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DAVID J. MALAN: All right, this is CS50, and this is the

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start of week two.

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Thank you.

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Let us begin here with a phone call.

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I'm about to dial 617-BUG-CS50.

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CS50: This is CS50.

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For Shuttle Boy, press 1.

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To start over, press 9.

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DAVID J. MALAN: So he said for Shuttle Boy, press 1.

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So we're going to press 1.

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CS50: What is your origin?

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For quad, press 1.

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Mather, press 2.

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Boylston, press 3.

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Lamont, press 4.

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Mem Hall, press 5.

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To start over, press 0.

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DAVID J. MALAN: We'll press 1 for quad.

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CS50: Next shuttle leaves this very minute at 1:10 PM, and

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then at 1:20 PM, 1:30 PM, 1:40 PM.

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This is CS50.

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>> DAVID J. MALAN: So this is CS50's voice.

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And it's an example of the sorts of final projects, for

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instance, you can bite off toward

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the end of the semester.

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For instance, that shuttleboy.cs50.net exists--

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actually a project that I first wrote after taking CS51

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back when I was an undergraduate.

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And the inspiration here was back then, all they had was

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the printed shuttle bus schedules, and there was no

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notion of looking things up online.

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And so I sort of dove in one weekend, poured through the

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printed schedule, and ported it to a computer program.

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At the time, the computer program happened to be written

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in C. And you actually ran it by typing Shuttle Boy at a

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blinking prompt like we've been doing thus far.

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But over the years, it's evolved into an instant

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messaging bot.

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It's evolved more recently into this website, into an

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SMS-based tool, as well as into this voice-based tool.

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And this is to hint at the sorts of things that you can

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do for yourself by semester's end.

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>> For instance, there, the SMS version of Shuttle Boy happens

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to operate as follows.

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If, on your cell phone, you send a text message to 41411 and

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then send the special symbol sboy, for Shuttle Boy,

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followed by A and B, where A is an origin and B is a

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destination--

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for instance, Boylston Space Quad--

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what you should get back within a few seconds is a text

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message from Shuttle Boy telling you exactly when the

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next few shuttles are, from that point A going to that

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point B.

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And this is a more general example of what's known as

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using an API.

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>> So for instance, this here is just shuttleboy.cs50.net, the

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actual web-based incarnation of this.

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But the data that underlines this and other apps that CS50

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has developed are all exposed to everyone here in the form

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of APIs, application programming interfaces.

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And that's just a fancy way of saying that people like we on

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the Internet and others have spent some time creating

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software that you can use in order to grab data from us and

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then build your own applications on top

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of that data set.

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So for instance, this Shuttle Boy API page here, which

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happens to be in the CS50 manual, essentially documents

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how you can go about asking CS50 servers for data.

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For instance, if you're familiar with CSV files, comma

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separated values, these are just sort of quick and dirty

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Excel-like files.

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So you can ask Shuttle Boy for all of the data on all of the

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houses and their GPS coordinates, and you'll get

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back, essentially, a spreadsheet like that that

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you can then read into a program of your own and then

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generate results, like Shuttle Boy itself

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happens to be doing.

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For those more familiar, more modern data representations

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include JSON, JavaScript Object Notation.

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Something will come back to you toward

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the end of the semester.

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>> But again, this is just one of several of CS50's own APIs.

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And the exciting thing is now, these days, Facebook and

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Twitter and Google and pretty much every popular website out

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there has some sort of API, which means if you read the

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documentation on their website, you sign up for an

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account, you can then start writing software on top of

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whatever tools or data that company there provides.

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And so one of our own teaching fellows a couple years back

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wrote a Mac version of this.

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So at the link titled Mac here at top left, you can actually

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download a Mac OS widget that runs on your own Mac to do the

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same kinds of things.

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So it's all about building on top of data sets like these.

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But more on that toward the end of the semester.

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>> So let's dive in real quick to a bug, just to kind of get

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things warmed up today, and think back on some of the

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things we looked at last week.

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In particular, let me go ahead and pull up, say,

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this example here.

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Buggy1.c, this is available on the course's website if you'd

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like to download it and poke around yourself.

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But let's zoom in here at this fairly short program, and just

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a super-fast recap of some of the basic building blocks that

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we really are going to just start taking for granted.

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>> So the blue stuff, in lines 1 through 9, are just

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softball questions.

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So these are just comments.

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They have no functional meaning.

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But they're comments in the sense that they're notes that

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I, the human, made to myself so that in lecture and after

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lecture, I can actually remember what this program

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does without having to read through it line by line and

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recreating history in my mind.

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Moreover, if I hand this program to someone else like

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you, it's much clearer to you, because of comments like this,

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what the program's actually doing, or at least what the

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program's supposed to be doing.

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Whether or not it's correct is another matter altogether.

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Now, in C, with multi-line comments, recall that on line

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one here is the magic symbol, /*.

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It means here comes the start of a comment.

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And nothing else matters until you reach the end terminator,

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which is */, the opposite.

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So the fact that I have 80-some odd stars here from

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left to right is really just an aesthetic detail.

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It has no functional meaning.

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>> Now how about line 11?

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What does this do in layman's terms?

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What's that?

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AUDIENCE: Includes the standard.

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DAVID J. MALAN: OK, good.

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So it includes the stdio.h library.

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So what does that mean?

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Well, inside that file, stdio.h, are a whole bunch of

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function declarations--

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that is, code that someone else wrote.

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And a perfect example of a function that's declared in

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stdio.h is--

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which favorite by now?

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So printf, one of the most common ones to use, certainly

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early on, from that library is there.

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If I exclude that line of code, Clang is going to yell

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at me something about using an undeclared symbol.

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Something undeclared is probably the keyword, because

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we haven't informed the compiler what printf looks

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like unless we include that line.

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And more down to Earth, really, what that line is saying is

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open up that file, stdio.h, wherever it is on the server's

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hard drive, or the appliance's hard drive, and copy-paste it

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right there into my file, without my

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having to do that manually.

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>> Now, once we get down here to main, before long we'll start

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teasing apart what int and what void is.

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But for now, let's look at the three lines

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within 15 through 17.

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This here I claim as buggy.

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Line 7 in my comments says "Should print 10

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asterisks but does not." Why does this not print, in fact,

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10 such stars?

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AUDIENCE: [INAUDIBLE].

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DAVID J. MALAN: Exactly.

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So notice that we're starting to count from 0.

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And this is actually a convention in programming and

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computer science more generally, starting to count

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from 0 instead of 1.

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And this really just derives from the fact that, for

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instance, when we had eight people up on the stage, when

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no one was raising their hand, they were

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all effectively zeros.

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And so it's just kind of a computer convention so,

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therefore, to start counting from 0.

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If that's the lowest number you can represent in binary.

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>> So here we've started initializing i to 0.

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We've set i equal to 0.

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But then I made this mistake here, saying i is less than or

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equal to 10.

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But if you think that through, if I start at 0 and then I go

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up to 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, I'm actually going to

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print out 11 stars to the screen, because I've gone up

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to and equal to 10.

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So the easy fix here then is what?

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AUDIENCE: [INAUDIBLE].

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DAVID J. MALAN: Just change it to less than.

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If you really want, you could do this.

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But in general, that's frowned upon.

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And so starting to count from 0 is just something you should

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typically get used to.

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Now, what about this whole construct in and of itself?

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This line 15 demarks a for loop.

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So for is not a function.

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It's just a statement.

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It's a looping construct, like we saw in Scratch.

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And it has three parts.

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Notice that there is the first part to

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the left of the semicolon.

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There's the middle part in between the two semicolons.

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And then there's the right-hand part to the right

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of the second semicolon.

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Now, the first of these does what?

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AUDIENCE: [INAUDIBLE].

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DAVID J. MALAN: Back there?

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Yeah?

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AUDIENCE: Initialization.

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>> DAVID J. MALAN: Initialization.

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So what does this mean?

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We have declared a variable called i.

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It is of type int, because I've specified int i.

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And I'm initializing i to the value of 0.

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So what's this really saying?

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It's effectively saying to the computer hey, give me enough

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memory, enough RAM, to fit a number, and then put the

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number 0 in that chunk of RAM.

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And as an aside, how big is an int typically, at least inside

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of the appliance?

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AUDIENCE: 32 bit.

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DAVID J. MALAN: 32 bits.

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So that means give me 32 bits, otherwise known as 4 bytes,

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and put the value 0 in it, which is pretty easy because

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it just means set all the bits to 0.

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>> So now, the second part here is the condition.

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And the condition, as the name suggests, is what is checked

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again and again and again as to whether it's true or false.

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So this is just saying do the following lines of code--

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namely line 16, because that's the only one indented

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underneath--

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so long as i is less than 10.

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And after each iteration through this loop, do the

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incrementation, which in this case is i++.

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Now, it doesn't have to be i++.

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It could be i--.

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But if I did this, what's the behavior going

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to be of this program?

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AUDIENCE: It's going to be an infinite loop.

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DAVID J. MALAN: It's going to be some kind of infinite loop,

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unless we get lucky after negative 2 billion or so.

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Maybe things will wrap around, just by nature of the finite

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number of bits that we have allocated for an int.

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But it's certainly going to iterate far more than 10 and

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certainly more than 11 times here.

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>> And now, just as an aside, realize that i++ and i--

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are really just syntactic sugar.

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It's just shorthand notation for what's a little more

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explicitly written as follows:

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i = i + 1.

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That is identical to i++.

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It just kind of looks prettier to say i++.

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It's more succinct, more readable.

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And so most people do that instead.

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But this is identical functionally to

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what we just saw.

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So in short, the quick fix here is just to say iterate i

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from 0 all the way up to less than 10.

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And then we'll indeed get 10 stars.

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>> So let's try this.

256
00:10:33,430 --> 00:10:35,310
Let me open up the terminal at the bottom.

257
00:10:35,310 --> 00:10:37,420
Let me go into the directory that this is in.

258
00:10:37,420 --> 00:10:40,030
And I'm going to compile it manually with Clang for now.

259
00:10:40,030 --> 00:10:44,880
And I'm going to compile this as buggy1.c, Enter.

260
00:10:44,880 --> 00:10:50,280
And now buggy1, why is there no such file or directory

261
00:10:50,280 --> 00:10:52,026
called buggy1?

262
00:10:52,026 --> 00:10:53,850
AUDIENCE: [INAUDIBLE].

263
00:10:53,850 --> 00:10:54,240
DAVID J. MALAN: Yeah.

264
00:10:54,240 --> 00:10:55,420
So it's actually called a.out.

265
00:10:55,420 --> 00:10:57,480
So recall if you just run Clang, where Clang is the

266
00:10:57,480 --> 00:11:00,420
compiler, and you don't specify the name you want to

267
00:11:00,420 --> 00:11:03,550
give to your program, it's going to default to a.out.

268
00:11:03,550 --> 00:11:05,030
So indeed if I do ls--

269
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oops.

270
00:11:05,510 --> 00:11:07,680
And I didn't--the black-and-white issue some of

271
00:11:07,680 --> 00:11:08,840
you faced has been fixed.

272
00:11:08,840 --> 00:11:10,670
But let me do this.

273
00:11:10,670 --> 00:11:14,570
There we have a.out on the left-hand side there.

274
00:11:14,570 --> 00:11:17,360
So we have to run a.out instead of buggy1.

275
00:11:17,360 --> 00:11:19,140
>> So let me go ahead and do this.

276
00:11:19,140 --> 00:11:22,840
./a.out, Enter.

277
00:11:22,840 --> 00:11:26,160
And I apparently did not do what?

278
00:11:26,160 --> 00:11:26,620
AUDIENCE: Save.

279
00:11:26,620 --> 00:11:27,390
DAVID J. MALAN: Save my file.

280
00:11:27,390 --> 00:11:31,520
So that is easily solved by hitting Control S, or going to

281
00:11:31,520 --> 00:11:33,450
File, Save, like in most programs.

282
00:11:33,450 --> 00:11:36,360
Let me go down here, clear the screen, run it again.

283
00:11:36,360 --> 00:11:37,960
And there's still a bug.

284
00:11:37,960 --> 00:11:41,618
So what is going--

285
00:11:41,618 --> 00:11:42,980
AUDIENCE: You didn't compile.

286
00:11:42,980 --> 00:11:44,040
DAVID J. MALAN: Ah, good.

287
00:11:44,040 --> 00:11:45,010
I didn't compile it.

288
00:11:45,010 --> 00:11:47,250
Like an idiot, I'm looking at the code to see what's wrong.

289
00:11:47,250 --> 00:11:51,275
So clang buggy1.c, now a.out.

290
00:11:51,275 --> 00:11:52,640
And phew, saved.

291
00:11:52,640 --> 00:11:55,090
>> So it looks a little ugly because there's no new line

292
00:11:55,090 --> 00:11:55,970
anywhere in the program.

293
00:11:55,970 --> 00:11:57,390
But again, that's just an aesthetic detail.

294
00:11:57,390 --> 00:11:59,430
And at least if we count those out, we should

295
00:11:59,430 --> 00:12:01,160
now see 10 such stars.

296
00:12:01,160 --> 00:12:03,260
Well, what about this second warm-up example?

297
00:12:03,260 --> 00:12:07,750
So in buggy2, I claim that this version, too, will print

298
00:12:07,750 --> 00:12:10,110
10 stars, one per line.

299
00:12:10,110 --> 00:12:12,340
So this time, I have a newline character, just to make

300
00:12:12,340 --> 00:12:13,520
things a little prettier.

301
00:12:13,520 --> 00:12:15,410
But instead what I get is this.

302
00:12:15,410 --> 00:12:19,590
So let me do clang buggy2.c, Enter.

303
00:12:19,590 --> 00:12:21,790
Now it's again called a.out.

304
00:12:21,790 --> 00:12:23,130
Enter.

305
00:12:23,130 --> 00:12:26,900
I only see one new line, only the very last new line that

306
00:12:26,900 --> 00:12:29,370
moves my prompt to the next line.

307
00:12:29,370 --> 00:12:32,660
And yet clearly I've been printing *, then a new line,

308
00:12:32,660 --> 00:12:33,990
*, then a new line.

309
00:12:33,990 --> 00:12:35,490
But what's the bug here?

310
00:12:35,490 --> 00:12:35,974
Yeah?

311
00:12:35,974 --> 00:12:37,224
AUDIENCE: [INAUDIBLE].

312
00:12:39,850 --> 00:12:40,470
>> DAVID J. MALAN: Exactly.

313
00:12:40,470 --> 00:12:44,110
So unlike some languages, like Python, where indentation

314
00:12:44,110 --> 00:12:46,030
actually has functional meaning, in a

315
00:12:46,030 --> 00:12:47,150
language like C--

316
00:12:47,150 --> 00:12:49,140
as we'll see, PHP, JavaScript--

317
00:12:49,140 --> 00:12:51,940
the indentation is really just for humans' benefit.

318
00:12:51,940 --> 00:12:55,690
So the fact that I've indented line 16 and 17 looks great,

319
00:12:55,690 --> 00:12:57,530
but it has no functional meaning here.

320
00:12:57,530 --> 00:13:00,700
If I want both lines to execute as part of the for

321
00:13:00,700 --> 00:13:05,140
loop, then I must enclose them in curly braces by doing this.

322
00:13:05,140 --> 00:13:08,540
You can only cut that corner and omit the curly braces if

323
00:13:08,540 --> 00:13:10,036
what's the case?

324
00:13:10,036 --> 00:13:10,410
AUDIENCE: Just one line.

325
00:13:10,410 --> 00:13:11,130
DAVID J. MALAN: Just one line.

326
00:13:11,130 --> 00:13:15,280
So that's just sort of a nice sort of syntax detail so that

327
00:13:15,280 --> 00:13:17,900
you don't waste time writing three lines, two of which are

328
00:13:17,900 --> 00:13:20,040
curly braces, just to write a single line of code.

