE: [INAUDIBLE].02:05:33
DAVID J. MALAN: Yeah, so
I didn't compile it again.02:05:35
So sort of newbie mistake, you're going
to make this mistake and many others02:05:38
before long. But now let me go ahead
and remake hello, enter.02:05:42
It's going to seemingly
make the same program.02:05:45
But this time when I run
it, it's, "Hello, CS50."02:05:49
Any other questions on some
of these building blocks?02:05:53
And we'll come back to all the
crazy syntax I typed before long.02:05:56
But for now, we're focusing
on just the output.02:05:58
Yeah?02:05:59
AUDIENCE: [INAUDIBLE].02:06:00
DAVID J. MALAN: When
I keep running make,02:06:02
it creates a new version
of the machine code.02:06:05
So it keeps changing the hello program
and the hello file, and that's it.02:06:09
There's no make file, per se.02:06:12
AUDIENCE: [INAUDIBLE].02:06:13
DAVID J. MALAN: Good question, no.02:06:15
If I open up that directory, you'll
see that there's just the one.02:06:18
And it doesn't matter how
many times I run make hello--02:06:21
three, four, five-- it just
keeps overwriting the original.02:06:25
So it's kind of like just saving in
the world of Google Docs or Microsoft02:06:28
Word or the like.02:06:29
But there's an additional step today.02:06:31
We have to then convert my words to
the computer's, the zeros and ones.02:06:36
Yeah, in front.02:06:37
AUDIENCE: [INAUDIBLE].02:06:38
DAVID J. MALAN: Oh, what
happens if I run hello.c?02:06:40
So let me go ahead and do ./hello.c,
which is a mistake you'll invariably02:06:45
make early on.02:06:46
Permission denied.02:06:47
So what does that mean?02:06:48
This is where the error
messages mean something02:06:50
to the people who designed the operating
system, but it's a little cryptic.02:06:53
It's not that you don't
have access to the file.02:06:55
It means that it's not executable.02:06:57
This is not something you
have permission to run,02:07:00
but you do have permission to read
or write it-- that is, change it.02:07:04
AUDIENCE: [INAUDIBLE].02:07:06
DAVID J. MALAN: Oh,
really good question.02:07:08
So if I have named my file, hello
dot C, or more generally something02:07:11
dot C, of the things that
Make does is it automatically02:07:15
picks the file name for me.02:07:17
And we'll discuss a bit--02:07:18
we'll discuss this a bit more next week.02:07:20
Make itself-- is kind of the
first of white lies today--02:07:23
itself is not a compiler.02:07:25
It's a program that knows how to find
and use the compiler on the system02:07:30
and automatically create the program.02:07:33
If I use, as we'll discuss next
week, the actual compiler myself,02:07:37
I have to type a much longer sequence
of commands to specify explicitly02:07:41
what do I want the name
of my program to be.02:07:43
Make is a nice program,
especially in week 1,02:07:45
because it just automates
all of that for us.02:07:47
And so here, we have now a
program that very simply prints02:07:51
something on the screen.02:07:52
So let's not put this
into the context of where02:07:54
we left off last time in the context
of Scratch and inputs and outputs.02:07:58
So we discuss the last time, of
course, functions and arguments.02:08:01
Functions, again, are those actions and
verbs like say, or ask, or the like.02:08:07
And the arguments were the
inputs to those functions,02:08:09
generally in those little white
ovals that, in Scratch, you02:08:11
could type words or numbers into.02:08:13
We'll see, in all of the languages
we're going to see this term,02:08:16
have that same capability.02:08:18
And let's just start to translate
one of these things to another.02:08:20
So for instance, let's put
this same program in C,02:08:24
in the context of Scratch.02:08:26
This is what Hello, World looked like
last week in the form of one function.02:08:29
This week, of course,
it looks like print.02:08:32
And then the parentheses,
notice, are kind of02:08:34
deliberately designed in the world of
Scratch to resemble that same shape.02:08:38
Even though this is a
white oval, you kind of get02:08:40
that it's kind of evoking that
same idea with the parentheses.02:08:45
Technically the function
in C, it's not called say.02:08:49
It's not even called print.02:08:50
It's called printf.02:08:51
The F stands for formatted, but we'll
see what that means in a moment.02:08:55
But printf is the closest
analogous function02:08:57
for say in the world of
C. Notice if, though, you02:09:00
want to print something like
Hello, World or Hello CS50 in C,02:09:05
you don't just write the
words as we did last week.02:09:08
You also had an add what,
if you notice already02:09:11
what's missing from this version.02:09:13
Yeah, so the double quotes
on the left and the right.02:09:15
So, that's necessary in C whenever
you have a string of words.02:09:20
And I'm using that word deliberately.02:09:22
Whenever you have multiple words
like this, this is known as a string02:09:26
as we'll see.02:09:27
And you have to put it in double
quotes, not single quotes.02:09:30
You have to put it in double quotes.02:09:32
There's one other stupid thing
that we need to have in my C code02:09:36
in order to get this function to do
something ultimately, which is what?02:09:40
Semicolon.02:09:41
So just like in our human
world, you eventually02:09:44
got into the habit of using, at
least in formal writing, periods.02:09:47
Semicolon is generally what
you use to finish your thought02:09:50
in the world of programming with C.02:09:53
All right, so we have
that function in place.02:09:55
Now, what does this really fit
into in terms of the mental model?02:09:59
Well, functions take arguments.02:10:01
And it turns out functions can
have different types of outputs.02:10:04
And we've actually seen
both already last week.02:10:07
One type of output from a function
can be something called a side effect.02:10:11
And it generally refers
to something visual,02:10:13
like something appearing on the screen
or a sound playing from your computer.02:10:17
It's sort of a side effect of
the function doing its thing.02:10:20
And indeed, last week we saw this in
the context of passing in something02:10:24
like Hello, World as
input to the say function.02:10:27
And we saw on the screen Hello,
World, but it was kind of a one off.02:10:31
It's one and done.02:10:32
You can't actually do anything
with that visual output02:10:35
other than consume it,
visually, with your human eyes.02:10:38
But sometimes, recall last week, we
had functions like the ask block that02:10:43
actually returned me some value.02:10:44
Remember the ask, what's your name.02:10:47
It handed me back whatever
answer the human typed in.02:10:50
It didn't just arbitrarily
display it on the screen.02:10:52
The cat didn't necessarily
say it on the screen.02:10:55
It was stored, instead, in that special
variable that was called answer.02:11:01
Because some functions have not
side effects but return values.02:11:05
They hand you back an output
that you can use and reuse,02:11:09
unlike the side effect, which,
again displays and that's it.02:11:11
You can't sort of catch
it and hold on to it.02:11:14
So, in the context of last
week, we had the ask block.02:11:17
And that had this special
answer return value.02:11:20
In C, we're going to
see in just a moment,02:11:22
we could translate this as follows.02:11:25
The closest match I can
propose for the ask block02:11:29
is a function that we're going
to start calling get string.02:11:31
String is, again, a word, a
set of words, like a phrase02:11:34
or a sentence in programming.02:11:36
It, too, is a function insofar as
it takes input and pretty much--02:11:41
this isn't always true--
but very often when02:11:43
you have a word in C followed by an open
parenthesis and a closed parenthesis,02:11:48
it's most likely the name of a function.02:11:50
And we're going to see that
there's some exceptions to that.02:11:53
But for now this indeed
looks like a function02:11:55
because it matches that pattern.02:11:57
If I want to ask the question,
what's your name, question mark--02:12:00
and I'm even going to deliberately
put a space there just to kind of move02:12:03
the cursor a little bit over so that
the human isn't typing literally02:12:07
after the question mark.02:12:08
So that's just the nitpicky aesthetic.02:12:10
This is perhaps the closest analog
to just asking that question.02:12:14
But because the ask
block returns a value,02:12:18
the analog here forget string is
that it, too, returns a value.02:12:22
It doesn't just print the human's input.02:12:23
It hands it back to you in the form
of a variable, a.k.a. return value,02:12:28
that I can then use and reuse.02:12:30
Now ideally it would be as
simple as this literally02:12:33
saying answer on the left equals.02:12:36
And this is where
things start to diverge02:12:38
from math and sort of our human world.02:12:40
This equal sign, henceforth,
is not the equal sign.02:12:44
It is the assignment operator.02:12:46
To assign a value means to
store a value in some variable.02:12:50
And you read these things,
weirdly, right to left.02:12:53
So here is a function called get string.02:12:55
I claim that it's going
to return to you whatever02:12:58
the human types in as their name.02:13:00
It's going to get stored
over here on the left because02:13:03
of this so-called assignment
operator, that yes is an equal sign.02:13:06
But it doesn't mean
equality in this context.02:13:08
It makes things equal.02:13:10
But it does so by copying the value on
the right into the thing on the left.02:13:14
Unfortunately, we're not
quite done yet with C.02:13:16
And this is where, again, it
gets a little annoying at first02:13:19
where Scratch just let us express
our ideas without so much syntax.02:13:23
In C when you have a
variable you don't just02:13:27
give it a name like you did in Scratch.02:13:29
You also have to tell the computer
in advance what type of value02:13:33
it is storing.02:13:34
String is one such type of value.02:13:37
Int, for integer, is
going to be another.02:13:40
And there's even more than that
we'll see today and beyond.02:13:42
And this is partly an answer to the
question that came up one or more times02:13:46
last week, which was how does a computer
distinguish this pattern of zeros02:13:49
and ones from this.02:13:51
Like is this a letter, a number,
a color, a piece of video.02:13:55
And I just claimed last week that
it totally depends on the program.02:13:58
It depends on the context.02:13:59
And that's true.02:14:00
But within those
programs, it often depends02:14:04
on what the human programmer
said the type of the value is.02:14:08
If this specifies that
the string, which means02:14:10
interpret the following
zeros and ones that02:14:12
are stored in my program as
words or letters, more generally.02:14:16
If it's an int for integer, it would
be implying, by the programmer,02:14:20