329
00:13:20,040 --> 00:13:21,650
But if you have two or more lines, we

330
00:13:21,650 --> 00:13:22,860
indeed need to do this.

331
00:13:22,860 --> 00:13:24,210
>> So now let me save this.

332
00:13:24,210 --> 00:13:27,140
Let me go ahead and re-run Clang.

333
00:13:27,140 --> 00:13:31,375
Then let me rerun a.out, and now I get them one per line.

334
00:13:31,375 --> 00:13:33,670
Now, a.out again is kind of a dumb name for a program.

335
00:13:33,670 --> 00:13:37,500
How can I tell Clang to actually give me a file name

336
00:13:37,500 --> 00:13:39,820
that's more user-friendly, like buggy2 itself?

337
00:13:43,200 --> 00:13:44,466
A little more clearly?

338
00:13:44,466 --> 00:13:45,220
AUDIENCE: [INAUDIBLE].

339
00:13:45,220 --> 00:13:47,810
DAVID J. MALAN: OK, so I can actually take the very

340
00:13:47,810 --> 00:13:51,000
user-friendly shortcut and just write make buggy2.

341
00:13:51,000 --> 00:13:53,840
I don't specify .c in this case, and hit Enter.

342
00:13:53,840 --> 00:13:58,530
And what make does is it compiles buggy2.c for me by

343
00:13:58,530 --> 00:14:00,260
asking Clang to do it.

344
00:14:00,260 --> 00:14:05,100
Specifically, it calls Clang, it runs Clang using way more

345
00:14:05,100 --> 00:14:08,540
command-line arguments or switches than I actually need.

346
00:14:08,540 --> 00:14:11,270
In time, we'll come back to what all of these various

347
00:14:11,270 --> 00:14:13,810
cryptic hyphenated expressions means.

348
00:14:13,810 --> 00:14:16,910
But for now, that's just saving me the trouble of

349
00:14:16,910 --> 00:14:19,600
having to remember and having to type out all of those

350
00:14:19,600 --> 00:14:21,150
various hyphenated expressions.

351
00:14:21,150 --> 00:14:23,240
And the upside of it ultimately is

352
00:14:23,240 --> 00:14:25,900
that now I have buggy2.

353
00:14:25,900 --> 00:14:27,480
If I want to do this manually, though, I

354
00:14:27,480 --> 00:14:28,775
can instead do this--

355
00:14:28,775 --> 00:14:34,110
clang -o buggy2 and then buggy2.c.

356
00:14:34,110 --> 00:14:37,980
And that will similarly give me a file called buggy2.

357
00:14:37,980 --> 00:14:39,710
>> So in short, Clang's the compiler.

358
00:14:39,710 --> 00:14:43,080
Make is just a user-friendly tool that we'll be using more

359
00:14:43,080 --> 00:14:44,420
and more, because it just starts to

360
00:14:44,420 --> 00:14:47,330
simplify things for us.

361
00:14:47,330 --> 00:14:49,430
And we return 0, lastly.

362
00:14:49,430 --> 00:14:51,890
For now, just because, but we'll start teasing that part

363
00:14:51,890 --> 00:14:52,930
today and on Wednesday.

364
00:14:52,930 --> 00:14:55,560
Any questions on any of this?

365
00:14:55,560 --> 00:14:56,450
Yeah?

366
00:14:56,450 --> 00:14:58,234
AUDIENCE: [INAUDIBLE]

367
00:14:58,234 --> 00:15:00,020
ls in quotes there?

368
00:15:00,020 --> 00:15:01,710
DAVID J. MALAN: OK.

369
00:15:01,710 --> 00:15:05,880
When I typed ls in quotes, that was me doing some magic

370
00:15:05,880 --> 00:15:07,160
behind the scenes to fix a bug.

371
00:15:07,160 --> 00:15:09,370
I forgot, like we've been telling many of you on the

372
00:15:09,370 --> 00:15:11,600
discussion boards, to do--

373
00:15:11,600 --> 00:15:17,220
we'll do this now--sudo yum -y update appliance50.

374
00:15:17,220 --> 00:15:19,790
Whoops, that that's spelled right.

375
00:15:19,790 --> 00:15:23,240
So the appliance is like an operating system.

376
00:15:23,240 --> 00:15:25,650
It's running this operating system called Fedora.

377
00:15:25,650 --> 00:15:27,450
And now because of my slow Internet connection, I've

378
00:15:27,450 --> 00:15:28,260
really hosed it.

379
00:15:28,260 --> 00:15:31,620
So running sudo yum update, as we tell you to do in the

380
00:15:31,620 --> 00:15:34,080
problem set, is essentially like running automatic updates

381
00:15:34,080 --> 00:15:35,420
in Mac OS or Windows.

382
00:15:35,420 --> 00:15:37,280
And the reason for running this at the very start of the

383
00:15:37,280 --> 00:15:39,640
problem set is because when we created the appliance, I

384
00:15:39,640 --> 00:15:42,480
messed up, and I accidentally made all of your programs look

385
00:15:42,480 --> 00:15:45,210
black on a black screen, which is why you're not seeing them

386
00:15:45,210 --> 00:15:45,860
by default.

387
00:15:45,860 --> 00:15:48,020
But the latest version of the appliance fixes this.

388
00:15:48,020 --> 00:15:50,500
And I'll fix that during break once I have Internet

389
00:15:50,500 --> 00:15:51,280
connectivity.

390
00:15:51,280 --> 00:15:54,630
>> So the quotes just hides my mistake, very discreetly,

391
00:15:54,630 --> 00:15:55,730
apparently.

392
00:15:55,730 --> 00:15:56,980
Other questions?

393
00:15:59,750 --> 00:16:00,668
Yes?

394
00:16:00,668 --> 00:16:02,012
AUDIENCE: Where does make come from?

395
00:16:02,012 --> 00:16:03,810
[INAUDIBLE]

396
00:16:03,810 --> 00:16:04,280
DAVID J. MALAN: Good question.

397
00:16:04,280 --> 00:16:05,460
Where does make come from?

398
00:16:05,460 --> 00:16:09,390
It is a Linux program that has existed for many years, long

399
00:16:09,390 --> 00:16:10,570
before CS50.

400
00:16:10,570 --> 00:16:12,930
And it comes with an operating system like Fedora.

401
00:16:12,930 --> 00:16:14,830
It does not come from the CS50 library.

402
00:16:14,830 --> 00:16:17,560
In fact, the only things that come from the CS50 library

403
00:16:17,560 --> 00:16:20,470
thus far, that we've seen, are GetString, GetInt, all of

404
00:16:20,470 --> 00:16:25,550
those Get functions, and the word string, and to some

405
00:16:25,550 --> 00:16:26,575
extent, the word bool.

406
00:16:26,575 --> 00:16:29,180
But we'll tease that apart when we dive into the CS50

407
00:16:29,180 --> 00:16:30,580
appliance itself.

408
00:16:30,580 --> 00:16:32,820
>> So yes, one more question here.

409
00:16:32,820 --> 00:16:36,636
AUDIENCE: When you said make and then buggy, how does the

410
00:16:36,636 --> 00:16:39,980
computer know [INAUDIBLE]?

411
00:16:39,980 --> 00:16:40,520
DAVID J. MALAN: Good question.

412
00:16:40,520 --> 00:16:43,830
So when you just run make buggy1 or make buggy2, how

413
00:16:43,830 --> 00:16:45,220
does make know?

414
00:16:45,220 --> 00:16:49,900
So by default, if you type make buggy1, make looks for a

415
00:16:49,900 --> 00:16:52,330
file called buggy1.c.

416
00:16:52,330 --> 00:16:55,570
And then it executes the appropriate Clang commands,

417
00:16:55,570 --> 00:17:00,380
thereby overriding the default output file called a.out.

418
00:17:00,380 --> 00:17:03,670
In fact, if we look at what make, what--

419
00:17:03,670 --> 00:17:04,839
let's quit this.

420
00:17:04,839 --> 00:17:09,530
If we look at what make was actually doing, make buggy2,

421
00:17:09,530 --> 00:17:10,470
it's already up to date.

422
00:17:10,470 --> 00:17:13,020
So let me remove the rm command, the

423
00:17:13,020 --> 00:17:14,319
program I wrote before.

424
00:17:14,319 --> 00:17:17,319
Typing Y-E-S to confirm that I want to remove it.

425
00:17:17,319 --> 00:17:20,480
If I now do make, notice that in this very long line,

426
00:17:20,480 --> 00:17:22,980
there's this last thing here, -o buggy2.

427
00:17:22,980 --> 00:17:26,959
All make is doing is passing that argument, so to speak, to

428
00:17:26,959 --> 00:17:29,665
Clang, so that I don't have to type it myself.

429
00:17:29,665 --> 00:17:32,660
>> All right, so a quick couple of administrative

430
00:17:32,660 --> 00:17:33,470
announcements.

431
00:17:33,470 --> 00:17:36,940
So for sections, which officially began this coming

432
00:17:36,940 --> 00:17:38,970
Sunday, you'll always want to bring, if

433
00:17:38,970 --> 00:17:40,120
you have one, a laptop.

434
00:17:40,120 --> 00:17:42,570
If you don't have a laptop, do reach out to me by

435
00:17:42,570 --> 00:17:43,580
dropping me an email.

436
00:17:43,580 --> 00:17:44,810
And we'll figure out a workflow.

437
00:17:44,810 --> 00:17:47,100
What generally you'll find in section is that they're part

438
00:17:47,100 --> 00:17:48,770
conceptual, part hands-on.

439
00:17:48,770 --> 00:17:52,190
We'll specifically use the section of questions, part of

440
00:17:52,190 --> 00:17:54,110
the week's problem set, to walk through some of the

441
00:17:54,110 --> 00:17:55,820
conceptual material from lecture.

442
00:17:55,820 --> 00:17:57,480
And that's all in the current problem set.

443
00:17:57,480 --> 00:18:00,010
And we'll also dive into some hands-on activities, sometimes

444
00:18:00,010 --> 00:18:02,190
of which will be required to be submitted, sometimes of

445
00:18:02,190 --> 00:18:02,830
which will not.

446
00:18:02,830 --> 00:18:04,630
For instance, this first week, they're meant just as a

447
00:18:04,630 --> 00:18:05,780
warm-up exercise.

448
00:18:05,780 --> 00:18:07,800
And you'll find that those problems are really just that.

449
00:18:07,800 --> 00:18:10,220
They're meant to be fairly small, but not necessarily

450
00:18:10,220 --> 00:18:13,100
trivial programs to write, that aren't necessarily

451
00:18:13,100 --> 00:18:16,230
exciting in and of themselves but are good opportunities to

452
00:18:16,230 --> 00:18:19,620
practice with syntax, with new functions, in the comfort of a

453
00:18:19,620 --> 00:18:21,900
section where you have a few of your classmates present as

454
00:18:21,900 --> 00:18:22,870
well as your TF.

455
00:18:22,870 --> 00:18:26,210
>> And what we'll do over time is use a tool called CS50 Spaces,

456
00:18:26,210 --> 00:18:28,940
whereby instead of just using the CS50 Appliance, you'll

457
00:18:28,940 --> 00:18:32,450
instead go to a web page in a browser, where you'll be able

458
00:18:32,450 --> 00:18:35,610
to write code in a browser window during section.

459
00:18:35,610 --> 00:18:38,240
And then if you opt in, your teaching fellow can then show

460
00:18:38,240 --> 00:18:40,980
whatever it is you're typing on your screen in your browser

461
00:18:40,980 --> 00:18:43,930
window up at the front of the class, whether anonymously or

462
00:18:43,930 --> 00:18:47,250
publicly, so that he or she can then walk through with

463
00:18:47,250 --> 00:18:50,080
your classmates what you did well, what you didn't do well.

464
00:18:50,080 --> 00:18:52,750
And again, rest assured all of this can be nicely anonymized.

465
00:18:52,750 --> 00:18:54,410
But it'll be a nice opportunity for much more

466
00:18:54,410 --> 00:18:56,810
interactivity than something like lecture allows.

467
00:18:56,810 --> 00:18:58,910
>> In the meantime, we'll have these things called super

468
00:18:58,910 --> 00:19:01,610
sections, which are optional but are open to everyone in

469
00:19:01,610 --> 00:19:04,030
the class, so that you can do this more collectively for

470
00:19:04,030 --> 00:19:05,190
problem set one.

471
00:19:05,190 --> 00:19:06,010
Here's the schedule.

472
00:19:06,010 --> 00:19:08,870
This is also posted on the homepage at cs50.net.

473
00:19:08,870 --> 00:19:10,850
Notice that there'll be a hacker-specific

474
00:19:10,850 --> 00:19:12,430
one tomorrow afternoon.

475
00:19:12,430 --> 00:19:15,460
And we will film one today and one tomorrow and post those

476
00:19:15,460 --> 00:19:16,720
online within 24 hours.

477
00:19:16,720 --> 00:19:19,370
So if you can't make any of these times, not to worry.

478
00:19:19,370 --> 00:19:22,720
And again, the schedule is online now at cs50.net.

479
00:19:22,720 --> 00:19:25,350
>> In terms of sectioning itself, you should have gotten an

480
00:19:25,350 --> 00:19:28,520
email instructing you to go to the course's homepage to find

481
00:19:28,520 --> 00:19:29,610
out your section.

482
00:19:29,610 --> 00:19:32,890
If life has changed and you need to change your section,

483
00:19:32,890 --> 00:19:33,560
not a problem.

484
00:19:33,560 --> 00:19:37,900
Go back to that same URL, cs50.net/section, singular,

485
00:19:37,900 --> 00:19:40,090
and you'll fill out the similar form so that you can

486
00:19:40,090 --> 00:19:41,260
then give us your preferences.

487
00:19:41,260 --> 00:19:43,360
And we will follow up by week's end as to what we can

488
00:19:43,360 --> 00:19:44,240
accommodate.

489
00:19:44,240 --> 00:19:49,530
Last week, recall that we proposed using CS50 Discuss,

490
00:19:49,530 --> 00:19:51,370
the course's discussion tool, in lecture.

491
00:19:51,370 --> 00:19:53,680
So we had 40 questions that were asked and

492
00:19:53,680 --> 00:19:54,720
answered during lecture.

493
00:19:54,720 --> 00:19:56,280
So it seemed to work well, so we'll continue

494
00:19:56,280 --> 00:19:57,180
trying to do this.

495
00:19:57,180 --> 00:19:59,600
If, during lecture, you don't just feel comfortable raising

496
00:19:59,600 --> 00:20:00,780
your hand, not a problem.

497
00:20:00,780 --> 00:20:04,640
Go to cs50.net/discuss, post there, and one of our teaching

498
00:20:04,640 --> 00:20:07,270
fellows will either answer it electronically or raise their

499
00:20:07,270 --> 00:20:10,620
hand on your behalf anonymously to ask, depending

500
00:20:10,620 --> 00:20:12,470
on the nature of the question.

501
00:20:12,470 --> 00:20:15,000
And in terms of feedback, generally psets will be

502
00:20:15,000 --> 00:20:16,760
returned within a week.

503
00:20:16,760 --> 00:20:19,080
Because it takes a little while for sections to achieve

504
00:20:19,080 --> 00:20:22,430
equilibrium, the first pset, 0 and 1, will be a little bit

505
00:20:22,430 --> 00:20:24,130
delayed as things settle down.

506
00:20:24,130 --> 00:20:27,130
But stay tuned for that in the weeks to come.

507
00:20:27,130 --> 00:20:29,750
>> All right, so let me put on my serious

508
00:20:29,750 --> 00:20:31,040
voice for just a moment.

509
00:20:31,040 --> 00:20:32,690
So this is actually an interesting climate to be

510
00:20:32,690 --> 00:20:35,210
having this discussion, what with all of the other things

511
00:20:35,210 --> 00:20:37,310
going on on campus related thereto.

512
00:20:37,310 --> 00:20:40,220
But CS50 has certainly had its history of this particular

513
00:20:40,220 --> 00:20:43,360
topic, in as much as every year, this course, for many

514
00:20:43,360 --> 00:20:46,910
years, Ad-Boards roughly 3% of the class.

515
00:20:46,910 --> 00:20:51,620
This most recent year, 2011, CS50 Ad-Boarded 35 students.

516
00:20:51,620 --> 00:20:54,120
This is not, I think, due to lack of clarity.

517
00:20:54,120 --> 00:20:56,760
Realize that in the course's syllabus, there is a page of

518
00:20:56,760 --> 00:20:59,020
statement explaining where the lines are.

519
00:20:59,020 --> 00:21:01,350
That same statement is repeated on every one of the

520
00:21:01,350 --> 00:21:03,070
problem sets on page one.

521
00:21:03,070 --> 00:21:05,760
>> So I mention this today really just to make

522
00:21:05,760 --> 00:21:07,030
folks mindful of this.

523
00:21:07,030 --> 00:21:08,080
And we've tried different things.

524
00:21:08,080 --> 00:21:10,630
And what I thought we would do today is just take a moment to

525
00:21:10,630 --> 00:21:13,830
actually look at some of past cases that have come up.

526
00:21:13,830 --> 00:21:16,160
Rather than keep these as dirty little secrets, actually

527
00:21:16,160 --> 00:21:19,170
point out what students have done and how we have detected

528
00:21:19,170 --> 00:21:21,630
it and really what the overarching motivation is for

529
00:21:21,630 --> 00:21:23,950
even having this conversation.

530
00:21:23,950 --> 00:21:27,060
So with that said, the line essentially is this--

531
00:21:27,060 --> 00:21:29,200
per the syllabus, you're welcome, you are encouraged, to

532
00:21:29,200 --> 00:21:30,210
talk with classmates.

533
00:21:30,210 --> 00:21:31,910
That's the whole purpose of having these collaborative

534
00:21:31,910 --> 00:21:34,470
office hours in Annenberg and encouraging people for the

535
00:21:34,470 --> 00:21:35,755
final project to work together.

536
00:21:35,755 --> 00:21:38,590
But the line is drawn when it comes time to actually write

537
00:21:38,590 --> 00:21:39,790
your final solution.

538
00:21:39,790 --> 00:21:42,590
Speaking in English, totally fine, speaking in pseudo code,

539
00:21:42,590 --> 00:21:43,400
totally fine.

540
00:21:43,400 --> 00:21:46,340
Emailing a classmate your pset, letting them look over

541
00:21:46,340 --> 00:21:48,610
the screen as the hands continue typing,

542
00:21:48,610 --> 00:21:50,110
over the line as well.

543
00:21:50,110 --> 00:21:52,250
Do look to the syllabus for the particular lines.

544
00:21:52,250 --> 00:21:55,850
>> But just to paint a picture of how this is unfortunately a

545
00:21:55,850 --> 00:21:58,370
reality, realize that there are websites out there that

546
00:21:58,370 --> 00:22:01,490
have solutions from this class and many other classes.

547
00:22:01,490 --> 00:22:04,940
The fact that you or some 3% of you know that this exists

548
00:22:04,940 --> 00:22:07,240
means that we know that this exists.

549
00:22:07,240 --> 00:22:09,340
The fact that there are websites like this where you

550
00:22:09,340 --> 00:22:11,540
can pay someone to actually do your problem sets-- this was

551
00:22:11,540 --> 00:22:13,450
an actual case that came up last year.

552
00:22:13,450 --> 00:22:15,790
This is a website called odesk.com.

553
00:22:15,790 --> 00:22:20,090
And Tim was the name of the person here who was posting on

554
00:22:20,090 --> 00:22:24,580
this website and asked someone to do his pset 7 in

555
00:22:24,580 --> 00:22:25,570
this particular case.

556
00:22:25,570 --> 00:22:28,350
Well, odesk.com is very Google-able, and we too are

557
00:22:28,350 --> 00:22:30,310
very good at Googling.

558
00:22:30,310 --> 00:22:32,360
Here, too, there are sites-- and this one's rather

559
00:22:32,360 --> 00:22:34,234
atrocious, frankly.

560
00:22:34,234 --> 00:22:38,050
[LAUGHTER]

561
00:22:38,050 --> 00:22:39,610
DAVID J. MALAN: The funny thing about this site is if

562
00:22:39,610 --> 00:22:41,970
you read the About page, they talk about their corporate

563
00:22:41,970 --> 00:22:44,390
culture and how customer service is their number-one

564
00:22:44,390 --> 00:22:46,190
priority, to make sure that your assignments get

565
00:22:46,190 --> 00:22:47,890
turned in on time.

566
00:22:47,890 --> 00:22:50,580
>> But in all seriousness, again, the fact that these sites

567
00:22:50,580 --> 00:22:52,910
exist, realize we, too, are cognizant of

568
00:22:52,910 --> 00:22:53,730
these kinds of sites.

569
00:22:53,730 --> 00:22:55,890
And to give you a sense of what forms this generally

570
00:22:55,890 --> 00:22:58,570
takes, we generally don't have great scandals where people

571
00:22:58,570 --> 00:23:01,080
are collaborating on any kind of massive scale, but rather

572
00:23:01,080 --> 00:23:04,540
it's these late-night moments of weakness, where you have so

573
00:23:04,540 --> 00:23:07,550
much to do, it's 4:00 AM, you're exhausted, and you

574
00:23:07,550 --> 00:23:09,770
think to yourself, well, let me just take a look at my

575
00:23:09,770 --> 00:23:11,770
roommate's or my friend's code or the like.

576
00:23:11,770 --> 00:23:14,800
And the manifestations of this unfortunately involve Student

577
00:23:14,800 --> 00:23:17,640
A submitting something like this and Student B submitting

578
00:23:17,640 --> 00:23:20,370
something like this, which certainly, in a computer

579
00:23:20,370 --> 00:23:23,830
science class, is remarkably easy for computer scientists

580
00:23:23,830 --> 00:23:25,690
to detect with software.

581
00:23:25,690 --> 00:23:28,410
This is another common paradigm, where you've sort of

582
00:23:28,410 --> 00:23:31,010
been working alongside of someone, maybe talking in

583
00:23:31,010 --> 00:23:32,360
English, just fine, pseudocode.

584
00:23:32,360 --> 00:23:35,080
But then it comes time to actually submit, and the psets

585
00:23:35,080 --> 00:23:37,910
just get exchanged via email or Dropbox or the like.

586
00:23:37,910 --> 00:23:41,120
But in an attempt to make it less apparent that this is

587
00:23:41,120 --> 00:23:44,830
what has happened, then this is what's submitted.

588
00:23:44,830 --> 00:23:47,880
This, too, does not trip up well-written pieces of

589
00:23:47,880 --> 00:23:50,360
software like we have to actually detect

590
00:23:50,360 --> 00:23:51,280
these kinds of things.

591
00:23:51,280 --> 00:23:53,760
And indeed what we do is run software that compares all of

592
00:23:53,760 --> 00:23:55,540
this year's submissions against all of past year's

593
00:23:55,540 --> 00:23:57,380
submissions, against everything we found on the

594
00:23:57,380 --> 00:24:00,060
Internet, against every job website out there.

595
00:24:00,060 --> 00:24:01,710
It's all very automated.

596
00:24:01,710 --> 00:24:05,665
And so we do this really in great fairness to the 97% who

597
00:24:05,665 --> 00:24:07,760
are really working their asses off in this and in other

598
00:24:07,760 --> 00:24:11,530
classes and putting in all of that effort so that the work

599
00:24:11,530 --> 00:24:13,030
they ultimately submit is their own.

600
00:24:13,030 --> 00:24:14,220
And I can go on for ages.

601
00:24:14,220 --> 00:24:16,030
>> These are just a handful of last year's cases.

602
00:24:16,030 --> 00:24:19,350
A few students submitted these files identically for pset 2,

603
00:24:19,350 --> 00:24:22,460
pset 3, pset 4, pset 5, pset 6, pset 9.

604
00:24:22,460 --> 00:24:26,360
In this case, this was quiz 0 and in last year, where two

605
00:24:26,360 --> 00:24:29,570
students submitted identically this sentence among many

606
00:24:29,570 --> 00:24:32,210
others, "The request of type--" dot, dot, dot.

607
00:24:32,210 --> 00:24:34,530
So even in a class of 600 did we detect this

608
00:24:34,530 --> 00:24:36,310
on submitted quizzes.

609
00:24:36,310 --> 00:24:39,080
So in short, this--frankly, I hate having this kind of

610
00:24:39,080 --> 00:24:41,300
conversation--but this is really a deliberate effort

611
00:24:41,300 --> 00:24:43,540
this year to try to drive down that number.

612
00:24:43,540 --> 00:24:45,650
Because even though we say these kinds of things every

613
00:24:45,650 --> 00:24:48,100
year, I think the reality of having dwelled on it for a few

614
00:24:48,100 --> 00:24:50,800
more seconds than usual and actually just pointing out

615
00:24:50,800 --> 00:24:53,600
that what might seem like, eh, not such a big deal, at least

616
00:24:53,600 --> 00:24:56,390
think back to this particular moment, both in fairness to

617
00:24:56,390 --> 00:25:00,100
yourself and to your classmates here.

618
00:25:00,100 --> 00:25:02,480
So if you ever have any questions as to where the line

619
00:25:02,480 --> 00:25:04,290
is, please just reach out to me personally.

620
00:25:04,290 --> 00:25:07,190
But the answer is always, completely stressed at the

621
00:25:07,190 --> 00:25:09,090
last minute, cash in a late day.

622
00:25:09,090 --> 00:25:11,080
Or if it's a matter of not having any late days,

623
00:25:11,080 --> 00:25:12,900
honestly, email me personally.

624
00:25:12,900 --> 00:25:14,110
We'll figure something out.

625
00:25:14,110 --> 00:25:17,480
Please do not put your time here at Harvard at risk.

626
00:25:17,480 --> 00:25:18,570
>> Now, I thought we should lighten the mood, so I

627
00:25:18,570 --> 00:25:20,264
included this as the next slide.

628
00:25:20,264 --> 00:25:22,770
[LAUGHTER]

629
00:25:22,770 --> 00:25:23,630
DAVID J. MALAN: This website was great.

630
00:25:23,630 --> 00:25:24,820
I actually got a little distracted.

631
00:25:24,820 --> 00:25:26,330
There's this one.

632
00:25:26,330 --> 00:25:29,590
And then this one was amazing.

633
00:25:29,590 --> 00:25:35,190
Okay, so think of that kitten late at night when making

634
00:25:35,190 --> 00:25:36,040
those decisions.

635
00:25:36,040 --> 00:25:40,110
All right, so back to more fun and less serious stuff, like

636
00:25:40,110 --> 00:25:40,780
conditions.

637
00:25:40,780 --> 00:25:43,900
>> All right, so we talked briefly about these.

638
00:25:43,900 --> 00:25:46,080
This is something that's probably quite familiar from

639
00:25:46,080 --> 00:25:47,260
the world of Scratch.

640
00:25:47,260 --> 00:25:49,860
And in the world of Scratch, we have this need sometimes to

641
00:25:49,860 --> 00:25:51,110
go make forks in the road.

642
00:25:51,110 --> 00:25:54,840
Either do this or that or this other thing here.

643
00:25:54,840 --> 00:25:58,990
And when we want to do this, we can use, in C now, this if

644
00:25:58,990 --> 00:26:00,860
else construct.

645
00:26:00,860 --> 00:26:06,420
And then here we have Boolean expressions.

646
00:26:06,420 --> 00:26:09,260
For instance, Boolean expressions here, we can OR

647
00:26:09,260 --> 00:26:12,260
them together, in the sense that we have this condition OR

648
00:26:12,260 --> 00:26:13,200
that condition.

649
00:26:13,200 --> 00:26:15,100
We can AND them together, in the sense that we want to

650
00:26:15,100 --> 00:26:17,710
check this condition AND that condition.

651
00:26:17,710 --> 00:26:22,440
And here we have a switch statement now, which is not so

652
00:26:22,440 --> 00:26:26,040
similar syntactically to those kinds of conditions, but it

653
00:26:26,040 --> 00:26:30,160
allows us to do the equivalent of if, else if, else if, else

654
00:26:30,160 --> 00:26:34,990
if, and the like by simply enumerating them case by case

655
00:26:34,990 --> 00:26:36,670
by case by case.

656
00:26:36,670 --> 00:26:38,370
So we saw those last time.

657
00:26:38,370 --> 00:26:40,430
And then we started touching upon things like loops.

658
00:26:40,430 --> 00:26:42,070
We saw one of these just a moment ago.

659
00:26:42,070 --> 00:26:44,180
But there are these other looping constructs.

660
00:26:44,180 --> 00:26:46,370
>> For instance, this one here.

661
00:26:46,370 --> 00:26:50,140
So while (condition), do this thing again and again.

662
00:26:50,140 --> 00:26:53,070
So fundamentally, what seems to be different between this

663
00:26:53,070 --> 00:26:55,760
for loop and this while loop here?

664
00:27:00,810 --> 00:27:05,045
This for loop and this while loop.

665
00:27:05,045 --> 00:27:07,840
Yeah?

666
00:27:07,840 --> 00:27:08,800
What's that?

667
00:27:08,800 --> 00:27:10,050
AUDIENCE: [INAUDIBLE].

668
00:27:14,632 --> 00:27:15,120
DAVID J. MALAN: Good.

669
00:27:15,120 --> 00:27:17,130
So whereas in the for loop condition, there's clearly

670
00:27:17,130 --> 00:27:17,940
more syntax.

671
00:27:17,940 --> 00:27:20,560
There's this initialization, there's this update.

672
00:27:20,560 --> 00:27:23,820
In a while loop, there's just this condition.

673
00:27:23,820 --> 00:27:26,630
So it seems that it's a little pared down versus the for

674
00:27:26,630 --> 00:27:28,770
loop, which means if we want to have variables and we want

675
00:27:28,770 --> 00:27:30,800
to have incrementation, we actually have to

676
00:27:30,800 --> 00:27:32,500
do this thing ourselves.

677
00:27:32,500 --> 00:27:34,420
>> So let me go ahead and open up gedit.

678
00:27:34,420 --> 00:27:36,320
Let me switch over to the appliance.

679
00:27:36,320 --> 00:27:38,110
And let's just do a quick little example that

680
00:27:38,110 --> 00:27:40,550
distinguishes one of these from the other.

681
00:27:40,550 --> 00:27:42,490
And in the back of my mind here, I should say one thing.

682
00:27:42,490 --> 00:27:44,120
I specifically mentioned the name Tim.

683
00:27:44,120 --> 00:27:47,740
Tim was actually someone that a student tried to find to do

684
00:27:47,740 --> 00:27:48,680
their homework for them.

685
00:27:48,680 --> 00:27:51,280
We had no Tim in that particular section.

686
00:27:51,280 --> 00:27:53,690
So realize, lest I disclosed a student, it was not a student.

687
00:27:53,690 --> 00:27:55,720
It was a random person on the Internet doing things

688
00:27:55,720 --> 00:27:57,180
by proxy last year.

689
00:27:57,180 --> 00:27:58,670
So we find that, too.

690
00:27:58,670 --> 00:28:00,630
So in this case here, let me go ahead and

691
00:28:00,630 --> 00:28:02,810
open up a new file.

692
00:28:02,810 --> 00:28:04,800
File, New.

693
00:28:04,800 --> 00:28:06,050
This gives me a tab here.

694
00:28:06,050 --> 00:28:09,940
Let me go ahead and save it as loop.c.

695
00:28:09,940 --> 00:28:11,810
Let me go and click Save.