treat the following zeros and
ones in my program as a number,02:14:24
an integer, not a string.02:14:26
So here's where this
week, unlike with Scratch,02:14:29
which is kind of figures out what you
mean, with C in a lot of languages02:14:33
you have to be this pedantic
and tell it what you mean.02:14:36
There's still one stupid thing
missing from my code here.02:14:39
What's still missing here?02:14:40
Yeah.02:14:41
AUDIENCE: [INAUDIBLE]02:14:42
DAVID J. MALAN: And we still
need the stupid semicolon.02:14:44
And I'm sort of impugning it here.02:14:45
Because honestly, these are
the kinds of stupid mistakes02:14:48
you're going to make today,
tomorrow, this weekend, next week,02:14:50
a few weeks from now, until you
start to notice this and recognize it02:14:54
as well as you do English or
whatever your spoken language is.02:14:58
Yeah, question.02:14:59
Good question.02:15:00
Suppose I mix apples and
oranges, so to speak,02:15:03
and I try to put a string in
an int or an int in a string,02:15:06
the compiler is going to complain.02:15:08
So when I run that make
command as I did earlier,02:15:10
it's not going to be nice and blissfully
quiet and just give me another prompt.02:15:14
It's going to yell at me
with honestly a very cryptic02:15:17
looking error message until we get
the muscle memory for reading it.02:15:20
Other questions.02:15:23
Ah, what happened to the backslash n.02:15:24
So, we'll come back to that
in just a moment, if we may.02:15:26
Because I have deliberately omitted
it here but we did have it earlier.02:15:29
And we'll see the different
behavior in a sec.02:15:32
Other questions.02:15:36
Yeah, not at all nitpicky.02:15:37
These are the kinds of
things that just matter.02:15:39
And it's going to take time to recognize
and develop this muscle memory.02:15:43
Everything I've typed here except,
for the W at the moment, is lowercase.02:15:48
And the W is capitalized
just because it's English.02:15:50
Everything else is lowercase.02:15:51
And this kind of varies by
language and also context.02:15:54
So, in many languages the convention
is to use all lowercase letters02:15:58
for your variable names.02:16:00
Other languages might use
some capitals, as well.02:16:02
But we'll talk about that before long.02:16:04
But this is the kind
of thing that matters02:16:05
and is hard to see at first, especially
when a little S doesn't look that02:16:09
different when it's on your tiny
laptop screen from a capital S.02:16:12
But you'll start to
develop these instincts.02:16:15
All right, so besides
this particular block,02:16:18
let's go ahead and consider how we can
go about implementing this now in code.02:16:22
So let me switch back to VS Code here.02:16:24
This was the program I had earlier.02:16:26
And let me go ahead and
undo my CS50 change.02:16:28
And this time just rerun it.02:16:30
Rerun Make on Hello with the original
version with the backslash n.02:16:35
Enter, nothing bad
seems to have happened.02:16:37
So dot slash Hello, enter Hello, World.02:16:40
Now, if you're curious,
this is a good instinct02:16:43
to start to acquire what
happens if I get rid of this.02:16:45
Well, I'm probably not going
to break things too badly.02:16:47
So let's try.02:16:48
Let me go ahead now and do Make Hello.02:16:51
Still compile.02:16:52
So it's not a really bad mistake.02:16:53
So let me go ahead and
run dot slash Hello.02:16:56
What's the difference here?02:16:58
Yeah, what do you see that's different?02:17:01
Yeah, the dollar sign, my so-called
prompt, stayed on the same line.02:17:05
Why? Well, we can presumably
infer now that the backslash02:17:08
n is some fancy notation for
saying create a new line,02:17:12
move the cursor, so to
speak, to the next line.02:17:14
Notice that the cursor will move to
the next line in my terminal window.02:17:18
If I keep hitting it,
it just automatically,02:17:20
by nature of hitting enter, does it.02:17:22
But it'd be kind of stupid if when
you run a program in this world,02:17:25
simple as it is, if
the next command is now02:17:28
weirdly spaced in the middle of
the terminal with the dollar sign,02:17:31
it just looks sloppy.02:17:33
It's really just an aesthetic argument.02:17:35
And notice that it's not acceptable or
correct to do this, to hit enter there.02:17:40
Let me go ahead and save that,
though, and see what happens.02:17:43
Let me go ahead now and
run Make Hello enter.02:17:47
Oh my god, like four errors.02:17:49
This is like, what, 10 lines of
errors for a one line program.02:17:53
And this is where, again, you'll start
to develop the instincts for just02:17:56
reading this stuff.02:17:57
These kinds of tools, like
the compiler tool we're using,02:18:00
were not designed necessarily
with user friendliness in mind.02:18:03
That's changed over the
decades, but certainly early02:18:06
on it's really just meant to be
correct and precise with its errors.02:18:09
So what did I do here?02:18:11
Missing terminating
close quote character,02:18:13
long story short, when
you have a string in C,02:18:17
your double quotes just have to
be on the same line just because.02:18:20
Now, there's the slight white lie.02:18:22
There's ways around this.02:18:23
But the best way around it is to
use this so-called escape sequence.02:18:29
To escape something means generally
to put a backslash, and then02:18:32
a special symbol like n for new line.02:18:34
And this is just the agreed upon way
that humans, decades ago, decided,02:18:39
OK you don't just hit your enter key.02:18:42
You instead put backslash n
and that tells the computer02:18:46
to move the cursor to the new line.02:18:49
So again, kind of cryptic.02:18:50
But once you know it, that's it.02:18:52
It's just another word
in our vocabulary.02:18:55
So now let me transition to making
my program a little more interactive.02:18:58
Instead of just saying
Hello, world, let me02:19:00
change it like last week
to say Hello, David,02:19:02
or whoever is interacting
with the program.02:19:04
So I'm going to do string
answer gets, get string,02:19:07
quote unquote, what's your name.02:19:11
I'm not going to bother
with a new line here.02:19:13
I could. This is now just a judgment call.02:19:15
I deliberately want the human to
type their name on the same line02:19:17
just because.02:19:18
And how do I now print this?02:19:20
Well last week recall we used say.02:19:22
And then we use the
other block called join.02:19:25
So the idea here is the same.02:19:27
But the syntax this week is
going to be a little different.02:19:30
It's going to be printf, which
prints something on the screen.02:19:33
I'm going to go ahead
and say Hello comma.02:19:38
And let me just go with this initially
with the backslash n, semicolon.02:19:43
Let me go ahead and recompile my code.02:19:46
Whoops, damn doesn't work still.02:19:50
And look at all these errors.02:19:51
There's more errors than code I wrote.02:19:54
But what's going on here?02:19:56
Well, this is actually
something, a mistake you'll see,02:19:58
somewhat often, at least initially.02:20:01
And let's start to glean
what's going on here.02:20:03
So here, if I look at the very first
line of output after the dollar sign--02:20:06
so even though it jumped
down the screen pretty fast,02:20:09
I wrote Make Hello at
the dollar sign, prompt.02:20:12
And then here's the first error.02:20:14
On Hello dot C, line 5--02:20:17
technically character 5, but generally
line is enough to get you going--02:20:21
there's an error, use of
undeclared identifier string.02:20:24
Did you mean standard in?02:20:27
So, I didn't.02:20:28
And this is not an
obvious solution at first.02:20:30
But you'll start to recognize
these patterns in error messages.02:20:34
It turns out that if I want to use
string, I actually have to do this.02:20:39
I have to include another library
up here, another line of code,02:20:43
rather, called CS50
dot H. We'll come back02:20:46
to what this means in just a moment.02:20:48
But if I now retroactively say,
all right, what does standard I/O02:20:54
do for us up here.02:20:55
Before I added that new line,
what is standard I/O doing?02:20:59
Well, if you think
back to Scratch, there02:21:01
were a few examples with the camera and
with the speech to-- the text to voice.02:21:07
Remember I had to poke around
in the extensions button.02:21:10
And then I had to load it into Scratch.02:21:12
It didn't come natively with Scratch.02:21:14
C is quite like that.02:21:15
Some functions come with the language.02:21:18
But for the most part, if you want to
use a function, an action or a verb02:21:22
like printf, you have to load
that extension, so to speak,02:21:26
that more traditionally
is called a library.02:21:29
So there is a standard I/O
library, STD I/O, standard I/O,02:21:35
where I/O just means input and output.02:21:37
Which means, just like
in MIT's World, there02:21:39
was an extension for doing text
to voice or for using your camera.02:21:43
In C, there's an extension, a.k.a.02:21:45
a library, for doing
standard input and output.02:21:49
And so if you want to use any functions
related to standard input and output,02:21:53
like text from a keyboard, you
have to include standard I/O dot02:21:58
H. And then can you use printf.02:22:02
Same goes here.02:22:03
Get string, it turns out, is a
function that CS50 wrote some time ago.02:22:08
And as we'll see over
the coming weeks, it just02:22:10
makes it way easier to
get input from a user.02:22:14
C is very good with printf at
printing output on the screen.02:22:18
C makes it really annoying and
hard, as we'll see in a few weeks,02:22:21
to just get input from the user.02:22:23
So we wrote a function
called get_string,02:22:25
but the only way you can use
that is to load the extension,02:22:29
a.k.a. load the library called CS50.02:22:32
And we'll come back in time, like,
why is it .h, why is it a hash symbol.02:22:35
But for now, standard
I/O is a library that02:22:39
gives you access to printf and
input- and output-related stuff.02:22:42
CS50 is a second library
that provides you02:22:45
with access to functions
that don't come with C02:22:48
that include something like get_string.02:22:51
So with that said, we've
now kind of teased apart02:22:55
at a high level what lines
2 and now 1 are doing.02:22:58
Let me go ahead and rerun make hello.02:23:00
Now it worked.02:23:01
So all those crazy error messages
were resolved by just one fix,02:23:05
so key takeaway is not to get
overwhelmed by the sheer number02:23:07
of errors.02:23:08
Let me now do ./hello and if I type
in my name, what am I going to say?02:23:16
What do you think?02:23:19
Yeah, hello answer, because the
computer is going to take me literally.02:23:24
And it turns out that if
you just write "hello,02:23:26
answer" all in the double quotes,
you're really just passing02:23:29
English as the input
to the printf function,02:23:32
you're not actually
passing in the variable.02:23:34
And unfortunately in
C, it's not quite as02:23:37
easy to plug things in to
other things that you've typed.02:23:40
Remember in Scratch, there
was not just the Save block02:23:43
but the Join block,
which was kind of pretty,02:23:45
you can combine apples and oranges--02:23:47
or was it apple and banana?02:23:48
Then we changed it to hello and then
the answer that the human typed in.02:23:52
In C, the syntax is going
to be a little different.02:23:54
You tell the computer inside of your
double quotes that you want to have02:23:59
a placeholder there, a so-called
format code. %s means, hey, computer,02:24:05
put a string here eventually.02:24:08
Then outside of your quotes, you
just add a comma and then you type02:24:12
in whatever variable you want the
computer to plug in at that %s location02:24:18
for you.02:24:19
So %s is a format code which
serves as a placeholder.02:24:23
And now the printf function
was designed by humans years02:24:26
ago to figure out how to
do the apple and banana02:24:28
thing of joining two words together.02:24:30
It's not nearly as user-friendly
as it is in Scratch,02:24:33
but it's a very common paradigm.02:24:35
So let me try and rerun
this now. make hello.02:24:38
No errors, that's good.02:24:40
./hello.02:24:41
What's my name, David?02:24:42
If I type Enter now, now it's hello.02:24:45
David.02:24:46
And the printf, here's the F in printf.02:24:48
It formats its input for you by using
these placeholders for things like02:24:53
strings, represented again by %s.02:24:58
So a quick question then, if I focus
here on line 7 for just a moment02:25:02
and even zoom in here, how many
inputs is printf taking as a function?02:25:09
A moment ago, I'll admit that it was
taking one input, "hello, world,"02:25:13
quote unquote.02:25:14
How many inputs might you
infer printf is taking now?02:25:19
2.02:25:20
And it's implied by this comma here,
which is separating the first one,02:25:25
quote, unquote, "hello, %s"
from the second one, answer.02:25:29
And then just as a quick safety
check here, why is it not 3?02:25:32
Because there's obviously
two commas here.02:25:35
Why is it not actually
3 arguments or inputs?02:25:38
AUDIENCE: [INAUDIBLE]02:25:42
DAVID J. MALAN: Exactly.02:25:43
The comma to the left is actually
part of my English grammar,02:25:46
that's all, so same syntax.02:25:47
And, again, here's where
programming can just be confusing02:25:50
early on because we're using the
same special punctuation to mean02:25:52
different things, it just
depends on the context.02:25:55
And so now is actually
a good time to point out02:25:57
all of the somewhat pretty colors that
have been popping up on the screen02:26:01
here--02:26:02
even though I wasn't going to a format
menu, I wasn't boldfacing things,02:26:06
I certainly wasn't changing
things to red or blue or whatnot--02:26:08
that's because a text editor like
VS Code syntax highlights for you.02:26:13
This is a feature of so many different
programming environments nowadays,02:26:17
VS Code does it as well.02:26:18
If your text editor understands the
language that you're programming in--02:26:23
C, in this case--02:26:24
it highlights in different colors the
different types of ideas in your code.02:26:28
So, for instance, string and
answer here are in black,02:26:31
but get_string a function is in
this sort of nasty brown-yellow02:26:35
here right now, but that's just
how it displays on the screen.02:26:38
The string, though, here in red
is kind of jumping out at me,02:26:41
and that's marginally useful.02:26:42
The %s is in blue.02:26:44
That's kind of nice, because
it's jumping out at me.02:26:46
And so it's just using different colors
to make different things on the screen02:26:49
pop so you can focus on
how these ideas interrelate02:26:53
and, honestly, when you
might make a mistake.02:26:55
For instance, let me accidentally
leave off this quote here.02:26:58
And now all of a sudden,
notice if I delete the quote,02:27:03
the colors start to get a little awry.02:27:05
But if I go back there and put it
back, now everything's back in place.02:27:09
What's another feature
of this text editor?02:27:11
Notice when my cursor is next
to this parenthesis, which02:27:15
demarcates the end of the
inputs to the function,02:27:18
notice that highlighted in green
here is the opening parenthesis.02:27:21
Why?02:27:22
It's just a visually useful
thing, especially when02:27:24
you start writing more and
more code, just to make02:27:26
sure your parentheses are lining up.02:27:28
And that's true for these curly braces
over here on the left and the right.02:27:31
We'll come back to those in a moment.02:27:33
If I put my cursor there, you can
see that these things correspond02:27:36
to one another.02:27:37
So it's nothing in your code
fundamentally, it's just the editor02:27:40
trying to help you, the human, program.02:27:42
And you can even see it,
though it's a little subtle--02:27:45
see these four dots here
and these four dots here?02:27:48
That's my indentation.02:27:49
I configured VS Code to
indent by four spaces, which02:27:53
is a very common convention.02:27:54
Any time I hit the Tab key, this
too can help you make sure--02:27:58
once we have more interesting
and longer programs--02:28:00
that everything lines
up nice and neatly.02:28:04
Phew.02:28:05
All right, any questions
then on printf or more?02:28:07
Yeah.02:28:08
AUDIENCE: [? Would ?]
the printf [INAUDIBLE]??02:28:11
DAVID J. MALAN: Short
answer, yes. printf02:28:12
can handle more than one
type of variable or value.02:28:16
%s is one.02:28:17
We're going to see %i is another
for plugging in an integer.02:28:20
You can have multiple i's, multiple
s's, and even other symbols too.02:28:24
We'll come back to that
in just a little bit.02:28:26
printf can take many more
arguments than just these two.02:28:29
This is just meant to be representative.02:28:32
Yeah, over here.02:28:34
Can you declare variables
within the printf?02:28:36
No.02:28:37
The only variable I'm
using right now is answer,02:28:39
and it's got to be done outside
the context of printf in this case.02:28:43
Good question, we'll see
more of that before long.02:28:46
Yeah, in back.02:28:47
AUDIENCE: [INAUDIBLE]02:28:49
DAVID J. MALAN: How do we
download the CS50 library?02:28:51
So we will show you in problems
set 1 exactly how to do that.02:28:55
It's automatically done for you in
our version of VS Code in the cloud.02:28:58
If, ultimately, you program on your own
Mac or PC, either initially or later02:29:01
on, it's also installable online.02:29:03
But if you want to ask that
via online or afterward,02:29:07
we can point you in the right direction.02:29:08
But PSet 1 will itself.02:29:10
Yeah.02:29:11
AUDIENCE: [INAUDIBLE]02:29:13
DAVID J. MALAN: String is the type
of the variable or, more properly,02:29:16
the data type of the variable.02:29:19
int is another keyword I alluded
to earlier, I haven't used it yet.02:29:22
int, for integer, is going to be
another type, or data type, of variable.02:29:26
AUDIENCE: OK. [? Thank you. ?]02:29:27
DAVID J. MALAN: Yeah.02:29:28
AUDIENCE: [INAUDIBLE]02:29:29
DAVID J. MALAN: Oh, good question.02:29:31
Could I go ahead and just
plug in this function,02:29:34
kind of like we did in Scratch,
getting rid of the variable altogether02:29:39
and just do this, which
recall, is reminiscent of what02:29:42
I did in Scratch by plopping
block on top of block on block?02:29:45
Am I answering that right?02:29:48
Can I put string in front of get_string?02:29:50
No. You only put the word string
in front of a variable02:29:53
that you want to make string.02:29:55
And even though I'm apparently
answering the wrong question,02:29:57
let me go ahead and zoom out,
save this, do make hello again.02:30:01
Seems to compile OK.02:30:03
If I run ./hello, type in David, voila.02:30:06
That, too, works.02:30:07
And so, actually, let's go down
this rabbit hole for just a moment.02:30:10
Clearly, it's still correct--02:30:12
at least, based on my limited testing.02:30:14
Is this better designed
or worse designed?02:30:18
Let's open that question
like we did last week.02:30:20
Yeah?02:30:21
Yeah, I kind of agree with that.02:30:23
Reasonable people could
disagree, but I do02:30:26
agree that this seems harder to
read because I start reading here,02:30:29
but wait a minute. get_string
is going to get used first,02:30:32
and then it's going to
give me back a value.02:30:34
So, yeah, it just feels like it
was nicer to read top to bottom,02:30:37
I would say.02:30:38
Your thoughts? AUDIENCE: [INAUDIBLE]02:30:39
DAVID J. MALAN: Yeah.02:30:40
And so this is useful if I only want
to print out the person's name once.02:30:44
If I want to use it later in a
longer program, I'm out of luck,02:30:47
and so I haven't saved it in a variable.02:30:49
So I think, long story short, we
could debate this all day long.02:30:53
But in this case, eh, if you can
make a reasonable argument one02:30:56
way or the other, that's a
pretty solid ground to stand on.02:30:59
But, invariably,
reasonable people are going02:31:01
to disagree, whether first-time
programmers or many years after that.02:31:05
So let's frame this one last example
in the context of the same process02:31:09
of taking inputs and outputs.02:31:11
The functions we've
been talking about all02:31:13
take inputs, otherwise now known as
arguments, or parameters, pretty much02:31:17
synonymous.02:31:18
That's just the fancy word
for an input to a function.02:31:21
And some functions have either
side effects, like we saw--02:31:25
printing something, saying
something on the screen,02:31:27
sort of visually or audibly--02:31:28
or they return a value, which is a
reusable value, like name or answer,02:31:33
in this case.02:31:35
If we look then at what we did last
time in the world of Scratch last week,02:31:39
the input was what's your
name, the function was ask,02:31:41
and the return value was answer.02:31:44
And now let's take a look at this block,
which is honestly a more user-friendly02:31:49
version of what we just did with the %s.02:31:51
Last week we said save, then
join, then hello and answer.02:31:54
But the interesting takeaway there
was not how to say hello anything.02:31:58
It was the fact that in Scratch
2, the output of one function,02:32:03
like the green join, could become
the input to another function,02:32:08
the purple say.02:32:09
The syntax in C is
admittedly pretty different,02:32:12
but the idea is essentially the same.02:32:14
Here, though, we have
hello, a placeholder,02:32:18
but we have to, in this
world of C, tell printf02:32:21
what we want to plug in
for that placeholder.02:32:25
It's just different. But that's the way to do it.02:32:27
When we get to Python and other
languages later in the term,02:32:29