696
00:28:11,810 --> 00:28:15,340
And then down here, let's go ahead and start writing

697
00:28:15,340 --> 00:28:16,605
#include <stdio.h>.

698
00:28:19,116 --> 00:28:20,520
Let me zoom in.

699
00:28:20,520 --> 00:28:23,410
Now we'll do int main(void).

700
00:28:23,410 --> 00:28:30,020
Now let me go ahead and do for (int i = 0; i <

701
00:28:30,020 --> 00:28:33,480
oh, 10; i++).

702
00:28:33,480 --> 00:28:36,620
And now I'm going to go ahead and do print the star that I

703
00:28:36,620 --> 00:28:37,890
did earlier.

704
00:28:37,890 --> 00:28:39,390
And then at the end of this program, we're just going to

705
00:28:39,390 --> 00:28:41,130
print a new line, just so that my prompt

706
00:28:41,130 --> 00:28:42,470
doesn't look all messy.

707
00:28:42,470 --> 00:28:45,170
return 0.

708
00:28:45,170 --> 00:28:47,610
Seem syntactically correct?

709
00:28:47,610 --> 00:28:48,040
So far.

710
00:28:48,040 --> 00:28:48,560
So let's see.

711
00:28:48,560 --> 00:28:51,290
>> So let me zoom out, go into my terminal window.

712
00:28:51,290 --> 00:28:54,570
And let me go ahead and run loop, because I called this

713
00:28:54,570 --> 00:28:55,690
thing loop.c.

714
00:28:55,690 --> 00:28:56,780
So make loop.

715
00:28:56,780 --> 00:28:57,840
Seems to compile OK.

716
00:28:57,840 --> 00:29:00,210
Let me run loop, and now Enter.

717
00:29:00,210 --> 00:29:01,820
And it seems to have printed 10 stars.

718
00:29:01,820 --> 00:29:03,780
So let's just convert this to a while loop and see what

719
00:29:03,780 --> 00:29:05,220
kinds of issues we trip over.

720
00:29:05,220 --> 00:29:11,290
So instead of this, let me go in here and say while i is

721
00:29:11,290 --> 00:29:12,760
less than 10--

722
00:29:12,760 --> 00:29:14,910
let me get rid of the for loop.

723
00:29:14,910 --> 00:29:17,170
OK, so we have a couple of problems already.

724
00:29:17,170 --> 00:29:21,110
So the condition is the same, but I'm obviously missing the

725
00:29:21,110 --> 00:29:21,680
initialization.

726
00:29:21,680 --> 00:29:23,840
I am missing the incrementation.

727
00:29:23,840 --> 00:29:28,020
So what should the compiler likely tell me when I try to

728
00:29:28,020 --> 00:29:29,170
compile this program?

729
00:29:29,170 --> 00:29:29,666
Yeah?

730
00:29:29,666 --> 00:29:31,154
AUDIENCE: [INAUDIBLE].

731
00:29:31,154 --> 00:29:31,650
>> DAVID J. MALAN: Good.

732
00:29:31,650 --> 00:29:34,180
So it's going to say something like undeclared--in this

733
00:29:34,180 --> 00:29:35,280
case, variable i.

734
00:29:35,280 --> 00:29:37,980
And indeed, use of undeclared identifier i.

735
00:29:37,980 --> 00:29:40,960
And so this is in contrast with languages like PHP and

736
00:29:40,960 --> 00:29:43,120
Python and Ruby, with which some of you might be familiar,

737
00:29:43,120 --> 00:29:44,820
where you can just kind of start using variables

738
00:29:44,820 --> 00:29:47,420
willy-nilly and not have to worry about declaring them

739
00:29:47,420 --> 00:29:48,560
explicitly always.

740
00:29:48,560 --> 00:29:51,770
In C and in languages like Java and C++, you have to be

741
00:29:51,770 --> 00:29:53,020
super explicit.

742
00:29:53,020 --> 00:29:55,760
And if you want a variable called i, you have to tell me

743
00:29:55,760 --> 00:29:57,360
what kind of variable it is.

744
00:29:57,360 --> 00:29:59,360
So we're going to have to fix this as follows.

745
00:29:59,360 --> 00:30:05,510
I'm going to have to go up here and type int i; therefore,

746
00:30:05,510 --> 00:30:07,860
I have declared a variable called i.

747
00:30:07,860 --> 00:30:09,180
Now, I've skipped one step.

748
00:30:09,180 --> 00:30:11,340
I've obviously not initialized it, but let's see if that at

749
00:30:11,340 --> 00:30:13,650
least makes Clang stop complaining.

750
00:30:13,650 --> 00:30:15,770
So let me remake this program.

751
00:30:15,770 --> 00:30:16,770
>> All right, now it's just complaining

752
00:30:16,770 --> 00:30:17,870
for a different reason.

753
00:30:17,870 --> 00:30:21,130
"Variable 'i' is uninitialized when used here." All right, so

754
00:30:21,130 --> 00:30:22,340
that's pretty explicit.

755
00:30:22,340 --> 00:30:25,510
Initialized just means setting it equal to a value.

756
00:30:25,510 --> 00:30:30,820
And we've not done that, so let me try equals 0.

757
00:30:30,820 --> 00:30:35,030
Now let's try this again and re-run Clang.

758
00:30:35,030 --> 00:30:36,140
Compiled this time.

759
00:30:36,140 --> 00:30:37,210
And I'm about to run it.

760
00:30:37,210 --> 00:30:43,120
But big old infinite loop, because I've done the

761
00:30:43,120 --> 00:30:45,530
initialization, I've done the condition, but I've never done

762
00:30:45,530 --> 00:30:47,030
any kind of incrementation.

763
00:30:47,030 --> 00:30:48,780
So how can I do the incrementation?

764
00:30:48,780 --> 00:30:51,170
Well, in a while loop, it feels like I'm going to have

765
00:30:51,170 --> 00:30:54,180
to do it inside of the loop, because much like the first

766
00:30:54,180 --> 00:30:56,570
week's examples of doing looping constructs, like with

767
00:30:56,570 --> 00:30:59,040
the socks and with the self-counting, we had to do

768
00:30:59,040 --> 00:31:01,550
something at the very end, like go back to the next line.

769
00:31:01,550 --> 00:31:05,040
What if I go ahead and do this i++ here?

770
00:31:05,040 --> 00:31:06,030
Let's not even compile this.

771
00:31:06,030 --> 00:31:06,890
Catch me already.

772
00:31:06,890 --> 00:31:09,704
What's wrong here?

773
00:31:09,704 --> 00:31:10,690
AUDIENCE: [INAUDIBLE].

774
00:31:10,690 --> 00:31:11,770
>> DAVID J. MALAN: So it's definitely not int.

775
00:31:11,770 --> 00:31:13,080
It's i.

776
00:31:13,080 --> 00:31:15,110
And the curly braces, like before,

777
00:31:15,110 --> 00:31:16,800
indentation is not enough.

778
00:31:16,800 --> 00:31:18,410
So now I have this construct.

779
00:31:18,410 --> 00:31:21,780
So while i is less than 10, print a star,

780
00:31:21,780 --> 00:31:23,220
then increment i.

781
00:31:23,220 --> 00:31:25,680
And the way a while loop works is that as soon as you hit the

782
00:31:25,680 --> 00:31:29,630
bottom of the loop, which in this case looks like line 10,

783
00:31:29,630 --> 00:31:32,880
it's going to go back to line 6, at which point the

784
00:31:32,880 --> 00:31:34,400
condition will be checked again.

785
00:31:34,400 --> 00:31:37,500
And if i is still less than 10, we'll do lines 8 and

786
00:31:37,500 --> 00:31:41,250
then 9, then we'll hit 10, and go back to 6, again and

787
00:31:41,250 --> 00:31:45,580
again and again and again, so long as i is less than 10.

788
00:31:45,580 --> 00:31:47,860
So let's re-run make here.

789
00:31:47,860 --> 00:31:48,940
>> Okay, we've compiled okay.

790
00:31:48,940 --> 00:31:50,300
Let me re-run loop.

791
00:31:50,300 --> 00:31:52,620
And now it actually seems to work.

792
00:31:52,620 --> 00:31:55,100
So pluses and minuses here?

793
00:31:55,100 --> 00:31:58,910
Well, so far there's actually not a whole of plo--

794
00:31:58,910 --> 00:31:59,420
so cute.

795
00:31:59,420 --> 00:32:01,870
All right, that was an--ah, that was an accident.

796
00:32:01,870 --> 00:32:05,200
All right, so let's go back to the for loop.

797
00:32:05,200 --> 00:32:07,530
So for loops are nice because they're super explicit.

798
00:32:07,530 --> 00:32:09,720
And even though they're a little clunky to write, it's

799
00:32:09,720 --> 00:32:11,500
very powerful and it allows you to do

800
00:32:11,500 --> 00:32:12,980
multiple things at once.

801
00:32:12,980 --> 00:32:16,240
While loops don't seem to have a huge amount of value just

802
00:32:16,240 --> 00:32:18,500
yet, because it feels like we just have to do more work.

803
00:32:18,500 --> 00:32:20,670
We have to put the initialization up here, the

804
00:32:20,670 --> 00:32:23,480
update down here, and we have to remember to do all that.

805
00:32:23,480 --> 00:32:26,260
So we'll see in time that while loops actually lend

806
00:32:26,260 --> 00:32:29,380
themselves to just different contexts, different data

807
00:32:29,380 --> 00:32:33,900
structures like lists and hash tables, things we'll get to

808
00:32:33,900 --> 00:32:34,970
mid-semester.

809
00:32:34,970 --> 00:32:37,900
But for now, know that there's this third type known as a do-

810
00:32:37,900 --> 00:32:38,480
while loop.

811
00:32:38,480 --> 00:32:39,540
And we've seen this briefly.

812
00:32:39,540 --> 00:32:41,830
And this might be super helpful with pset 1.

813
00:32:41,830 --> 00:32:45,570
Any time you want to do something and then check if

814
00:32:45,570 --> 00:32:48,940
the user cooperated, and if they didn't, do it again, a do-

815
00:32:48,940 --> 00:32:51,460
while loop lends itself to that kind of logic.

816
00:32:51,460 --> 00:32:55,640
Because as the ordering from top to bottom here suggests,

817
00:32:55,640 --> 00:32:57,750
do literally means do this.

818
00:32:57,750 --> 00:32:59,830
And do this again and again, what might that be?

819
00:32:59,830 --> 00:33:03,000
Maybe it means calling GetInt or GetString and then

820
00:33:03,000 --> 00:33:05,830
checking the value of GetInt or GetString and then yelling

821
00:33:05,830 --> 00:33:08,260
at the user if they haven't cooperated by asking them

822
00:33:08,260 --> 00:33:10,100
again and again and again.

823
00:33:10,100 --> 00:33:11,730
Where you want to do something once,

824
00:33:11,730 --> 00:33:13,210
then check some condition.

825
00:33:13,210 --> 00:33:14,110
>> So let's try this.

826
00:33:14,110 --> 00:33:17,130
Let me actually change this now to a do-while loop.

827
00:33:17,130 --> 00:33:18,830
And I'm going to go ahead and do the following.

828
00:33:18,830 --> 00:33:21,830
So do the following.

829
00:33:21,830 --> 00:33:26,870
Let's do int i = GetInt(); but let's first tell the user

830
00:33:26,870 --> 00:33:27,410
what to do.

831
00:33:27,410 --> 00:33:29,050
So a little different this time.

832
00:33:29,050 --> 00:33:31,270
"Give me an int".

833
00:33:31,270 --> 00:33:32,910
So I'll use printf for that.

834
00:33:32,910 --> 00:33:35,740
And now I'm going to go down here, and I'm going to do this

835
00:33:35,740 --> 00:33:41,520
while i is, let's say, greater than--

836
00:33:41,520 --> 00:33:47,540
let's see, i is, let's say, less than 0, or i is

837
00:33:47,540 --> 00:33:48,730
greater than 10.

838
00:33:48,730 --> 00:33:51,810
In other words, I want a number from 1 to 9, just

839
00:33:51,810 --> 00:33:52,720
arbitrarily.

840
00:33:52,720 --> 00:33:55,290
So I'm using a combined Boolean expression here to

841
00:33:55,290 --> 00:33:59,930
make sure that i is less than 0 or greater than 10, in which

842
00:33:59,930 --> 00:34:02,530
case I will do this loop here again.

843
00:34:02,530 --> 00:34:04,400
So again, do this--

844
00:34:04,400 --> 00:34:08,480
while i is less than 0 or i is greater than 10.

845
00:34:08,480 --> 00:34:11,440
>> So now let's go ahead and do this once we've done that.

846
00:34:11,440 --> 00:34:13,270
Let's just do a quick sanity check.

847
00:34:13,270 --> 00:34:18,929
printf("Thanks, i is %d", i).

848
00:34:18,929 --> 00:34:21,350
So this simple program asks the user for an int,

849
00:34:21,350 --> 00:34:24,000
makes sure it's within some range, 1 to 9 inclusive,

850
00:34:24,000 --> 00:34:26,280
and then thanks the user by reminding them what they just

851
00:34:26,280 --> 00:34:27,940
typed in, just as a little sanity check.

852
00:34:27,940 --> 00:34:30,659
But let's see if this works as intended.

853
00:34:30,659 --> 00:34:34,533
Let me go head down here and re-run make loop.

854
00:34:34,533 --> 00:34:35,350
Hmm.

855
00:34:35,350 --> 00:34:38,600
"Use of undeclared identifier 'i'". That's strange.

856
00:34:38,600 --> 00:34:41,509
I thought we resolved that.

857
00:34:45,489 --> 00:34:47,560
Same symptom but different code.

858
00:34:47,560 --> 00:34:47,899
Yeah?

859
00:34:47,899 --> 00:34:50,191
AUDIENCE: [INAUDIBLE] inside the two, we have to

860
00:34:50,191 --> 00:34:52,639
[INAUDIBLE].

861
00:34:52,639 --> 00:34:53,260
>> DAVID J. MALAN: Exactly.

862
00:34:53,260 --> 00:34:55,989
So this actually leads us to a topic known as scope.

863
00:34:55,989 --> 00:34:59,350
It turns out that C, again, it really takes you literally.

864
00:34:59,350 --> 00:35:02,970
And if you do something like this where you declare an int

865
00:35:02,970 --> 00:35:06,120
and then assign it some value, but you do that inside of a

866
00:35:06,120 --> 00:35:09,840
pair of curly braces, what C does is it assumes that you

867
00:35:09,840 --> 00:35:14,800
only want those 32 bits known as i to exist within the

868
00:35:14,800 --> 00:35:18,200
context of those curly braces, within the context of lines

869
00:35:18,200 --> 00:35:22,420
6 through 9. So i is declared, and it is assigned a

870
00:35:22,420 --> 00:35:26,530
value in line 8, but as soon as you get outside of

871
00:35:26,530 --> 00:35:30,270
line 9 below the curly brace, i is no longer in

872
00:35:30,270 --> 00:35:31,910
scope, so to speak.

873
00:35:31,910 --> 00:35:35,030
S-C-O-P-E. It's no longer in the right context.

874
00:35:35,030 --> 00:35:37,940
So now there is no i, so it's as though we hadn't even

875
00:35:37,940 --> 00:35:38,770
declared it at all.

876
00:35:38,770 --> 00:35:41,520
>> So what's a fix then for something like this, if the

877
00:35:41,520 --> 00:35:44,300
reason is that i is declared within the curly braces, which

878
00:35:44,300 --> 00:35:45,120
is apparently bad?

879
00:35:45,120 --> 00:35:45,410
Here?

880
00:35:45,410 --> 00:35:46,820
AUDIENCE: [INAUDIBLE].

881
00:35:46,820 --> 00:35:47,150
DAVID J. MALAN: Yeah.

882
00:35:47,150 --> 00:35:48,710
So we can initialize it outside.