there's actually easier ways to do this.02:32:31
But this is a very common
paradigm, particularly when02:32:34
you want to format
your data in some way.02:32:37
All right, let's then take a
step back to where we began,02:32:40
which was with that whole
program, which had the include02:32:43
and it had int main(void) and
all of this other cryptic syntax.02:32:47
This Scratch piece last week
was kind of like the go-to02:32:51
whenever you want to have a
main part of your program.02:32:53
It's not the only way to
start a Scratch program.02:32:55
You could listen for clicks or other
things, not just the green flag.02:32:59
But this was probably the most popular
place to start a program in Scratch.02:33:03
In C, the closest analog is
to literally write this out.02:33:07
So just like last week, if you were
in the habit of dragging and dropping02:33:10
when green flag clicked,
as a C programmer,02:33:13
the first thing you would do is
after creating an empty file,02:33:15
like I did with hello.c,
you'd probably type int02:33:18
main(void) open curly
brace, closed curly brace,02:33:22
and then you can put all of your
code inside of those curly braces.02:33:26
So just like Scratch had
this sort of magnetic nature02:33:29
to it where the puzzle pieces would snap
together, C, as a text-based language,02:33:33
tends to use these curly braces, one
of them opened, the other one closed.02:33:38
And anything inside of
those braces, so to speak,02:33:41
is part of this puzzle piece, a.k.a.02:33:44
main.02:33:45
So what was atop them?02:33:47
We went down this rabbit hole moment ago
with these things called header files,02:33:50
even though I didn't
call them by this name.02:33:52
But, indeed, when we have a whole
program in Scratch, super easy.02:33:56
Just have the one green flag
clicked and then say hello, world.02:33:59
There's no special syntax.02:34:00
After all, it's meant to be very
user-friendly and graphical.02:34:03
In C, though, you technically can't just
put int main(void) printf hello, world.02:34:09
You also need this.02:34:10
Because, again, you need to tell
the compiler to load the library--02:34:16
code that someone else wrote-- so that
the compiler knows what printf even is.02:34:22
You have to load the
CS50 library whenever02:34:24
you want to use get_string or
other functions, like get_int,02:34:28
as we'll soon see.02:34:29
Otherwise, the compiler won't
know what get_string is.02:34:32
You just have to do it this way.02:34:35
The specific file name
I'm mentioning here,02:34:37
stdio.h, cs50.h, is what C programmers
called a call a header file.02:34:44
We'll see eventually what's
inside of those files.02:34:46
But long story short, it's like a menu
of all of the available functions.02:34:51
So in cs50.h, there's a menu
mentioning get_string, get_int,02:34:54
and a bunch of other stuff.02:34:56
And in stdio.h, there's a menu of
functions, among which are printf.02:35:01
And that menu is what
prepares the compiler02:35:04
to know how to implement
those same functions.02:35:08
All right, let me pause here.02:35:10
Question.02:35:11
AUDIENCE: [INAUDIBLE]02:35:15
DAVID J. MALAN: Not quite.02:35:16
A library provides all of the
functionality we're talking about.02:35:20
A header file is the very specific
mechanism via which you include it.02:35:25
And we'll discuss this more next week.02:35:27
For now, they're essentially
the same, but we'll discuss02:35:30
nuances between the two next week.02:35:32
Yeah, the library would be standard
I/O. The library would CS50.02:35:36
The corresponding header
file is stdio.h, cs50.h.02:35:41
Indeed.02:35:42
Other questions. Yeah.02:35:43
AUDIENCE: [INAUDIBLE]02:35:45
DAVID J. MALAN: Indeed.02:35:46
That, too, is on the menu.02:35:48
We'll come back to that.02:35:49
But the word string--02:35:50
incredibly common in the world of
programming, it's not a CS50 idea--02:35:54
but in C, there's technically no
such data type as string by default.02:35:59
We have sort of conjured it up
to simplify the first few weeks.02:36:02
That's a training wheel that we'll
very deliberately, in a few weeks,02:36:05
take away, and we'll see why we've
been using get_string and string.02:36:09
Because C otherwise makes things
quite more challenging early on,02:36:14
which then gets besides
the point for us.02:36:16
Yeah.02:36:17
AUDIENCE: [INAUDIBLE]02:36:19
DAVID J. MALAN: Yes.02:36:20
Early on, you will have to use whatever
is prescribed by the specification.02:36:23
That will include CS50's functions.02:36:24
Long story short, you referred, I
think, a moment ago to another function02:36:28
called scanf, we won't
talk about for a few weeks.02:36:30
Long story short, in C, it's pretty easy
and possible to get input from a user.02:36:36
The catch is that it's really
easy to do it dangerously.02:36:40
And C, because it's an older,
lower-level language, so to speak,02:36:45
that gives you pretty much ultimate
control over your computer's hardware.02:36:49
It's very easy to make mistakes.02:36:51
And, indeed, that's too
why we use the library,02:36:55
so your code won't crash unintendedly.02:36:58
All right, so with this in
mind, we have this now mapping02:37:01
between the Scratch
version and the other.02:37:03
Let me just give you a quick tour of
some of the other placeholders and data02:37:06
types that students will start seeing as
we assemble more interesting programs.02:37:10
In the world of Linux, here
is a non-exhaustive list02:37:13
of commands with which you'll get
familiar over the next few weeks02:37:16
by playing with problem sets.02:37:18
We've only seen two of these so
far, ls for list, rm for others.02:37:22
But I mention them now
just so that it doesn't02:37:24
feel too foreign when you see them
on screen or online in a problem set.02:37:30
cp is going to stand for copy.02:37:32
mkdir is going to stand
for make directory. mv is02:37:35
going to stand for move or rename.02:37:38
rmdir is going to be remove directory,
and cd is going to be for change /02:37:44
and let me show you this
last one here first,02:37:46
only because it's something
you'll use so commonly.02:37:49
If I go back to my code here on
the screen, I'm going to go ahead02:37:53
and re-open the little GUI on the
left-hand side, the so-called Explorer,02:37:58
revealing that I've got two
files, hello and hello.c02:38:01
so nothing has changed since there.02:38:02
Suppose now that it's
a few weeks into class02:38:05
and I want to start
organizing the code I'm02:38:07
writing so that I have a folder
for this week or next week,02:38:10
or maybe a folder for
problem set 1, problem set 2.02:38:13
I can do this in a few ways.02:38:15
In the GUI, I can go up
here and do what most of you02:38:18
would do instinctively on a Mac or PC.02:38:19
You look for like a
folder icon, you click it,02:38:22
and then you name a
folder like PSet1, Enter.02:38:25
Voila, you've got a folder called PSet1.02:38:28
I can confirm as much with my command
line interface by typing what command?02:38:34
How can I list what's in my folder?02:38:36
Yeah, so ls for list.02:38:38
And now I see hello--02:38:39
and it's green with an
asterisk because that's02:38:41
my executable, my runnable program--02:38:43
hello.c, which is my source
code, and now PSet1 with a slash02:38:46
at the end, which just implies
that it's indeed a folder.02:38:50
All right, I didn't really
want to do it that way.02:38:52
I'd like to do it more advanced.02:38:53
So let me go ahead and right-click
on PSet1, delete permanently.02:38:57
I get a scary irreversible
error message.02:38:59
But there's nothing
in it, so that's fine.02:39:01
Now I've deleted it using the GUI.02:39:03
But now let me go ahead and start doing
the same thing from the command line.02:39:08
And if you're wondering how
things keep disappearing,02:39:11
if you hit Control-L in your terminal
window or explicitly type clear,02:39:15
it will delete everything you previously
typed just to clean things up.02:39:18
In practice, you don't need
to be doing this often.02:39:20
I'm doing it just to keep our
focus on my latest commands.02:39:23
If I do-- what was the command
to make a new directory?02:39:26
AUDIENCE: [INAUDIBLE]02:39:28
DAVID J. MALAN: Yeah, so
mkdir, make directory.02:39:30
Let me create PSet1, Enter.02:39:32
And notice at left, there's my PSet1.02:39:34
If I want to get a little
overzealous, plan for next week,02:39:37
here's my PSet2 directory.02:39:39
Suppose now I want to open those
folders on a Mac or PC or in this GUI,02:39:44
I could double-click on
it like this, and you'd02:39:46
see this little arrow is moving.02:39:48
It's not doing anything because there's
nothing in there, but that's fine.02:39:51
But suppose again I want to get more
comfortable with my command line.02:39:55
Notice if I type ls now, I
see all four same things.02:39:59
Let me change directories
with cd space PSet1 Enter.02:40:05
And now notice two things
will have happened.02:40:08
One, my prompt has changed
slightly to remind me where I am,02:40:14
just to keep me sane so that I don't
forget what folder I'm actually in.02:40:17
So here is just a visual reminder
of what folder I'm currently in.02:40:21
If I type ls now, what should
I see after hitting Enter?02:40:26
Nothing, because I've only
created empty folders so far.02:40:29
And, indeed, I see nothing.02:40:31
If I wanted to create a folder called
Mario for a program that might be02:40:35
called Mario this week, I can do that.02:40:37
Now if I type ls, there is Mario.02:40:40
Now if I do cd Mario,
notice my prompt's going02:40:42
to change to be a little more precise.02:40:44
Now I'm in PSet1/Mario.02:40:47
And notice what's happening at top left.02:40:49
Nothing now, because these
folders are collapsed.02:40:51
But if I click the little
triangle, there I see Mario.02:40:54
Nothing's going on in there
because there's no files yet.02:40:56
But suppose now I want to
create a file called mario.c.02:41:00
I could go up here, I could click the
little plus icon, and use the GUI.02:41:04
Or I can just type code mario.c.02:41:07
Voila.02:41:08
That creates a new tab for me.02:41:10
I'm not going to write any code in here
yet, but I am going to save the file.02:41:13
And now at top left, you'll
see that mario.c appears.02:41:16
So at some point, you can
eventually just close the Explorer.02:41:19
Because, again, it's not providing
you with any new information.02:41:22
It's maybe more
user-friendly, but there's02:41:23
nothing you can't do at the command
line that you could do with the GUI.02:41:27
All right, but now I'm kind of stuck.02:41:29
How do I get out of this folder?02:41:32
In my Mac or PC world,
I'd probably click02:41:34
the Back button or something like that
or just close it and start all over.02:41:37
In the terminal window,
I can do cd dot dot.02:41:42
Dot dot is a nickname, if you
will, for the parent directory.02:41:46
That is, the previous directory.02:41:48
So if I hit Enter now, notice I'm
going to close the Mario folder,02:41:52
a.k.a. directory, and
now I'm back in PSet1.02:41:56
Or, if I want to be fancy, let me
go back into Mario temporarily.02:42:00
If I type ls, there's
mario.c, just to orient us.02:42:03
If I want to do multiple things
at a time, I could do cd../..02:42:09
which goes to my parent to my
grandparent all in one breath.02:42:13
And voila, now I'm back in my
default folder, if you will.02:42:16
And one last little trick of the
trade, if I'm in PSet1/Mario like I02:42:20
was a moment ago, and you're
just tired of all the navigation,02:42:24
if you just type cd and
hit Enter, it'll whisk you02:42:26
away back to your default
folder, and you don't have02:42:29
to worry about getting there manually.02:42:31
Recall a bit ago, though, that I
was running hello as this, ./hello.02:42:38
If dot refers to my parent,
perhaps infer here syntactically,02:42:42
what does a single dot mean instead?02:42:46
It means this directory,
your current directory.02:42:49
Why is that necessary?02:42:50
It just makes super
explicit to the computer02:42:52
that I want the program called
hello that's installed here,02:42:55
not in some random other folder
on my hard drive, so to speak.02:42:59
I want the one that's
right here instead.02:43:02
All right, so besides
these commands, there's02:43:04
going to be others that
we encounter over time.02:43:06
Those are kind of the basics.02:43:07
That allows you to wean yourself off
of a GUI, Graphical User Interface,02:43:11
and start using more comfortably,
with practice and time,02:43:14
a command line interface instead.02:43:16
Well, what about those other
types, now back in the world of C?02:43:19
Those commands were not C. Those are
just command-specific to a command line02:43:23
interface, like in Linux, which,
again, we're using in the cloud.02:43:28
It's an alternative
to Mac OS and Windows.02:43:30
Back in the world of C now, we've
seen strings, which are words.02:43:34
I mentioned int or integer,
but there's others as well.02:43:38
In the world of C, we've
seen string, we will see int.02:43:42
If you want a bigger integer, there's
something literally called a long.02:43:46
If you want a single character,
there's something called a char.02:43:49
If you want a Boolean value,
true or false, there is a bool.02:43:53
And if you want a floating-point value--02:43:56
a fancy way of saying a real number,
something with a decimal point in it--02:43:59
that is what C and other
languages call a float.02:44:03
And if you want even more numbers
after the decimal point that02:44:06
is more precision, you can
use something called a double.02:44:10
That is to say, here is, again,
an example in programming02:44:14
where it's up to you now to provide
the computer with hints, essentially,02:44:17
that it will rely on to know what
is this pattern of zeros and ones.02:44:21
Is it a number, a letter?02:44:23
Is it a sound, an image,
a color, or the like?02:44:26
These are the types of data types
that provide exactly those hints.02:44:30
What are the functions that come in
the menu that is the CS50 library?02:44:35
We talked about standard I/O, and
that's just one function so far, printf.02:44:39
In the CS50 library, you can
see that it follows a pattern.02:44:42
The C50 library exists largely
for the first few weeks02:44:45
of the class to make our lives easier
when you just want to get user input.02:44:50
So if you want to get a string,
like a word or words from the human,02:44:53
you use get_string.02:44:54
If you want to get an integer from
the user, you're going to use get_int.02:44:57
When you want to get any of those
other data types, for the most part,02:45:01
you use get_ something else.02:45:03
And they're indeed all
lowercase by convention.02:45:06
What about printf?02:45:07
If we have the ability now to
store different types of data02:45:10
and we have functions with which
to get different types of data,02:45:13
how might you go about printing
different types of data?02:45:16
Well, we've seen %s for string,
%i for integer, %c for char,02:45:23
%f for a float or a double, those
real numbers I described earlier,02:45:30
and then %li for a long integer.02:45:33
So here's the first
example of inconsistencies.02:45:36
In an ideal world, that would
just be %l and we'd move on.02:45:38
It's %li instead in this case.02:45:42
That's printf and some
of its format codes.02:45:45
What more might we do?02:45:47
Well, in C, as we'll see--02:45:49
no pun intended-- there is
a whole bunch of operators.02:45:51
And, indeed, computers, one
of the first things they did02:45:54
was a lot of math and calculations, so
there's a lot of operators like these.02:45:57
Computers, and in turn, C, really
good at addition, subtraction,02:46:01
multiplication, division,
and even the percent sign,02:46:04
which is the remainder operator.02:46:05
There's a special symbol
in C and other languages02:46:08
just for getting the remainder, when
you divide one number by another.02:46:13
There are other features in the world
of C, like variables, as we've seen.02:46:18
And there's also what is of
playfully called syntactic sugar that02:46:22
makes it easier over time
to write fewer characters02:46:27
but express your thoughts the same.02:46:29
So just as a single example
of this, as a single example,02:46:33
consider this use of
a variable last week.02:46:37
Here in Scratch is how you might
set a variable called counter to 0.02:46:41
In C, it's going to be similar.02:46:43
If you want the variable
to be called counter,02:46:46
you literally write the word counter,
or whatever you want it to be called.02:46:49
You then use the assignment
operator, a.k.a. the equals sign,02:46:53
and you assign it whatever its initial
value should be here on the right.02:46:56
So, again, the 0 is going to get copied
from right to left into the variable02:47:01
because of that single equal sign.02:47:02
But this isn't sufficient
in C. What else02:47:05
is missing on the right-hand
side, instinctively now?02:47:08
Even if you've never
programmed in this before.02:47:11
Yeah, in front.02:47:12
AUDIENCE: Semicolon.02:47:13
DAVID J. MALAN: A semicolon at the end.02:47:14
And one other thing, I
think, is probably missing.02:47:16
Again.02:47:17
AUDIENCE: A data type.02:47:18
DAVID J. MALAN: A data type.02:47:19
So if we can keep going
back and forth here,02:47:22
what data type seems appropriate
intuitively for counter?02:47:26
int for integer.02:47:27
So, indeed, we need to
tell the computer when02:47:29
creating a variable what
type of data we want,02:47:32
and we need to finish our
thought with the semicolon.02:47:35
So there might be a counterpart there.02:47:38
What about in Scratch if we wanted
to increment that counter variable?02:47:43
We had this very user-friendly
puzzle piece last time02:47:45
that was change counter
by 1, or add 1 to counter.02:47:49
In C, here's where things get
a little more interesting.02:47:54
And pretty commonly done, you might
do this. counter = counter + 1;02:47:58
with a semicolon.02:47:59
And this is where, again, it's
important to note, the equal sign,02:48:02
it's not equality.02:48:03
Otherwise, this makes no sense.02:48:05
counter cannot equal
counter plus 1, right?02:48:08
That just doesn't work if we're
talking about integers here.02:48:11
That's because the equal
sign is assignment.02:48:13
So it can certainly be the
case that you calculate02:48:15
counter plus 1, whatever that is,
then you update the value of counter02:48:19
from right to left to be that new value.02:48:22
This, as we'll see,
is a very common thing02:48:25
to do in programming just to kind of
count upward, for whatever reason.02:48:29
You can write this more succinctly.02:48:30
This code here is what we'll
call syntactic sugar, sort02:48:34
of a fancy way of saying the same thing
with fewer words or fewer characters02:48:39
on the screen.02:48:40
This also adds 1, or whatever
number you type over here,02:48:44
to the variable on the left.02:48:46
And there's one other form of syntactic
sugar we're going to start seeing too,02:48:49
and it's even more terse than this.02:48:51
That too will increment counter by 1
by literally changing its value by 1.02:48:56
Or if you change it to minus
minus, subtracting 1 from it.02:48:59
You can't do that with
2 and 3 and 4, but you02:49:02
can do it by default with just plus plus
or minus minus adding or subtracting 1.02:49:07
Yeah.02:49:08
AUDIENCE: [INAUDIBLE]02:49:12
DAVID J. MALAN: Ah, so when you are
changing a variable that already02:49:15
has been created, as we did with
the code that looked like this,02:49:20
you no longer need to remind the
computer what the data type is.02:49:23
Thankfully, the computer is
at least as smart as that.02:49:27
It will remember the type of
the data that you intended.02:49:31
Other questions or comments on this?02:49:34
All right, that's quite a lot.02:49:36
Why don't we go ahead and
here take a 10-minute break?02:49:38
And we'll be back, we'll
start writing some code.02:49:41
All right, so we are back.02:49:44
We've just looked at some
of the basics of compiling,02:49:48
even if it doesn't
quite feel that basic.02:49:50
But now, let's actually
start focusing really02:49:52
on writing more and more code,
more and more interesting02:49:55
code, kind of like we dove
into Scratch last week.02:49:58
So here I have these code open.02:50:00
I've closed the GUI.02:50:01
I'm going to focus more on my
terminal window and my code editors.02:50:04
Many different ways I can create new
files, but I want to create something02:50:07
called a calculator.02:50:08
So, again, within this
environment of VS Code,02:50:11
I can literally write the code
command which is VS Code specific,02:50:15
and it just creates a new
file for me automatically.02:50:18
Or I could do that in the GUI.02:50:20
I'm going to go ahead and create
this file called calculator.c02:50:23
and I'm going to go ahead and
include some familiar things.02:50:25
So I'm just going to go ahead and
proactively include cs50.h, stdio.h.02:50:30
I'm going to go ahead from
memory and do the int void main--02:50:33
more on that next week, why it's
int, why it's void, and so forth.02:50:38
And now let me just implement
a very simple calculator.02:50:40
We saw some mathematical
operators, like plus and the like.02:50:44
So let's actually use this.02:50:45
So let me go ahead and
first give myself a variable02:50:48
called x, sort of like grade
school math or algebra.02:50:52
Let me go ahead then and
get an int, which is new,02:50:55
but I mentioned this exists.02:50:57
And then let me just ask the user
for whatever their x value is.02:51:00
The thing in the quotes is
just the English, or the string02:51:03
that I'm printing on the screen.