883
00:35:48,710 --> 00:35:53,530
So let me go ahead and delete the declaration parts whereby

884
00:35:53,530 --> 00:35:55,820
I specify the type, and let me do it up here.

885
00:35:55,820 --> 00:36:00,220
So in line 5, it now says "Give me an int." Call it i.

886
00:36:00,220 --> 00:36:03,400
Notice in line 9, I do not want to do this, because I

887
00:36:03,400 --> 00:36:04,830
already have the 32 bits.

888
00:36:04,830 --> 00:36:06,140
I don't want to ask the computer for a

889
00:36:06,140 --> 00:36:07,630
different 32 bits.

890
00:36:07,630 --> 00:36:09,850
I want to use those same 32 bits.

891
00:36:09,850 --> 00:36:13,190
And now because i is declared in line 5, it's still legit

892
00:36:13,190 --> 00:36:16,550
to use it in line 11 and line 12.

893
00:36:16,550 --> 00:36:19,310
>> So let me try to recompile this and see

894
00:36:19,310 --> 00:36:20,490
if Clang stops yelling.

895
00:36:20,490 --> 00:36:22,380
make loop.

896
00:36:22,380 --> 00:36:25,470
So now it is "implicit declaration of function

897
00:36:25,470 --> 00:36:29,880
'GetInt' is invalid in C99." What is that?

898
00:36:29,880 --> 00:36:30,400
Yeah?

899
00:36:30,400 --> 00:36:32,330
AUDIENCE: [INAUDIBLE].

900
00:36:32,330 --> 00:36:32,690
DAVID J. MALAN: Yeah.

901
00:36:32,690 --> 00:36:35,520
So now that I'm actually using GetInt, this is not something

902
00:36:35,520 --> 00:36:38,190
that just comes with C. This comes from CS50.

903
00:36:38,190 --> 00:36:39,890
So we need this here.

904
00:36:39,890 --> 00:36:41,450
And let me go back to the prompt down

905
00:36:41,450 --> 00:36:43,500
here and re-run make.

906
00:36:43,500 --> 00:36:44,160
Okay, finally.

907
00:36:44,160 --> 00:36:46,270
Now we've resolved that and the other error.

908
00:36:46,270 --> 00:36:48,930
Let me now run loop and see what happens.

909
00:36:48,930 --> 00:36:51,740
"Give me an int." I'll give it 11.

910
00:36:51,740 --> 00:36:53,100
I'll give it -1.

911
00:36:53,100 --> 00:36:54,470
I'll give it foo.

912
00:36:54,470 --> 00:36:56,080
I'll give it 5.

913
00:36:56,080 --> 00:36:57,450
And now it indeed works.

914
00:36:57,450 --> 00:36:59,410
But the prompt changed for a reason here.

915
00:36:59,410 --> 00:37:02,800
Why did it say retry one of these times but give me an

916
00:37:02,800 --> 00:37:03,920
int the other three times?

917
00:37:03,920 --> 00:37:05,500
Why is that behavior different?

918
00:37:05,500 --> 00:37:06,980
AUDIENCE: Gave it a string.

919
00:37:06,980 --> 00:37:07,830
DAVID J. MALAN: Sorry?

920
00:37:07,830 --> 00:37:08,570
AUDIENCE: You gave it a string.

921
00:37:08,570 --> 00:37:08,990
DAVID J. MALAN: Yeah.

922
00:37:08,990 --> 00:37:11,450
So we gave it a string in this third attempt,

923
00:37:11,450 --> 00:37:12,490
when I typed foo.

924
00:37:12,490 --> 00:37:13,200
Foo is a string.

925
00:37:13,200 --> 00:37:14,340
It's obviously not an int.

926
00:37:14,340 --> 00:37:17,500
And the way that CS50 has implemented GetInt is that we

927
00:37:17,500 --> 00:37:20,330
don't check if something's less than 0 or greater than 10

928
00:37:20,330 --> 00:37:22,260
for you, because how do we know in advance what kind of

929
00:37:22,260 --> 00:37:23,130
int you want?

930
00:37:23,130 --> 00:37:25,880
But we can minimally check for you, did the user at least

931
00:37:25,880 --> 00:37:26,730
type an integer?

932
00:37:26,730 --> 00:37:30,500
And if they didn't, we yell at the user by typing "retry" on

933
00:37:30,500 --> 00:37:31,440
the screen.

934
00:37:31,440 --> 00:37:34,210
>> So now we have a program that's looping. Okay.

935
00:37:34,210 --> 00:37:37,070
Now, which of these is sort of the better construct?

936
00:37:37,070 --> 00:37:39,060
So this is where things start to get a little messy, the

937
00:37:39,060 --> 00:37:42,360
fact that you have to remember to declare a variable up here

938
00:37:42,360 --> 00:37:44,630
if you want to use it inside of some

939
00:37:44,630 --> 00:37:46,460
curly braces and outside.

940
00:37:46,460 --> 00:37:48,490
But even if this looks a little cryptic at first

941
00:37:48,490 --> 00:37:50,750
glance, just again, remember the simple logic.

942
00:37:50,750 --> 00:37:54,630
In order to use anything in C, whether it's a function or

943
00:37:54,630 --> 00:37:57,680
it's a variable, you have to include it if it's a function

944
00:37:57,680 --> 00:37:59,770
in some library, or you need to declare it.

945
00:37:59,770 --> 00:38:02,405
But now you need to be extra mindful of the fact that

946
00:38:02,405 --> 00:38:04,940
you're declaring it in the right scope.

947
00:38:04,940 --> 00:38:08,150
You're not putting it too tightly inside of parentheses.

948
00:38:08,150 --> 00:38:09,410
>> So let me actually roll back.

949
00:38:09,410 --> 00:38:12,660
If we go back to our for example from earlier, and I go

950
00:38:12,660 --> 00:38:19,245
back to for int, int i = 0; i < 10; i++, and I

951
00:38:19,245 --> 00:38:26,370
do printf stars, like this, and then close paren, and now

952
00:38:26,370 --> 00:38:30,410
printf i is now--

953
00:38:30,410 --> 00:38:33,500
according to the same logic, what will happen when I try to

954
00:38:33,500 --> 00:38:35,500
compile this program?

955
00:38:35,500 --> 00:38:36,790
AUDIENCE: Invalid identifier.

956
00:38:36,790 --> 00:38:38,560
DAVID J. MALAN: So it's another invalid identifier,

957
00:38:38,560 --> 00:38:39,470
undeclared identifier.

958
00:38:39,470 --> 00:38:41,810
Now, the reason's a little different.

959
00:38:41,810 --> 00:38:44,370
There's obviously no curly braces here, but the same

960
00:38:44,370 --> 00:38:46,790
idea, the same story of scope applies.

961
00:38:46,790 --> 00:38:50,340
If you have declared a variable like i inside of a

962
00:38:50,340 --> 00:38:53,960
for loop, even if you have not explicitly written the curly

963
00:38:53,960 --> 00:38:56,980
braces, think of them mentally as still being there, in which

964
00:38:56,980 --> 00:39:00,310
case i is only valid inside of the for loop.

965
00:39:00,310 --> 00:39:03,080
It is not valid once you get to the next line, which in

966
00:39:03,080 --> 00:39:05,090
this case is now 10.

967
00:39:05,090 --> 00:39:09,060
So just a few issues of scope and the like.

968
00:39:09,060 --> 00:39:12,620
All right, any questions?

969
00:39:12,620 --> 00:39:15,310
>> All right, so this is kind of a trivial little program,

970
00:39:15,310 --> 00:39:17,450
printing just little stars.

971
00:39:17,450 --> 00:39:20,400
But let's see if you remember this song here.

972
00:39:20,400 --> 00:39:22,530
This is an incredibly annoying song the kids would sing on

973
00:39:22,530 --> 00:39:23,550
the school bus and the like.

974
00:39:23,550 --> 00:39:25,990
But what's nice about it is that it has this cyclicity,

975
00:39:25,990 --> 00:39:27,910
whereby it's "99 bottles of beer on the wall,

976
00:39:27,910 --> 00:39:28,520
99 bottles of beer.

977
00:39:28,520 --> 00:39:31,030
Take one down, pass it around, 98 bottles of beer on the

978
00:39:31,030 --> 00:39:34,330
wall." And then the song repeats the 97, then 96, then

979
00:39:34,330 --> 00:39:38,040
95, then 94, all the way down to 0 if you actually got that

980
00:39:38,040 --> 00:39:38,920
far on the bus.

981
00:39:38,920 --> 00:39:41,960
So this is a nice program to sort of implement, because my

982
00:39:41,960 --> 00:39:43,650
God, if you could just implement this with a few

983
00:39:43,650 --> 00:39:46,660
lines of code, you could spit out the entire lyrics to this

984
00:39:46,660 --> 00:39:48,240
song quite quickly.

985
00:39:48,240 --> 00:39:50,420
But along the way, we can start to now tease apart some

986
00:39:50,420 --> 00:39:52,460
of these basic looping constructs and now also

987
00:39:52,460 --> 00:39:55,830
introduce functions that we write ourselves, return values

988
00:39:55,830 --> 00:39:57,020
that we pass around.

989
00:39:57,020 --> 00:39:58,910
But first, why don't we go ahead and take our five-minute

990
00:39:58,910 --> 00:39:59,320
break here?

991
00:39:59,320 --> 00:40:01,480
And when we get back, we will sing this song.

992
00:40:05,680 --> 00:40:08,760
>> All right, so we are back.

993
00:40:08,760 --> 00:40:11,850
And when I say we will now sing this song, I mean

994
00:40:11,850 --> 00:40:13,250
programmatically, not verbally.

995
00:40:13,250 --> 00:40:19,370
So here we have beer1.c, which is one implementation of this

996
00:40:19,370 --> 00:40:20,580
particular song.

997
00:40:20,580 --> 00:40:23,110
And just to be clear, for those unfamiliar with what

998
00:40:23,110 --> 00:40:24,460
this thing looks like, let me go ahead

999
00:40:24,460 --> 00:40:27,070
and make beer1, Enter.

1000
00:40:27,070 --> 00:40:30,400
Now let me run beer1, and what we'll see--how many bottles

1001
00:40:30,400 --> 00:40:31,140
of beer will there be?

1002
00:40:31,140 --> 00:40:33,240
I'll type in 99, like the song says.

1003
00:40:33,240 --> 00:40:34,040
Enter.

1004
00:40:34,040 --> 00:40:35,650
And now if we scroll through--

1005
00:40:35,650 --> 00:40:38,280
oops--if we scroll through all of this, we'll see that

1006
00:40:38,280 --> 00:40:41,350
this did indeed sing the whole song.

1007
00:40:44,050 --> 00:40:44,540
Wait a minute.

1008
00:40:44,540 --> 00:40:46,240
My scroll bar's a little messed up.

1009
00:40:46,240 --> 00:40:47,940
Let's use the bigger window.

1010
00:40:47,940 --> 00:40:53,500
So beer1, 99, there we go.

1011
00:40:53,500 --> 00:40:56,420
So here we have the whole song, sung much faster by the

1012
00:40:56,420 --> 00:40:58,450
computer than it could have been by us.

1013
00:40:58,450 --> 00:41:00,340
So notice, though, the cyclical nature here.

1014
00:41:00,340 --> 00:41:03,380
It says 99, then 99, then "take one down, pass it

1015
00:41:03,380 --> 00:41:04,740
around," then 98.

1016
00:41:04,740 --> 00:41:06,640
And now it repeats again and again.

1017
00:41:06,640 --> 00:41:08,840
>> So this is actually a perfect opportunity for some kind of

1018
00:41:08,840 --> 00:41:10,400
looping construct.

1019
00:41:10,400 --> 00:41:12,950
Notice that I'm kind of cutting a corner here.

1020
00:41:12,950 --> 00:41:15,960
Notice that I'm saying "98 bottles of beer on the wall,

1021
00:41:15,960 --> 00:41:19,010
97 bottles of beer on the wall," and that was just so

1022
00:41:19,010 --> 00:41:21,640
that when we get to one bottles of beer, I don't have

1023
00:41:21,640 --> 00:41:23,300
to worry about the English grammar.

1024
00:41:23,300 --> 00:41:25,790
But we can also fix this with a little bit of an if

1025
00:41:25,790 --> 00:41:26,910
condition, perhaps.

1026
00:41:26,910 --> 00:41:30,110
If this number is singular, go ahead and say "bottle",

1027
00:41:30,110 --> 00:41:31,840
otherwise if it's plural, say "bottles".

1028
00:41:31,840 --> 00:41:34,150
But for now, I'm completely cutting that corner.

1029
00:41:34,150 --> 00:41:35,520
So let's see what we've got here.

1030
00:41:35,520 --> 00:41:37,150
So we've got some comments at the top.

1031
00:41:37,150 --> 00:41:39,450
I'm including these two libraries, as

1032
00:41:39,450 --> 00:41:41,140
we've commonly been.

1033
00:41:41,140 --> 00:41:43,610
And now let me scroll down to the first

1034
00:41:43,610 --> 00:41:44,990
actual lines of code.

1035
00:41:44,990 --> 00:41:47,020
Line 17 kicks off main.

1036
00:41:47,020 --> 00:41:50,610
Line 21 and 20 has how many bottles of beer will there be?

1037
00:41:50,610 --> 00:41:52,060
And then I call GetInt.

1038
00:41:52,060 --> 00:41:53,800
And now I have a bit of a sanity check.

1039
00:41:53,800 --> 00:41:57,030
>> So this is a convention that we'll now start adopting to

1040
00:41:57,030 --> 00:41:59,620
more rigorously check the user's input.

1041
00:41:59,620 --> 00:42:01,710
Sometimes you just don't want to prompt them again

1042
00:42:01,710 --> 00:42:02,630
and again and again.

1043
00:42:02,630 --> 00:42:05,620
If the user screws up and doesn't cooperate, fine.

1044
00:42:05,620 --> 00:42:07,430
Quit and just don't deal with them.

1045
00:42:07,430 --> 00:42:08,990
And so that's what I'm doing here.

1046
00:42:08,990 --> 00:42:12,520
If n is less than 1, I'm just going to yell at the user,

1047
00:42:12,520 --> 00:42:14,330
"Sorry, that makes no sense." And then I'm going to

1048
00:42:14,330 --> 00:42:16,650
arbitrarily return 1.

1049
00:42:16,650 --> 00:42:18,550
So again, this is just a convention to get used to.

1050
00:42:18,550 --> 00:42:19,850
For now, take it on faith.

1051
00:42:19,850 --> 00:42:22,700
But up until now, we've always been returning 0, because

1052
00:42:22,700 --> 00:42:25,204
we've said returning 0 denotes what?

1053
00:42:25,204 --> 00:42:26,000
AUDIENCE: Success.

1054
00:42:26,000 --> 00:42:27,320
DAVID J. MALAN: Success, that's all.

1055
00:42:27,320 --> 00:42:29,740
So now that we're finally starting to think about

1056
00:42:29,740 --> 00:42:31,330
non-successes--

1057
00:42:31,330 --> 00:42:33,760
in other words, corner cases, error conditions--

1058
00:42:33,760 --> 00:42:37,520
now I have an infinite supply, or at least four billion

1059
00:42:37,520 --> 00:42:39,790
possible things that can go wrong in my programs.

1060
00:42:39,790 --> 00:42:42,300
And I can start assigning them individual numbers.

1061
00:42:42,300 --> 00:42:44,340
Now, generally it suffices to just return

1062
00:42:44,340 --> 00:42:45,730
something other than 0.

1063
00:42:45,730 --> 00:42:48,040
So we're going to simply return 1 for now.

1064
00:42:48,040 --> 00:42:51,440
But the reason for returning 1 is that as soon as you return

1065
00:42:51,440 --> 00:42:55,110
1, guess what happens to the rest of the program?