so I could say anything I want.02:51:06
I'm just going to say x colon
to prompt the user accordingly.02:51:09
Now I'm going to go ahead and
get another variable called y.02:51:12
I'm going to get int again.02:51:13
And now, I'm going to
prompt the user for y.02:51:16
And I'm just very nitpickly
using a space just02:51:18
to move the cursor so it doesn't
look too messy on the screen.02:51:22
And then lastly, let me go ahead and
just print out the sum of x and y.02:51:27
In an ideal world, I would just
say something like printf x + y.02:51:31
But that is not valid in C. The
first argument, recall, in printf02:51:36
has to be a string in double quotes.02:51:39
So if I want to print out
the value of an integer,02:51:43
I need to put something in quotes
here, maybe followed by a newline,02:51:47
if I want to move the cursor as well.02:51:49
So, again, we only glimpsed
it briefly, but what02:51:51
do I replace these question marks with
if I want a placeholder for an integer?02:51:55
AUDIENCE: [INAUDIBLE]02:51:56
DAVID J. MALAN: Yeah, so %i.02:51:57
Just like %s was string, %i is integer.02:52:00
So I change this %i.02:52:02
And now if I want to add x and y, for
instance, super-- simple calculator,02:52:06
doesn't do much of anything other
than addition of two integers--02:52:09
I think this works.02:52:11
And, again, it looks definitely
cryptic at first glance.02:52:14
It would be if programming
weren't this cryptic.02:52:16
Other languages will
clean this up for us.02:52:18
But, again, if you focus on the
basics, printf takes one input first--02:52:22
which is a format string with
English or whatever language,02:52:26
some placeholders, maybe--02:52:27
then it takes potentially more
arguments after the comma,02:52:31
like the value of x plus y.02:52:33
All right, let me go ahead
now and make calculator,02:52:36
which, again, compiles
my source code in C,02:52:41
pictured above, and converts
it into corresponding machine02:52:44
code, or zeros and ones.02:52:46
No error messages.02:52:47
so that's already good.02:52:48
Now I do ./calculator.02:52:50
Let's do 1 plus 1 and Enter.02:52:53
Voila.02:52:54
Now I have the makings of a calculator.02:52:57
Now let's start to tinker
with this a little bit.02:53:00
What if I instead had done this?02:53:02
int z = x + y and then plug-in z here.02:53:08
If I rerun make calculator, Enter,
rerun ./calculator, type in 1 plus 1,02:53:14
still equals 2, and let me claim that
it will work for other values as well--02:53:20
which of these versions
is better designed?02:53:22
If both seem to be correct at very
cursory glance, is this version better02:53:27
or is the previous one without the z?02:53:30
OK, so this one is arguably better
because I've now got a reusable02:53:34
variable called z that I
cannot only print but, heck,02:53:36
if my program is longer,
I can use it elsewhere.02:53:39
Counterthoughts?02:53:40
AUDIENCE: [INAUDIBLE]02:53:41
DAVID J. MALAN: Yeah.02:53:42
Debatable, like before, because
it depends on my intent.02:53:44
And, honestly, I think
a pretty good argument02:53:46
can be made for the first version.02:53:48
Because if I have no
intention of-- as you note--02:53:50
using that variable
again, you know what?02:53:53
Maybe I might as well do
this, just because it's02:53:55
one less thing to think about.02:53:57
It's one less distraction.02:53:58
It's one less line of code
to have to understand.02:54:00
It's just a little tighter.02:54:02
So here, again, it does
depend on your intention.02:54:04
But this field is pretty reasonable.02:54:07
And I think, as someone
noted earlier, when02:54:09
I did the same thing with get_string,
that, yeah, maybe kind of crossed02:54:13
s line because get_string and the
what's your name inside of it,02:54:16
it was just so much longer.02:54:17
But x + y, eh, it's not that hard
to wrap our mind around what's02:54:20
going on inside of the printf argument.02:54:23
So, again, these are the kinds
of thoughts that hopefully you'll02:54:25
acquire the instinct for
on not necessarily reaching02:54:28
the same answer as someone
else, but, again, the thought02:54:30
process is what matters here.02:54:33
All right, so how might I enhance
this program a little bit?02:54:36
Let's just talk about
style for just a moment.02:54:38
So x and y, at least in this case,
are pretty reasonable variable names.02:54:42
Why?02:54:43
Because that's the go-to
variable names in math02:54:45
when you're adding two things together.02:54:47
So x and y seem pretty reasonable.02:54:48
I could have done something like,
well, maybe my first variable02:54:53
should be called first
number and my next variable02:54:56
should be called second number.02:54:58
And then down here, I
would have to change this02:55:00
to first number plus second number.02:55:04
Like, eh, this isn't really
adding anything semantically02:55:07
to help my comprehension.02:55:08
But that would be one other
direction we could have taken things.02:55:11
So if you have very simple
ideas that are conventionally02:55:14
expressed with common variable names
like x and y, totally fine here.02:55:18
What if I want to annotate this program
and remind myself what it is it does?02:55:22
Well, I can add in C
what are called comments.02:55:25
With a slash slash, two forward slashes,
you can write a note to yourself,02:55:30
like prompt user for x.02:55:32
And then down here, I could
do something like prompt user02:55:35
for y, just to remind
myself what I'm doing there.02:55:37
And down here, perform addition.02:55:40
Now, in this case, I'm not
sure these commands are really02:55:42
adding all that much.02:55:44
Because in the time it took me to write
and eventually read these comments,02:55:47
I could have just read
the three lines of code.02:55:49
But as our programs
get more sophisticated02:55:52
and you start to learn more syntax--02:55:55
that, honestly, you might forget
the next day, the next week,02:55:58
the next month-- might be useful
to have these notes to self that02:56:01
reminds you of what your
code is doing or maybe even02:56:04
how it is doing that thing.02:56:06
With these early programs,
not really necessary,02:56:09
doesn't really add all that
much to our comprehension,02:56:11
but it is a mechanism
you have in place that02:56:14
can help you actually remind
yourself or remind someone02:56:18
else what it is that's going on.02:56:20
Well, let me go ahead and rerun
this again in this current version,02:56:23
make calculator.02:56:24
And here, too, you might
think I'm typing crazy fast--02:56:27
not really. I'm hitting Tab a lot.02:56:28
So it turns out that
Linux, the operating system02:56:32
we're using here in the cloud--02:56:33
but, actually, Windows and Mac
OS nowadays support this too--02:56:36
supports autocomplete.02:56:38
So if you only have one
program that starts with C-A-L,02:56:42
you don't have to finish writing
calculator, you can just hit Tab,02:56:45
and the computer will
finish your thought for you.02:56:47
The other thing you can do is
if you hit Up and keep going up,02:56:51
you'll scroll through your
entire history of commands.02:56:54
So there too, I've been
saving some keystrokes02:56:56
by hitting Up quickly rather than
retyping the same darn thing again02:56:59
and again. So, again, just another
little convenience02:57:02
to make programming and interacting with
the command line interface even faster.02:57:05
All right, let me go ahead and just make
sure it's compiled in the current form.02:57:09
The comments have no functional impact.02:57:11
These green things are
just notes to self.02:57:13
Let me run calculator with
maybe-- how about this?02:57:15
Instead of 1 plus 1,
how about 1 billion--02:57:22
whoops, let's do that again.02:57:23
Wa, da, da.02:57:24
1 million, 1 billion, and another 1
billion, and that answer is 2 billion.02:57:30
All right, so that seems correct.02:57:31
Let's run this program one more time.02:57:33
How about 2 billion
plus another 2 billion?02:57:39
Did you know that?02:57:42
So, apparently, it's not so correct.02:57:45
And, clearly, running 1 plus 1 was
not the most robust testing of my code02:57:49
here.02:57:50
What might have gone wrong?02:57:53
What might have gone wrong?02:57:54
Yeah.02:57:55
AUDIENCE: [INAUDIBLE]02:57:57
DAVID J. MALAN: Yeah.02:57:58
The computer probably ran
out of space with bits.02:58:00
So it turns out with these data types--
we've been talking about string and int02:58:05
and also float and char and those
other things-- they all use a specific,02:58:09
and, most importantly, finite
number of bits to represent them.02:58:13
It can vary by computer.02:58:14
Newer computers use more bits, older
computers tended to use fewer bits.02:58:18
It's not necessarily standardized
for all of these data types.02:58:21
But in this case, in this environment,
it is using 32 bits for an integer.02:58:28
That's a lot. So with 32 bits, you
can count pretty high.02:58:30
This is 64 light bulbs on the
stage and could count even higher.02:58:34
An int is only using half of these, or
we have two integers here on the stage.02:58:38
Now, if you think back to last week,
we talked about 8 bits at one point.02:58:42
And if you have 8 bits, 8 zeros and
ones, you can count as high as 256--02:58:47
just a good number to
generally remember as trivia.02:58:49
8 bits gives you 256
permutations of zeros and ones.02:58:53
32 gives you roughly how
many, if anyone knows?02:58:57
It's 2 to the 32 power.02:58:59
So it's roughly 4 billion, 2 to the 32.02:59:02
If you don't know that, it's fine.02:59:04
Most programmers, though, eventually