1066
00:42:55,110 --> 00:42:55,720
It stops.

1067
00:42:55,720 --> 00:42:56,560
That's it.

1068
00:42:56,560 --> 00:42:59,150
So the fact that I'm returning 1 is effectively

1069
00:42:59,150 --> 00:43:02,950
short-circuiting this program's execution so that

1070
00:43:02,950 --> 00:43:06,780
nothing below line 27 will continue executing.

1071
00:43:06,780 --> 00:43:09,210
As soon as main returns, that is it.

1072
00:43:09,210 --> 00:43:13,160
>> All right, so if the user does cooperate and we reach line 30

1073
00:43:13,160 --> 00:43:15,680
because they typed in a legitimate number, here is my

1074
00:43:15,680 --> 00:43:16,990
implementation of this song.

1075
00:43:16,990 --> 00:43:19,050
So I first print out a newline character, just for

1076
00:43:19,050 --> 00:43:19,880
aesthetics.

1077
00:43:19,880 --> 00:43:21,500
I now have a for loop.

1078
00:43:21,500 --> 00:43:22,930
And notice I'm doing things in a bit

1079
00:43:22,930 --> 00:43:23,880
of a different direction.

1080
00:43:23,880 --> 00:43:26,210
I don't have to do less than, I don't have to do ++.

1081
00:43:26,210 --> 00:43:30,660
I can instead say initialize a variable i, set it equal to

1082
00:43:30,660 --> 00:43:35,080
n, the number the user typed in, then do the following, so

1083
00:43:35,080 --> 00:43:39,590
long as i is greater than 0, then i--

1084
00:43:39,590 --> 00:43:42,070
once you've finished one iteration of this loop.

1085
00:43:42,070 --> 00:43:44,310
So we can count down using a for loop as well.

1086
00:43:44,310 --> 00:43:47,520
Now, this is pretty much week one stuff now, with printf.

1087
00:43:47,520 --> 00:43:51,770
So print "%d bottles of beer on the wall." Print "%d

1088
00:43:51,770 --> 00:43:54,990
bottles of beer." "Take one down, pass it around." Print

1089
00:43:54,990 --> 00:43:58,880
"%d bottles of beer on the wall." So it's still %d, but

1090
00:43:58,880 --> 00:44:02,770
notice that the argument to printf is changing.

1091
00:44:02,770 --> 00:44:05,840
After the comma, I have i, because I want to say 99.

1092
00:44:05,840 --> 00:44:08,590
After this comma, I have i, because I want to say 99.

1093
00:44:08,590 --> 00:44:14,030
After this comma, I have i - 1, because I want to say 98 in

1094
00:44:14,030 --> 00:44:16,260
this first iteration, and so forth.

1095
00:44:16,260 --> 00:44:18,520
And now down here, I just have some stupid little remark.

1096
00:44:18,520 --> 00:44:22,270
And then line 42, I return 0 by convention, signifying that

1097
00:44:22,270 --> 00:44:23,650
everything is okay.

1098
00:44:23,650 --> 00:44:24,490
>> So what if I goofed?

1099
00:44:24,490 --> 00:44:26,350
What might a common mistake here be?

1100
00:44:26,350 --> 00:44:29,200
Well, what if I accidentally said well, I do want to count

1101
00:44:29,200 --> 00:44:31,640
down to 0, I want 0 bottles of beer on the wall?

1102
00:44:31,640 --> 00:44:34,620
So I say, i is greater than or equal to 0.

1103
00:44:34,620 --> 00:44:38,920
What's going to be the symptom that I now see if I recompile

1104
00:44:38,920 --> 00:44:41,173
beer1 and run it?

1105
00:44:41,173 --> 00:44:42,120
AUDIENCE: Negative.

1106
00:44:42,120 --> 00:44:43,590
DAVID J. MALAN: Yeah, it's gonna go negative.

1107
00:44:43,590 --> 00:44:45,950
This is an off-by-one error, an incredibly

1108
00:44:45,950 --> 00:44:47,270
common mistake to make.

1109
00:44:47,270 --> 00:44:48,960
Let's actually go back to the terminal window and do it

1110
00:44:48,960 --> 00:44:50,620
here, so we can see more at a time.

1111
00:44:50,620 --> 00:44:53,280
Enter, 99 bottles of beer.

1112
00:44:53,280 --> 00:44:56,580
Close, but we went ever so slightly too far.

1113
00:44:56,580 --> 00:45:00,500
We sang the song too far down, such that we now hit the

1114
00:45:00,500 --> 00:45:01,510
negative number.

1115
00:45:01,510 --> 00:45:03,680
So it doesn't quite work.

1116
00:45:03,680 --> 00:45:06,450
>> All right, so we can easily fix that by going back to the

1117
00:45:06,450 --> 00:45:07,650
way it once was.

1118
00:45:07,650 --> 00:45:10,360
But what are some opportunities now for

1119
00:45:10,360 --> 00:45:11,190
improvement?

1120
00:45:11,190 --> 00:45:17,200
Well, let me open beer2.c and scroll down here and take a

1121
00:45:17,200 --> 00:45:19,310
look at this version.

1122
00:45:19,310 --> 00:45:21,370
What's the first thing that jumps out at you as different

1123
00:45:21,370 --> 00:45:23,715
in this version here?

1124
00:45:23,715 --> 00:45:24,190
AUDIENCE: [INAUDIBLE].

1125
00:45:24,190 --> 00:45:26,510
DAVID J. MALAN: Yeah, so no more i, because it occurred to

1126
00:45:26,510 --> 00:45:29,350
me you know what, I'm asking the user for n, and then I'm

1127
00:45:29,350 --> 00:45:33,580
setting i equal to n, and then I'm changing i, but I'm never

1128
00:45:33,580 --> 00:45:34,590
touching n again.

1129
00:45:34,590 --> 00:45:37,390
So what the heck was the point of you allocating another 32

1130
00:45:37,390 --> 00:45:45,210
bits called i just so that I can have a different variable?

1131
00:45:45,210 --> 00:45:47,960
So in this case, I sort of recognized that unnecessary

1132
00:45:47,960 --> 00:45:49,190
design feature.

1133
00:45:49,190 --> 00:45:52,730
And I'm now going to say while n is greater than 0, go

1134
00:45:52,730 --> 00:45:56,180
ahead and print the same song, passing an n to printf as the

1135
00:45:56,180 --> 00:46:00,210
second argument, and n - 1 as the second argument down here.

1136
00:46:00,210 --> 00:46:02,930
And then on each iteration of this loop, go ahead and just

1137
00:46:02,930 --> 00:46:05,080
decrement n itself.

1138
00:46:05,080 --> 00:46:06,960
Now, functionally, this program

1139
00:46:06,960 --> 00:46:08,010
is going to be identical.

1140
00:46:08,010 --> 00:46:10,730
If I type in 99, n starts at 99.

1141
00:46:10,730 --> 00:46:12,890
I decrement, decrement, decrement, decrement.

1142
00:46:12,890 --> 00:46:15,875
I'm going to get all the way down to "One bottle of beer on

1143
00:46:15,875 --> 00:46:16,740
the wall, one bottle of beer.

1144
00:46:16,740 --> 00:46:18,020
Take one down, pass it around.

1145
00:46:18,020 --> 00:46:21,480
0 bottles of beer on the wall." The end, because I did

1146
00:46:21,480 --> 00:46:23,200
get the condition correct.

1147
00:46:23,200 --> 00:46:24,280
It's greater than 0.

1148
00:46:24,280 --> 00:46:26,220
I didn't make this mistake.

1149
00:46:26,220 --> 00:46:28,470
>> So which is better, version one or version two?

1150
00:46:31,380 --> 00:46:33,480
So I heard a bunch of murmurings for two.

1151
00:46:33,480 --> 00:46:34,730
Why two?

1152
00:46:37,210 --> 00:46:38,225
What's that?

1153
00:46:38,225 --> 00:46:39,215
AUDIENCE: [INAUDIBLE].

1154
00:46:39,215 --> 00:46:40,070
DAVID J. MALAN: Oh, okay.

1155
00:46:40,070 --> 00:46:42,870
So it won't go below 0, but remember, in version one, the

1156
00:46:42,870 --> 00:46:45,870
original correct version didn't go below 0 either.

1157
00:46:45,870 --> 00:46:48,340
So remember that this is the correct version.

1158
00:46:48,340 --> 00:46:51,630
So let's at least compare the two correct versions.

1159
00:46:51,630 --> 00:46:53,300
What's an argument in favor of version

1160
00:46:53,300 --> 00:46:55,146
two being, mmm, better?

1161
00:46:55,146 --> 00:46:55,642
Yeah?

1162
00:46:55,642 --> 00:46:57,630
AUDIENCE: It uses less space.

1163
00:46:57,630 --> 00:46:59,530
DAVID J. MALAN: Okay, so it uses less space, right?

1164
00:46:59,530 --> 00:47:02,900
Whereas version one used 32 bits for n, and then another

1165
00:47:02,900 --> 00:47:07,680
32 bits for i.

1166
00:47:07,680 --> 00:47:10,060
Version two only uses 32 bits for n, so that

1167
00:47:10,060 --> 00:47:11,700
seems to be a plus.

1168
00:47:11,700 --> 00:47:12,950
Other thoughts?

1169
00:47:18,070 --> 00:47:21,520
Does anyone want to argue in favor of one?

1170
00:47:21,520 --> 00:47:22,070
Yeah?

1171
00:47:22,070 --> 00:47:25,240
AUDIENCE: You have to use extra line of code for n--.

1172
00:47:25,240 --> 00:47:26,090
>> DAVID J. MALAN: Okay, sure.

1173
00:47:26,090 --> 00:47:26,960
So that's fair.

1174
00:47:26,960 --> 00:47:29,040
So this just, at least to me--

1175
00:47:29,040 --> 00:47:31,940
I mean, this actually feels a little messier, the fact that

1176
00:47:31,940 --> 00:47:35,120
I can't sort of encapsulate all of my logic in one

1177
00:47:35,120 --> 00:47:38,030
beautiful line, the for loop, as the for loop can.

1178
00:47:38,030 --> 00:47:40,240
Here, I kind of have to tack on this n--

1179
00:47:40,240 --> 00:47:41,120
at the end of the loop, because

1180
00:47:41,120 --> 00:47:42,550
it's logically necessary.

1181
00:47:42,550 --> 00:47:45,190
But it kind of rubs me the wrong way, just because it

1182
00:47:45,190 --> 00:47:48,260
seems separate from the logic of up here, even though,

1183
00:47:48,260 --> 00:47:49,430
again, it's necessary.

1184
00:47:49,430 --> 00:47:50,990
Other thoughts?

1185
00:47:50,990 --> 00:47:51,490
Yeah?

1186
00:47:51,490 --> 00:47:52,740
AUDIENCE: [INAUDIBLE].

1187
00:47:57,990 --> 00:47:58,350
>> DAVID J. MALAN: Yeah.

1188
00:47:58,350 --> 00:48:00,730
So what if you instead, at the end of the song, wanted to

1189
00:48:00,730 --> 00:48:02,950
print out again the name of the song?

1190
00:48:02,950 --> 00:48:05,660
Like "Thanks for playing 99 bottles of beer", or something

1191
00:48:05,660 --> 00:48:06,690
silly like that?

1192
00:48:06,690 --> 00:48:09,750
But the point is, you wanted access to the original value.

1193
00:48:09,750 --> 00:48:13,180
The fact that you've mutated or changed n on every

1194
00:48:13,180 --> 00:48:16,330
iteration and therefore have destroyed its original value

1195
00:48:16,330 --> 00:48:18,650
means you just can't do that at the end.

1196
00:48:18,650 --> 00:48:20,660
Now, arguably, we clearly don't want to do

1197
00:48:20,660 --> 00:48:21,450
that in this program.

1198
00:48:21,450 --> 00:48:22,350
So who cares?

1199
00:48:22,350 --> 00:48:23,630
But that's a very valid point.

1200
00:48:23,630 --> 00:48:25,520
And to be honest, there's really no one

1201
00:48:25,520 --> 00:48:26,630
right answer here.

1202
00:48:26,630 --> 00:48:28,740
They're both equally correct.

1203
00:48:28,740 --> 00:48:30,210
I could be convinced either way.

1204
00:48:30,210 --> 00:48:33,310
I will say that, in general, it's a good principle if you

1205
00:48:33,310 --> 00:48:36,030
asked the user for some value and you stored in a variable

1206
00:48:36,030 --> 00:48:38,730
like n, just sort of on principle, it's probably good

1207
00:48:38,730 --> 00:48:40,160
to keep that around.

1208
00:48:40,160 --> 00:48:43,400
And any data you want to mutate again and again, just

1209
00:48:43,400 --> 00:48:46,030
give yourself a copy of that variable, just so that you

1210
00:48:46,030 --> 00:48:47,830
have access to the original.

1211
00:48:47,830 --> 00:48:51,040
You are spending 32 more bits, but the reality is this

1212
00:48:51,040 --> 00:48:53,490
computer has, like, two gigabytes of RAM these days,

1213
00:48:53,490 --> 00:48:55,310
and we're quibbling over 32 bits?

1214
00:48:55,310 --> 00:48:56,320
Really not such a big deal.

1215
00:48:56,320 --> 00:48:58,550
And even on this device here, with a half a gig or a

1216
00:48:58,550 --> 00:49:01,700
gigabyte of RAM, 32 bits versus 64 bits,

1217
00:49:01,700 --> 00:49:02,920
not such a big deal.

1218
00:49:02,920 --> 00:49:05,890
Certainly today, it's going to be way overwhelmed by the size

1219
00:49:05,890 --> 00:49:08,400
of the program itself, which is going to be several hundred

1220
00:49:08,400 --> 00:49:10,890
kilobytes, if not a few megabytes, these days.

1221
00:49:10,890 --> 00:49:13,550
>> So reasonable concerns, no one right answer.

1222
00:49:13,550 --> 00:49:15,490
But at least those are the thoughts that should start to

1223
00:49:15,490 --> 00:49:16,790
go through your mind?

1224
00:49:16,790 --> 00:49:19,600
Because in pset 0, even though we really only expected

1225
00:49:19,600 --> 00:49:22,340
correctness, or at least disclaiming various bugs that

1226
00:49:22,340 --> 00:49:25,440
you might have encountered, as we move forward, design is

1227
00:49:25,440 --> 00:49:27,910
going to be another key aspect, both of writing code

1228
00:49:27,910 --> 00:49:29,770
and also our evaluating code.

1229
00:49:29,770 --> 00:49:32,310
And so at least give thought to things like this.

1230
00:49:32,310 --> 00:49:35,590
And just because something works doesn't mean it's good,

1231
00:49:35,590 --> 00:49:37,130
doesn't mean it's well-designed.

1232
00:49:37,130 --> 00:49:38,820
And that's one of the things the teaching fellows and

1233
00:49:38,820 --> 00:49:41,990
problem sets will help us tease part over time.

1234
00:49:41,990 --> 00:49:45,020
>> Well, what about, let's say, this version here?

1235
00:49:45,020 --> 00:49:49,090
Let me do something a little sexy here in a moment.

1236
00:49:49,090 --> 00:49:50,740
First let me get rid of this.

1237
00:49:50,740 --> 00:49:54,120
And now let's fix this grammatical issue.

1238
00:49:54,120 --> 00:49:58,780
So in this version, I want to fix the grammar so that,

1239
00:49:58,780 --> 00:50:02,460
rather than just say parenthetical s, like "bottle"

1240
00:50:02,460 --> 00:50:03,360
or "bottles"--

1241
00:50:03,360 --> 00:50:04,900
I don't want to cut that corner--

1242
00:50:04,900 --> 00:50:08,350
I also want to dynamically print out the word "bottles"

1243
00:50:08,350 --> 00:50:12,820
or "bottle", thereby using these %s placeholders today.

1244
00:50:12,820 --> 00:50:16,550
So I need to conditionally check what is the value of i.

1245
00:50:16,550 --> 00:50:19,590
And if it's 1, I want to say "bottle", and if it's anything

1246
00:50:19,590 --> 00:50:23,115
else, I want to say "bottles". So let's try to do this.

1247
00:50:23,115 --> 00:50:31,340
So if i == 1, then let me go ahead and declare--

1248
00:50:31,340 --> 00:50:34,080
I need a string, so let me do string s1, because it's the

1249
00:50:34,080 --> 00:50:36,070
first string I care about right now.

1250
00:50:36,070 --> 00:50:40,980
I'm going to say "bottle". And then, let's see, string

1251
00:50:40,980 --> 00:50:43,110
s2--and I'll explain where I'm going in a moment--

1252
00:50:43,110 --> 00:50:47,650
"bottles." So recall that, in this song, we need to be able

1253
00:50:47,650 --> 00:50:50,580
to print things, two different words potentially.

1254
00:50:50,580 --> 00:50:53,590
So if we look back here, notice that when we get to

1255
00:50:53,590 --> 00:50:56,440
this example here, "two bottles of beer on the wall,

1256
00:50:56,440 --> 00:50:59,490
two bottles of beer, take one down, pass it around", I want

1257
00:50:59,490 --> 00:51:02,380
this fourth line to now say "one bottle of beer on the

1258
00:51:02,380 --> 00:51:04,900
wall". So I need to decide, do I want to say "bottles" or

1259
00:51:04,900 --> 00:51:07,780
"bottle"? So I'm going to arbitrarily say, all right,

1260
00:51:07,780 --> 00:51:10,530
I'm going to now declare a variable called s1, string

1261
00:51:10,530 --> 00:51:13,830
one, that's going to get plugged in here and also here,

1262
00:51:13,830 --> 00:51:16,070
because those words are always identical, just because of the

1263
00:51:16,070 --> 00:51:17,290
nature of the song.

1264
00:51:17,290 --> 00:51:20,100
And I'm going to call s2 whatever word I want to

1265
00:51:20,100 --> 00:51:21,560
eventually appear down here.

1266
00:51:21,560 --> 00:51:25,530
Now, literally, 99 times out of 100, it's going to be the

1267
00:51:25,530 --> 00:51:28,820
same in both of those cases, because 3 is plural, 2 is

1268
00:51:28,820 --> 00:51:30,200
plural, 4 is plural.

1269
00:51:30,200 --> 00:51:34,640
But in this corner case, where we get to 2 and then 1, or

1270
00:51:34,640 --> 00:51:37,250
even 1 and then 0, I need this logic.

1271
00:51:37,250 --> 00:51:41,020
So I have to spend some time in my code getting that right.

1272
00:51:41,020 --> 00:51:47,530
So if I do this, if i == 1, then set s1 equal to "bottle"

1273
00:51:47,530 --> 00:51:52,010
and s2 equal to "bottles", because this will be for 1

1274
00:51:52,010 --> 00:51:56,340
bottle, and this will be for 0 bottles.

1275
00:51:56,340 --> 00:51:58,250
And this here, what does this represent?

1276
00:51:58,250 --> 00:51:59,780
Just to be clear.

1277
00:51:59,780 --> 00:52:00,620
This is just a comment.

1278
00:52:00,620 --> 00:52:03,730
So the fact that you can have single-line comments means you

1279
00:52:03,730 --> 00:52:06,110
can comment your code like this, but another common

1280
00:52:06,110 --> 00:52:09,050
paradigm, too, is that if you have a super-short phrase that

1281
00:52:09,050 --> 00:52:11,410
you want to put yourself and it's just more readable to put

1282
00:52:11,410 --> 00:52:13,270
it right at the end of the line of code, you can

1283
00:52:13,270 --> 00:52:15,230
absolutely do something like this.

1284
00:52:15,230 --> 00:52:20,150
>> So now what if I do this? Else if i is not equal to 1.

1285
00:52:20,150 --> 00:52:21,620
So bang equals--

1286
00:52:21,620 --> 00:52:23,150
exclamation point is known as "bang".

1287
00:52:23,150 --> 00:52:25,080
So bang = 1.

1288
00:52:25,080 --> 00:52:28,530
So if i is not equal to 1, what do I instead want to do?

1289
00:52:28,530 --> 00:52:31,700
Well, the first word I want to be what?

1290
00:52:34,620 --> 00:52:40,030
So string 1 should be "bottles" for plural bottles,

1291
00:52:40,030 --> 00:52:42,440
and then this will be plural "bottles" as well, for now.

1292
00:52:42,440 --> 00:52:43,800
And we'll see if this actually gets us to

1293
00:52:43,800 --> 00:52:44,870
where we want to go.

1294
00:52:44,870 --> 00:52:47,680
So now if I scroll down here, notice that I'm plugging in

1295
00:52:47,680 --> 00:52:50,170
not only i, but s1.

1296
00:52:50,170 --> 00:52:51,860
I'm plugging in i and s1.

1297
00:52:51,860 --> 00:52:54,440
And then down here, I'm minus 1, which is the same as

1298
00:52:54,440 --> 00:52:55,920
before, but s2.

1299
00:52:55,920 --> 00:52:58,730
In other words, I want the English word to change based

1300
00:52:58,730 --> 00:52:59,930
on this logic.

1301
00:52:59,930 --> 00:53:03,310
Now, there's already some problems in this code.

1302
00:53:03,310 --> 00:53:08,460
What is broken already out of the gate here?

1303
00:53:08,460 --> 00:53:10,796
Yeah?

1304
00:53:10,796 --> 00:53:13,210
AUDIENCE: [INAUDIBLE].

1305
00:53:13,210 --> 00:53:13,800
>> DAVID J. MALAN: Exactly.

1306
00:53:13,800 --> 00:53:16,030
So I've already violated the lesson of scope.

1307
00:53:16,030 --> 00:53:19,610
So I've declared s1 and s2, but I've done it inside of

1308
00:53:19,610 --> 00:53:22,980
curly braces, which means yeah, this code will work up

1309
00:53:22,980 --> 00:53:27,430
until line 42, but as soon as I hit line 43, guess what no

1310
00:53:27,430 --> 00:53:28,900
longer exists?

1311
00:53:28,900 --> 00:53:32,600
Well, guess what's no longer in scope--neither s1 or s2.

1312
00:53:32,600 --> 00:53:33,780
So we have to fix this.

1313
00:53:33,780 --> 00:53:36,180
So let me delete the declarations.

1314
00:53:36,180 --> 00:53:39,320
And I'll leave the variable names and delete this here

1315
00:53:39,320 --> 00:53:40,120
and delete this here.

1316
00:53:40,120 --> 00:53:43,557
And in what lines should I really declare these things?

1317
00:53:43,557 --> 00:53:44,960
AUDIENCE: [INAUDIBLE].

1318
00:53:44,960 --> 00:53:45,410
DAVID J. MALAN: Yeah, so probably

1319
00:53:45,410 --> 00:53:46,850
right up here, 33-ish.

1320
00:53:46,850 --> 00:53:50,670
So string s1 and then string s2.

1321
00:53:50,670 --> 00:53:51,950
And it turns out, I can do this.

1322
00:53:51,950 --> 00:53:54,260
If you're declaring two variables of the same type,

1323
00:53:54,260 --> 00:53:57,420
you can actually just use a comma and do that in C. All

1324
00:53:57,420 --> 00:53:58,970
right, so now I have two variables--

1325
00:53:58,970 --> 00:53:59,860
s1 and s2.

1326
00:53:59,860 --> 00:54:02,000
I'm assigning them values in these

1327
00:54:02,000 --> 00:54:04,210
conditions here, or in here.

1328
00:54:04,210 --> 00:54:06,690
And then I'm using them down below.

1329
00:54:06,690 --> 00:54:10,960
How well is this now going to work?

1330
00:54:10,960 --> 00:54:12,740
>> Well, it's still a little buggy, but let's at least see

1331
00:54:12,740 --> 00:54:13,730
how far we've gotten.

1332
00:54:13,730 --> 00:54:16,500
So let me go ahead and make beer3.

1333
00:54:16,500 --> 00:54:17,695
Is this beer3?

1334
00:54:17,695 --> 00:54:19,410
Yep, this is beer3.

1335
00:54:19,410 --> 00:54:21,510
And now let me go ahead and run beer3.

1336
00:54:21,510 --> 00:54:23,820
399 99.

1337
00:54:23,820 --> 00:54:25,840
We can probably skip most of them.

1338
00:54:25,840 --> 00:54:29,100
And down here, look at that.

1339
00:54:29,100 --> 00:54:31,110
"One bottle of beer on the wall, one bottle of beer, take

1340
00:54:31,110 --> 00:54:34,306
one down, pass it around, 0 bottles of beer on the wall."

1341
00:54:34,306 --> 00:54:37,570
But I'm drawing your attention to only half of the solution.

1342
00:54:37,570 --> 00:54:39,620
Kind of screwed up here.

1343
00:54:39,620 --> 00:54:43,030
So it seems that the corner cases arise when i equals what

1344
00:54:43,030 --> 00:54:44,030
two values?

1345
00:54:44,030 --> 00:54:45,020
AUDIENCE: 2, 1.

1346
00:54:45,020 --> 00:54:46,190
DAVID J. MALAN: 2 and 1.

1347
00:54:46,190 --> 00:54:48,180
It's not 1 and not 1.

1348
00:54:48,180 --> 00:54:51,890
It's really just these last two stanzas of this song.

1349
00:54:51,890 --> 00:54:53,890
So what do I instead want to do?

1350
00:54:53,890 --> 00:54:58,890
So I seem to have caught the case where if i is == to 1,

1351
00:54:58,890 --> 00:55:02,240
then the first word is "bottle", but the second word

1352
00:55:02,240 --> 00:55:07,230
is "bottles". But here, I want to change this to be == 2.

1353
00:55:07,230 --> 00:55:08,570
And if this is the case, what do I want the

1354
00:55:08,570 --> 00:55:09,620
first word to be?

1355
00:55:09,620 --> 00:55:10,430
AUDIENCE: "Bottles".

1356
00:55:10,430 --> 00:55:12,890
DAVID J. MALAN: "Bottles", so for two bottles.

1357
00:55:12,890 --> 00:55:14,690
And then this word here should be--

1358
00:55:14,690 --> 00:55:15,340
AUDIENCE: "Bottle".

1359
00:55:15,340 --> 00:55:17,316
DAVID J. MALAN: "Bottle", singular.

1360
00:55:20,430 --> 00:55:25,160
>> All right, let's zoom out, go back over here, re-run make,

1361
00:55:25,160 --> 00:55:28,590
re-run beer3, type 99 again.

1362
00:55:28,590 --> 00:55:30,710
Okay, "Segmentation fault (core dumped)."

1363
00:55:30,710 --> 00:55:32,780
What have I done wrong?

1364
00:55:36,050 --> 00:55:38,722
AUDIENCE: You don't have a value [INAUDIBLE].

1365
00:55:38,722 --> 00:55:40,480
DAVID J. MALAN: Ah, excellent point.

1366
00:55:40,480 --> 00:55:43,310
All right, so what's wrong here?

1367
00:55:43,310 --> 00:55:45,450
So segmentation fault, and we're actually going to see

1368
00:55:45,450 --> 00:55:49,020
this quite a few times in the future, deliberately.

1369
00:55:49,020 --> 00:55:51,030
But for now, what does this actually mean?

1370
00:55:51,030 --> 00:55:53,620
A segmentation fault almost always means that you have

1371
00:55:53,620 --> 00:55:56,760
somehow tried to access memory, RAM in your computer,

1372
00:55:56,760 --> 00:56:00,600
that you don't own, that you have not actually asked the

1373
00:56:00,600 --> 00:56:02,050
operating system for.

1374
00:56:02,050 --> 00:56:04,440
So in this case, notice what I've done, which is

1375
00:56:04,440 --> 00:56:05,870
flawed in my logic.

1376
00:56:05,870 --> 00:56:09,500
I have assigned s1 and s2 a value if i equals 1.

1377
00:56:09,500 --> 00:56:11,590
I've also done that if i equals 2.

1378
00:56:11,590 --> 00:56:13,710
But I haven't done it in the infinite number of other

1379
00:56:13,710 --> 00:56:14,690
possibilities--

1380
00:56:14,690 --> 00:56:17,940
in particular, 3 or 4 or dot, dot, dot, 99.

1381
00:56:17,940 --> 00:56:20,100
So one fix for this could just be let's

1382
00:56:20,100 --> 00:56:22,190
have an else condition.

1383
00:56:22,190 --> 00:56:26,780
And let me go in here and say s1 equals--

1384
00:56:26,780 --> 00:56:28,180
what should it be here?

1385
00:56:28,180 --> 00:56:28,750
AUDIENCE: [INAUDIBLE].

1386
00:56:28,750 --> 00:56:30,460
>> DAVID J. MALAN: "Bottles", because in the common case,

1387
00:56:30,460 --> 00:56:32,020
it's just the same thing.

1388
00:56:32,020 --> 00:56:35,580
So equals quote, unquote, "bottles." So for plural

1389
00:56:35,580 --> 00:56:41,010
bottles, and then up here, for plural bottles.

1390
00:56:41,010 --> 00:56:44,580
Okay, so now let me go back to my terminal window,

1391
00:56:44,580 --> 00:56:47,200
recompile, re-run it.

1392
00:56:47,200 --> 00:56:48,440
99.

1393
00:56:48,440 --> 00:56:49,150
Whew.

1394
00:56:49,150 --> 00:56:50,610
And let's do a quick sanity check.

1395
00:56:50,610 --> 00:56:52,400
Technically, we'd want to read all of these to make sure

1396
00:56:52,400 --> 00:56:53,370
they're correct, but let's look at

1397
00:56:53,370 --> 00:56:54,640
least the known culprits.

1398
00:56:54,640 --> 00:56:57,370
3 bottles, 2 bottles, 2 bottles, 1

1399
00:56:57,370 --> 00:57:00,380
bottle, 1 bottle, 0 bottles.

1400
00:57:00,380 --> 00:57:03,080
We seem to have at least fixed it for now.

1401
00:57:03,080 --> 00:57:06,010
But the catch here is that what a god awful mess this is

1402
00:57:06,010 --> 00:57:07,470
just to solve a stupid

1403
00:57:07,470 --> 00:57:09,540
one-character grammatical detail.

1404
00:57:09,540 --> 00:57:11,150
So there's kind of a reason that I cut this corner

1405
00:57:11,150 --> 00:57:14,090
earlier, because it's just completely annoying to have to

1406
00:57:14,090 --> 00:57:15,020
write this much code.

1407
00:57:15,020 --> 00:57:17,530
But it turns out that there's slightly more elegant ways of

1408
00:57:17,530 --> 00:57:20,110
expressing the exact same thing.

1409
00:57:20,110 --> 00:57:22,040
And we can do this as follows.

1410
00:57:22,040 --> 00:57:23,890
>> Let me leave this on the screen for a moment and

1411
00:57:23,890 --> 00:57:27,320
introduce something known as a ternary operator.

1412
00:57:27,320 --> 00:57:29,600
This is kind of a one-liner that's just meant to make our

1413
00:57:29,600 --> 00:57:31,880
lives a little sexier, as promised.

1414
00:57:31,880 --> 00:57:33,130
And I'm going to do this as follows.

1415
00:57:33,130 --> 00:57:35,810
Give me a string called s1, and let me

1416
00:57:35,810 --> 00:57:37,200
assign it as follows.

1417
00:57:37,200 --> 00:57:39,900
(i == 1) ?

1418
00:57:39,900 --> 00:57:47,820
"bottle", otherwise "bottles". String s2 gets (i == 2) ?