remember these kinds of heuristics.02:59:07
So it's roughly 4 billion.02:59:08
So that feels like enough.02:59:10
2 billion plus 2 billion
is exactly 4 billion.02:59:13
And that actually should
fit in a 32-bit integer.02:59:17
The catch is that my Mac,
your PC, and the like02:59:20
also like to support negative numbers.02:59:22
And if you want to support both positive
and negative numbers, that technically02:59:26
means with 32-bit integers,
you can count as high02:59:29
as roughly 2 billion positive
or 2 billion negative02:59:33
in the other direction.02:59:34
That's still 4 billion, give or
take, but it's only half as many02:59:38
in one direction or the other.02:59:39
So how could I go about implementing
a correct calculator here?02:59:44
What might the solution be?02:59:47
Yeah, so not just li,
which was for long integer.02:59:50
I have to make one more change,
which is to the data type itself.02:59:54
So let me go back up here and change
x from an int to a long, a.k.a.02:59:59
long integer.03:00:00
And then let me change y as well.03:00:02
And then let me change the format code
per the little cheat sheet we had up03:00:05
a few minutes ago to li.03:00:07
Let me recompile the calculator--03:00:10
seems to work OK.03:00:11
Let's rerun it.03:00:12
Now let's do 1 plus 1.03:00:14
That's should obviously be the same.03:00:15
Now let's do 2 billion
and another 2 billion03:00:20
and cross our fingers this time.03:00:22
Now we're counting as high as 4 billion.03:00:24
And we can go way higher than
4 billion, but we're only03:00:27
kicking the can down the street a bit.03:00:29
Even though we're now using--03:00:31
with a long--03:00:32
64 bits, which is as long as this
stage now, that's still a finite value.03:00:37
It might be a really big
value, but it's still finite.03:00:40
And we'll come back at the
end of today to these kinds03:00:42
of fundamental limitations.03:00:44
Because arguably now, my calculator
is correct for like millions,03:00:48
billions of possible inputs but not all.03:00:51
And that's problematic
if you actually want03:00:53
to use my calculator for any
possible inputs, not just03:00:58
ones that are roughly less than,
say, 2 billion, as in this case.03:01:03
All right, any questions then on that?03:01:05
But it's really just a
precursor for all the problems03:01:07
that we're going to have to
eventually deal with later on.03:01:10
AUDIENCE: [INAUDIBLE]03:01:15
DAVID J. MALAN: A good question.03:01:17
Yes. If we were still using z, we would
also have to change it to a long.03:01:20
Otherwise, we'd be ignoring
32 of the bits that03:01:23
had been added together via the longs.03:01:25
Good question.03:01:27
All right, so how about we spice things
up with maybe not just addition here,03:01:32
how about something
with some conditions?03:01:35
Let's start to ask
some actual questions.03:01:37
So a moment ago, recall that we had
just the declaration of variables.03:01:44
Now let's look back at
something in Scratch that03:01:46
looked a little something like
this, a bunch of puzzle pieces03:01:48
asking questions by way
of these conditionals03:01:50
and then these Boolean expressions
here in green, maybe saying something03:01:53
like x is less than y.03:01:55
In C, this actually maps pretty cleanly.03:01:58
It's much cleaner from left to right
than it was with printf and join.03:02:02
Here, we have just code
that looks like this.03:02:04
If, a space, two parentheses
and then x less than y,03:02:09
and then we have something like
printf there in the middle.03:02:12
So here, it's actually
kind of a nice mapping.03:02:14
Notice that, just as the
yellow puzzle piece in Scratch03:02:16
is kind of hugging the
purple puzzle piece,03:02:18
that's effectively the role that
these curly braces are playing.03:02:21
They're sort of encapsulating
all of the code on the inside.03:02:24
The parentheses represent
the Boolean expression03:02:27
that needs to be asked and answered to
decide whether or not to do this thing.03:02:32
And here's an exception to
what I alluded to earlier.03:02:34
Usually, when you see a word and
then a parenthesis, something,03:02:38
and then closed parenthesis, I
claimed that's usually a function.03:02:42
And I'm still feeling pretty
good about that claim.03:02:44
But there are exceptions.03:02:45
And the word if is not a function.03:02:48
It's just a programming construct.03:02:50
It's a feature of the C language
that similarly uses parentheses, just03:02:55
for different purposes
for a Boolean expression.03:02:58
How about something like this?03:02:59
Last week, if you wanted to
have a two-way fork in the road,03:03:02
go this way or that way,
you can have if and else.03:03:05
In C, that would look a
little something like this.03:03:08
And if we add in the printf's,
it now looks quite like the same,03:03:11
but it adds, of course, the word else
and then a couple of more curly braces.03:03:15
As an aside, in C, It's not strictly
necessary to have curly braces03:03:21
if you have only one line
of code indented underneath.03:03:25
For best practice, though, do so anyway,
because it makes super clear to you03:03:30
and ultimately anyone
else reading your code03:03:31
that you intend for just that one
or more line of code to execute.03:03:35
How about this from last week?03:03:37
Here was a three-way fork in the road.03:03:38
If x is less than y, else if x is
greater than y, else if x equals y.03:03:45
Now, here's where you have some
disparities between Scratch03:03:47
and C. Scratch uses an equals sign
for equality, to compare two values.03:03:52
C uses a single equals sign
for assignment from right03:03:55
to left, minor difference
between the two worlds.03:03:58
In C, we could implement the same
code like this, the addition being03:04:02
just this additional else if.03:04:04
And if we add in the printf's, it
looks a little something now like this.03:04:08
This is correct both in the
Scratch world and in the C world.03:04:12
But could someone make a claim that
this is not, again, well-designed?03:04:16
Exactly.03:04:17
We don't need the last if.03:04:18
We need the else, at least,
but we don't need the last if.03:04:21
Because, at least in the
world of comparing integers,03:04:24
it's either going to be less
than, greater than, or equal to.03:04:27
There is no other case.03:04:28
So you can save a few
seconds, if you will,03:04:31
of your program running-- a blink of
the eye-- by only asking two questions03:04:35
and then inferring what
the answer to the third03:04:37
must be just by nature of
your own human logic here.03:04:41
Now, why is that a good thing?03:04:42
If, for instance, x and y
happen to equal each other--03:04:46
I type in 1 and 1 for both values,
either in Scratch or in the C world--03:04:51
in the case of this version,
you're sort of stupidly03:04:55
asking three questions, all of
which are going to get asked03:04:59
even though the answer is no, no, yes.03:05:02
That is false, false, true.03:05:05
That seems to be unnecessary because
if we instead optimize this code,03:05:09
get rid of the unnecessary if and
just do as you proposed logically--03:05:13
else print that x is equal to y--03:05:16
now if x indeed equals y because
they're both 1 or some other value,03:05:20
now you're only going to ask two
questions, so 2/3 as many questions,03:05:26
and then you're going to get
your same correct result.03:05:29
So, again, a minor detail,
but, again, the kinds of things03:05:32
you should be thinking
about, not only as03:05:33
you write your code to
be correct but also write03:05:36
it to be well-designed as well.03:05:38
All right, so why don't we
go ahead and translate this03:05:41
into the context of an
actual program here?03:05:44
I'll create a blank window here.03:05:46
And let's do something with points,
like points on my own very first CS5003:05:50
problem set.03:05:51
Let me go ahead and
run code of points.c.03:05:54
That's just going to
give me a new text file.03:05:56
And then up here, I'm going to
do my usual, include cs50.h.03:06:00
include stdio.h.03:06:03
int main void.03:06:04
So a lot of boilerplate, so to
speak, in these early programs.03:06:08
And now, let's see.03:06:09
Let's ask the user, how
many points did they03:06:12
lose on their most recent CS50 PSet?03:06:15
So sort of evoke my photograph of
my own very first PSet last week03:06:19
where I lost a couple of points myself.03:06:20
So int points = get_int.03:06:23
Then I'll ask a question in English
like, how many points did you lose,03:06:27
question mark, space?03:06:29
And then once I have this answer,
let's now ask some questions of it.03:06:33
So if points is less than 2--03:06:36
borrowing the syntax that we
saw on the screen a moment ago--03:06:40
let's go ahead and print
out something explanatory03:06:42
like you lost fewer points
than me, backslash n.03:06:49
else if points greater than 2--03:06:52
which is, again how many I lost--03:06:53
I'm going to go ahead and print out you
lost more points than me, backslash n.03:06:59
else if-- wait a minute, else seems
to be sufficient logically here.03:07:03
I'm just going to go ahead
and print out something03:07:05
like you lost the same number
of points as me, backslash n.03:07:12
So, really, just a straightforward
application of that simple idea03:07:16
but to a concrete scenario here.03:07:18
So let me go ahead and save this.03:07:21
Let me go ahead and
run make points, Enter.03:07:24
No errors, that's good.03:07:26
Run points.03:07:27
And then, how many points did you lose?03:07:29
How about, it's 1 point?03:07:31
All right, you lost
fewer points than me.03:07:32
How about 0 points?03:07:34
Even better. How about 3 points?03:07:36
And so forth.03:07:37
So, again, we have the ability to
express in C now pretty basic idea03:07:40
from last week in reality, which is
this notion of conditionals and asking03:07:44
questions.03:07:46
There's something subtle here, though,
that's maybe not super well-designed03:07:50
that someone might call a magic number.03:07:53
This is programming speak
for something I've done here.03:07:56
There's a bit of redundancy unrelated
to the if and the else and the else.03:08:01
But is there something I typed twice
just to ask, perhaps, for the obvious?03:08:07
Exactly, I've hard-coded, so to speak,
manually typed out the number 2--03:08:11
in two locations, in this case--03:08:13
that did not come from the user.03:08:15
So, apparently, once I
compile this, this is it.03:08:18
You're always comparing
yourself to me in like, 1996,03:08:21
which for better or for worse,
is all the program can do.03:08:24
But this is an example too of
a magic number in the sense03:08:27
like, wait, where did that 2 come
from, and why is it in two places?03:08:31
It feels like we are setting the
stage for just a higher probability03:08:35
of screwing up down the road.03:08:36
Because the longer this code gets,
suppose I'm comparing against 2 points03:08:39
elsewhere--03:08:40
2, 3, 4, 5 places--03:08:42
am I going to keep typing the number 2?03:08:45
Like, yeah, that's fine.03:08:46
It's correct. It's going to work.03:08:47
But, honestly, eventually,
you're going to screw up,03:08:49
and you're going to miss one of the
2's, you're going to change it to a 3,03:08:52
because maybe I did worse the
next year, or 1, I did better.03:08:54
And you don't want these
numbers to get out of sync.03:08:57
So what would be a logical
improvement to this design,03:09:01
rather than hard-coding the
same number sort of magically03:09:04
in two or more places?03:09:07
Yeah, why don't I make a
variable that I can use in there?03:09:09
So, for instance, I
could create a variable03:09:11
like this, another integer called mine.03:09:14
And I'm just going to
initialize it to 2.03:09:16
And then I'm going to change
mentions of 2 to this.03:09:19
And mine is a pretty reasonable
name for a variable insofar03:09:23
as it refers to exactly
whose points are in question.03:09:27
There's a risk here,
though, minor though it is.03:09:29
I could accidentally
change mine at some point.03:09:32
Maybe I forget what mine represents,
and I do some addition or subtraction.03:09:36
So there's a way to tell the
computer "don't trust me,03:09:39
because I'm going to
screw up eventually"03:09:40
by making a variable constant too.03:09:43
So a constant in a
programming language--03:09:45
this did not exist in Scratch--03:09:47
is just an additional hint to the
computer that essentially enables03:09:50
you to program more defensively.Description:
Learn the basics of computer science from Harvard University. This is CS50, an introduction to the intellectual enterprises of computer science and the art of programming. 💻 Slides, source code, and more at https://cs50.harvard.edu/x. ❤️ Support for this channel comes from our friends at Scrimba – the coding platform that's reinvented interactive learning: https://scrimba.com/freecodecamp ⭐️ Course Contents ⭐️ ⌨️ (00:00:00) Lecture 0 - Scratch ⌨️ (01:45:08) Lecture 1 - C ⌨️ (04:13:23) Lecture 2 - Arrays ⌨️ (06:20:43) Lecture 3 - Algorithms ⌨️ (08:37:55) Lecture 4 - Memory ⌨️ (11:03:17) Lecture 5 - Data Structures ⌨️ (13:15:36) Lecture 6 - Python ⌨️ (15:39:25) Lecture 7 - SQL ⌨️ (18:00:55) Lecture 8 - HTML, CSS, JavaScript ⌨️ (20:23:38) Lecture 9 - Flask ⌨️ (22:39:01) Lecture 10 - Emoji ⌨️ (24:02:50) Cybersecurity Recorded in 2021. --- HOW TO JOIN CS50 COMMUNITIES Discord: https://discord.com/invite/cs50 Ed: https://edstem.org/us/join/gEcnkZ Facebook Group: https://www.facebook.com/unsupportedbrowser Faceboook Page: https://www.facebook.com/unsupportedbrowser GitHub: https://github.com/cs50 Gitter: https://app.gitter.im/#/room/#cs50_x:gitter.im Instagram: https://www.facebook.com/unsupportedbrowser LinkedIn Group: https://www.linkedin.com/uas/login?session_redirect=https%3A%2F%2Fwww.linkedin.com%2Fgroups%2F7437240%2F LinkedIn Page: https://www.linkedin.com/school/cs50/ Medium: https://cs50.medium.com/ Quora: https://www.quora.com/topic/CS50 Reddit: https://www.reddit.com/r/cs50/ Slack: https://cs50x.slack.com/join/shared_invite/zt-30lzxiehd-p8chC8fLQyf4u4feEXMpYg Snapchat: https://www.snapchat.com/add/cs50 SoundCloud: https://soundcloud.com/cs50 Stack Exchange: https://cs50.stackexchange.com/ TikTok: https://www.tiktok.com/@cs50 Twitter: https://twitter.com/cs50 YouTube: https://www.youtube.com/cs50 HOW TO FOLLOW DAVID J. MALAN Facebook: https://www.facebook.com/unsupportedbrowser GitHub: https://github.com/dmalan Instagram: https://www.facebook.com/unsupportedbrowser LinkedIn: https://www.linkedin.com/in/malan/ TikTok: https://www.tiktok.com/@davidjmalan Twitter: https://twitter.com/davidjmalan LICENSE CC BY-NC-SA 4.0 Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International Public License https://creativecommons.org/licenses/by-nc-sa/4.0/ 🎉 Thanks to our Champion and Sponsor supporters: 👾 Raymond Odero 👾 Agustín Kussrow 👾 aldo ferretti 👾 Otis Morgan 👾 DeezMaster -- Learn to code for free and get a developer job: https://www.freecodecamp.org/ Read hundreds of articles on programming: https://www.freecodecamp.org/news
Mediafile available in formats
* — If the video is playing in a new tab, go to it, then right-click on the video and select "Save video as..."
** — Link intended for online playback in specialized players
Questions about downloading video
How can I download "Harvard CS50 – Full Computer Science University Course" video?
http://uniloader.pro/ website is the best way to download a video or a separate audio track if you want to do without installing programs and extensions.
The UDL Helper extension is a convenient button that is seamlessly integrated into YouTube, Instagram and OK.ru sites for fast content download.
UDL Client program (for Windows) is the most powerful solution that supports more than 900 websites, social networks and video hosting sites, as well as any video quality that is available in the source.
UDL Lite is a really convenient way to access a website from your mobile device. With its help, you can easily download videos directly to your smartphone.
Which format of "Harvard CS50 – Full Computer Science University Course" video should I choose?
The best quality formats are FullHD (1080p), 2K (1440p), 4K (2160p) and 8K (4320p). The higher the resolution of your screen, the higher the video quality should be. However, there are other factors to consider: download speed, amount of free space, and device performance during playback.
Why does my computer freeze when loading a "Harvard CS50 – Full Computer Science University Course" video?
The browser/computer should not freeze completely! If this happens, please report it with a link to the video. Sometimes videos cannot be downloaded directly in a suitable format, so we have added the ability to convert the file to the desired format. In some cases, this process may actively use computer resources.
How can I download "Harvard CS50 – Full Computer Science University Course" video to my phone?
You can download a video to your smartphone using the website or the PWA application UDL Lite. It is also possible to send a download link via QR code using the UDL Helper extension.
How can I download an audio track (music) to MP3 "Harvard CS50 – Full Computer Science University Course"?
The most convenient way is to use the UDL Client program, which supports converting video to MP3 format. In some cases, MP3 can also be downloaded through the UDL Helper extension.
How can I save a frame from a video "Harvard CS50 – Full Computer Science University Course"?
This feature is available in the UDL Helper extension. Make sure that "Show the video snapshot button" is checked in the settings. A camera icon should appear in the lower right corner of the player to the left of the "Settings" icon. When you click on it, the current frame from the video will be saved to your computer in JPEG format.
How do I play and download streaming video?
- in video formats, hover your mouse over "Streaming Video**";
- right-click on "Copy link";
- open VLC-player;
- select Media - Open Network Stream - Network in the menu;
- paste the copied link into the input field;
- click "Play".
- copy the video address (URL);
- select "Open Network Stream" in the "Media" item of VLC player and paste the link to the video into the input field;
- click on the arrow on the "Play" button and select "Convert" in the list;
- select "Video - H.264 + MP3 (MP4)" in the "Profile" line;
- click the "Browse" button to select a folder to save the converted video and click the "Start" button;
- conversion speed depends on the resolution and duration of the video.
For this purpose you need VLC-player, which can be downloaded for free from the official website https://www.videolan.org/vlc/.
How to play streaming video through VLC player:
To download streaming video via VLC player, you need to convert it:
Warning: this download method no longer works with most YouTube videos.
What's the price of all this stuff?
It costs nothing. Our services are absolutely free for all users. There are no PRO subscriptions, no restrictions on the number or maximum length of downloaded videos.