1419
00:57:47,820 --> 00:57:52,670
"bottle", otherwise "bottles".

1420
00:57:52,670 --> 00:57:57,050
So what then is the difference here?

1421
00:57:57,050 --> 00:57:59,370
These two lines of code, I argue, can

1422
00:57:59,370 --> 00:58:02,200
replace this whole mess.

1423
00:58:02,200 --> 00:58:04,280
So I call it a mess, just because it kind of rubs me the

1424
00:58:04,280 --> 00:58:05,940
wrong way that it's so many lines of code.

1425
00:58:05,940 --> 00:58:06,650
Not wrong.

1426
00:58:06,650 --> 00:58:07,700
It's not bad design.

1427
00:58:07,700 --> 00:58:10,140
Like, this is perfectly correct and perfectly fine.

1428
00:58:10,140 --> 00:58:12,970
But coding gets tedious if you have to express yourself so

1429
00:58:12,970 --> 00:58:15,530
damn specifically again and again and again with a simple

1430
00:58:15,530 --> 00:58:16,620
scenario like this.

1431
00:58:16,620 --> 00:58:19,470
So C has some shortcuts, like this.

1432
00:58:19,470 --> 00:58:24,270
So this essentially is saying declare a string called s1 and

1433
00:58:24,270 --> 00:58:32,610
assign it either this value or this value if i is ==--

1434
00:58:32,610 --> 00:58:35,290
sorry, I should say this more clearly.

1435
00:58:35,290 --> 00:58:41,680
Declare a variable s1, assign it this value if this is true.

1436
00:58:41,680 --> 00:58:44,280
Otherwise, assign it this value.

1437
00:58:44,280 --> 00:58:47,220
So in other words, this is sort of a one-line way of

1438
00:58:47,220 --> 00:58:51,490
saying if else but doing an assignment along the way.

1439
00:58:51,490 --> 00:58:55,540
So if i is 1, then go ahead and call this "bottle". And

1440
00:58:55,540 --> 00:58:59,830
then this else, call it "bottles". Meanwhile, s2, the

1441
00:58:59,830 --> 00:59:04,060
second word that we need to define, if i equals 2, we'll

1442
00:59:04,060 --> 00:59:08,350
set s2 to "bottle". Otherwise, set it to "bottles". And what

1443
00:59:08,350 --> 00:59:11,460
this means now is I can go through this and delete all of

1444
00:59:11,460 --> 00:59:12,860
those lines of code.

1445
00:59:12,860 --> 00:59:17,220
And when I say, somewhat ridiculously, that this is now

1446
00:59:17,220 --> 00:59:20,060
sexier, it's sexier in the sort of stylistic sense.

1447
00:59:20,060 --> 00:59:22,660
The fact that functionally, this code is actually going to

1448
00:59:22,660 --> 00:59:24,610
do the exact same thing.

1449
00:59:24,610 --> 00:59:26,890
And even though it might look a little cryptic at first

1450
00:59:26,890 --> 00:59:29,250
glance, because we've not seen this construct before, I'd

1451
00:59:29,250 --> 00:59:31,850
argue that it's ultimately going to be so much more

1452
00:59:31,850 --> 00:59:34,820
readable and so much easier for we humans to sort of

1453
00:59:34,820 --> 00:59:36,830
understand, because now you can just read the

1454
00:59:36,830 --> 00:59:38,830
code all on one line.

1455
00:59:38,830 --> 00:59:41,550
It's still similar in spirit to an if, where this is the

1456
00:59:41,550 --> 00:59:44,920
condition and then this is what's inside the if and this

1457
00:59:44,920 --> 00:59:46,480
is what's inside the else.

1458
00:59:46,480 --> 00:59:49,450
But we can do this just much more elegantly.

1459
00:59:49,450 --> 00:59:52,650
>> And if I now go back to my terminal, having deleted all

1460
00:59:52,650 --> 00:59:55,530
of those lines and replaced them with just those two,

1461
00:59:55,530 --> 01:00:00,150
recompile, re-run bottles of beer with 99, notice that my

1462
01:00:00,150 --> 01:00:03,350
grammar is, in fact, still correct.

1463
01:00:03,350 --> 01:00:06,160
So again, something to start.

1464
01:00:06,160 --> 01:00:08,840
2 bottles of beer, 1 bottle of beer.

1465
01:00:08,840 --> 01:00:09,370
Looks right.

1466
01:00:09,370 --> 01:00:10,100
Yeah.

1467
01:00:10,100 --> 01:00:13,900
So there we have a much more succinct solution.

1468
01:00:13,900 --> 01:00:16,020
So this, too, as you get more comfortable with C, not

1469
01:00:16,020 --> 01:00:18,630
necessarily with the first pset or even second, but

1470
01:00:18,630 --> 01:00:21,170
realize that these constructs can allow us to do things ever

1471
01:00:21,170 --> 01:00:22,810
more elegantly.

1472
01:00:22,810 --> 01:00:25,200
Now let's do one other thing here.

1473
01:00:25,200 --> 01:00:31,460
Let me go ahead and open up return1.c.

1474
01:00:31,460 --> 01:00:34,340
Now let's start to solve another problem in a way that

1475
01:00:34,340 --> 01:00:37,140
allows us to write more sophisticated code.

1476
01:00:37,140 --> 01:00:39,960
>> So here's a simple little program whose purpose in life

1477
01:00:39,960 --> 01:00:41,870
is to increment values.

1478
01:00:41,870 --> 01:00:43,100
And actually, let's take a step back.

1479
01:00:43,100 --> 01:00:44,400
Let me do this manually.

1480
01:00:44,400 --> 01:00:52,200
Let me do include and int main(void).

1481
01:00:52,200 --> 01:00:53,450
And let me call this increment.c.

1482
01:00:57,150 --> 01:00:58,570
And what do I want to do?

1483
01:00:58,570 --> 01:01:02,240
I'm going to go ahead and say something like--

1484
01:01:02,240 --> 01:01:04,280
will we call the numbers the same--

1485
01:01:04,280 --> 01:01:04,980
int x.

1486
01:01:04,980 --> 01:01:13,670
So int x gets 2; printf x is %d, new line, x.

1487
01:01:13,670 --> 01:01:16,780
So I'm typing fast, but sort of familiar stuff now.

1488
01:01:16,780 --> 01:01:18,830
Then I'm going to do x++.

1489
01:01:18,830 --> 01:01:21,710
Then I'm going to print that same sentence again.

1490
01:01:21,710 --> 01:01:24,550
And then I'm going to return 0 just to quit the program.

1491
01:01:24,550 --> 01:01:25,960
All right, so this is a program that

1492
01:01:25,960 --> 01:01:26,960
increments a number.

1493
01:01:26,960 --> 01:01:29,830
It's first going to initialize something to 2, and then it's

1494
01:01:29,830 --> 01:01:33,470
going to increment it and print it again.

1495
01:01:33,470 --> 01:01:36,930
>> So let's run increment, incredibly simple program.

1496
01:01:36,930 --> 01:01:40,940
But suppose now that I want to cube the value, so do

1497
01:01:40,940 --> 01:01:42,490
something somewhat arbitrary.

1498
01:01:42,490 --> 01:01:45,140
And I actually want to do x gets the cube of it.

1499
01:01:45,140 --> 01:01:47,570
So I could use what's called the pow function, but I don't

1500
01:01:47,570 --> 01:01:48,650
really know where that is yet.

1501
01:01:48,650 --> 01:01:50,580
So I'm going to do this the old-fashioned way.

1502
01:01:50,580 --> 01:01:54,550
x times this equals x times x times x.

1503
01:01:54,550 --> 01:01:56,880
So I'm cubing the value, multiplying it by itself again

1504
01:01:56,880 --> 01:02:00,440
and again and again, so that we get the power

1505
01:02:00,440 --> 01:02:02,050
of 3 in this case.

1506
01:02:02,050 --> 01:02:06,000
So now the numbers I should print should be, as we'll see

1507
01:02:06,000 --> 01:02:08,450
here--make increment, so it's actually not really increment

1508
01:02:08,450 --> 01:02:10,260
anymore, but we'll leave the name alone--

1509
01:02:10,260 --> 01:02:11,590
2 and then 8.

1510
01:02:11,590 --> 01:02:17,670
Now, we have the beginnings of an opportunity for refinement

1511
01:02:17,670 --> 01:02:21,570
here, whereby this cubing thing of multiplying a number

1512
01:02:21,570 --> 01:02:24,680
by itself by itself by itself feels like this might just be

1513
01:02:24,680 --> 01:02:27,920
useful to have as a function, much like someone decided

1514
01:02:27,920 --> 01:02:30,430
years ago--you know, kind of useful if one of us sits down

1515
01:02:30,430 --> 01:02:33,120
and writes printf so that the rest of the world can use it,

1516
01:02:33,120 --> 01:02:36,160
why don't we sit down and write a function called cube

1517
01:02:36,160 --> 01:02:39,250
that does this cubing for us so we don't have to manually

1518
01:02:39,250 --> 01:02:41,500
implement the notion of cubing values here?

1519
01:02:41,500 --> 01:02:44,040
>> So a simple example, but let's go ahead and use this is as an

1520
01:02:44,040 --> 01:02:46,280
opportunity to write our own function.

1521
01:02:46,280 --> 01:02:49,110
So thus far, we've only used main, and we've used other

1522
01:02:49,110 --> 01:02:51,780
people's functions, but we haven't written our own.

1523
01:02:51,780 --> 01:02:52,650
So here we go.

1524
01:02:52,650 --> 01:02:56,330
I'm going to go ahead and write a function called cube.

1525
01:02:56,330 --> 01:02:58,490
And I'm going to have it take an input.

1526
01:02:58,490 --> 01:03:02,070
So its input is going to be an integer.

1527
01:03:02,070 --> 01:03:03,570
And what is it going to do?

1528
01:03:03,570 --> 01:03:08,500
It's going to declare int output = input times input

1529
01:03:08,500 --> 01:03:09,880
times input.

1530
01:03:09,880 --> 01:03:12,190
And then it's going to return that output.

1531
01:03:12,190 --> 01:03:14,500
And then I have to be specific now.

1532
01:03:14,500 --> 01:03:16,820
This function is going to return an int.

1533
01:03:16,820 --> 01:03:19,130
So here then is how you'd write your own functions.

1534
01:03:19,130 --> 01:03:20,850
You first decide what's the name of your

1535
01:03:20,850 --> 01:03:21,720
function going to be.

1536
01:03:21,720 --> 01:03:23,964
And generally, something explanatory is good, so I'll

1537
01:03:23,964 --> 01:03:25,060
call it cube.

1538
01:03:25,060 --> 01:03:27,180
Then you have to specify what it's going to return, what's

1539
01:03:27,180 --> 01:03:28,240
its output going to be.

1540
01:03:28,240 --> 01:03:29,595
And we don't have that many options yet.

1541
01:03:29,595 --> 01:03:32,260
Int, char, float, bool, string.

1542
01:03:32,260 --> 01:03:34,260
For now, I'm going to stick with an int, because I want it

1543
01:03:34,260 --> 01:03:35,880
to return an integer.

1544
01:03:35,880 --> 01:03:38,770
Then you have to specify what its inputs, if any, are.

1545
01:03:38,770 --> 01:03:41,570
And if cube takes an argument, takes something between

1546
01:03:41,570 --> 01:03:44,970
parentheses, you have to give that argument a name so that

1547
01:03:44,970 --> 01:03:47,860
you can call it something as you're implementing or writing

1548
01:03:47,860 --> 01:03:50,550
this function, and you have to give it a type, which in this

1549
01:03:50,550 --> 01:03:51,810
case is going to be int.

1550
01:03:51,810 --> 01:03:54,690
So in short, cube is a function that takes an integer

1551
01:03:54,690 --> 01:03:57,560
as input and returns an integer as output.

1552
01:03:57,560 --> 01:03:59,240
>> So what does it do with that input?

1553
01:03:59,240 --> 01:04:03,710
Well, in line 14, I declare a variable called output, and I

1554
01:04:03,710 --> 01:04:07,410
assign it the value, input times input times input.

1555
01:04:07,410 --> 01:04:11,490
And then I return output.

1556
01:04:11,490 --> 01:04:14,890
So how do I use this then?

1557
01:04:14,890 --> 01:04:19,210
What do I change these highlighted characters on line

1558
01:04:19,210 --> 01:04:21,006
7 to be, do you think?

1559
01:04:21,006 --> 01:04:21,800
AUDIENCE: [INAUDIBLE].

1560
01:04:21,800 --> 01:04:25,570
DAVID J. MALAN: Yeah, so cube of x.

1561
01:04:25,570 --> 01:04:28,290
So x is a variable, which means it holds some value.

1562
01:04:28,290 --> 01:04:30,190
Fortunately, it's of type integer.

1563
01:04:30,190 --> 01:04:34,280
And because x is an int, that means I can pass it into cube.

1564
01:04:34,280 --> 01:04:39,500
And even though I'm overriding the value of x with the value

1565
01:04:39,500 --> 01:04:42,780
of cube x, as has been the case thus far, any time you

1566
01:04:42,780 --> 01:04:46,150
have equal sign and a line of code, the stuff on the right

1567
01:04:46,150 --> 01:04:49,090
gets executed and then gets assigned to the

1568
01:04:49,090 --> 01:04:50,150
value on the left.

1569
01:04:50,150 --> 01:04:52,950
So the order of operations is as we would hope.

1570
01:04:52,950 --> 01:04:56,620
So does this work?

1571
01:04:56,620 --> 01:04:58,410
Well, let me go down here.

1572
01:04:58,410 --> 01:04:59,970
Let me open up my terminal window.

1573
01:04:59,970 --> 01:05:03,610
Let me do make increment, Enter.

1574
01:05:03,610 --> 01:05:07,140
"Implicit declaration of function 'cube' is invalid in

1575
01:05:07,140 --> 01:05:13,700
C99." As an aside, C99 refers to the language C as it was

1576
01:05:13,700 --> 01:05:16,790
defined in 1999, which was an update over the version from

1577
01:05:16,790 --> 01:05:19,360
1989, which is an update over the original.

1578
01:05:19,360 --> 01:05:21,740
So that's all that means.

1579
01:05:21,740 --> 01:05:24,250
>> So what does it mean that "implicit declaration of

1580
01:05:24,250 --> 01:05:26,790
function 'cube' is invalid?" It's right here.

1581
01:05:26,790 --> 01:05:28,430
It's right there in line 12.

1582
01:05:28,430 --> 01:05:30,460
AUDIENCE: [INAUDIBLE].

1583
01:05:30,460 --> 01:05:30,730
DAVID J. MALAN: What's that?

1584
01:05:30,730 --> 01:05:32,470
AUDIENCE: It's not before.

1585
01:05:32,470 --> 01:05:33,540
DAVID J. MALAN: It's not before.

1586
01:05:33,540 --> 01:05:34,740
So this is the thing.

1587
01:05:34,740 --> 01:05:38,190
C is kind of stupid, or C compilers are kind of stupid.

1588
01:05:38,190 --> 01:05:41,060
They really only do what you tell them to do.

1589
01:05:41,060 --> 01:05:44,770
And they, in particular, only read your code top to bottom,

1590
01:05:44,770 --> 01:05:45,620
left to right.

1591
01:05:45,620 --> 01:05:49,140
So if the compiler, Clang, is reading your code, line 1, it

1592
01:05:49,140 --> 01:05:50,120
figures out how to do this.

1593
01:05:50,120 --> 01:05:50,940
Oh, here comes main.

1594
01:05:50,940 --> 01:05:53,000
Okay, let me go ahead and declare a variable x.

1595
01:05:53,000 --> 01:05:54,160
Let me print something.

1596
01:05:54,160 --> 01:05:55,890
Line 7, what the heck is cube?

1597
01:05:55,890 --> 01:05:58,230
It's not declared in stdio.h.

1598
01:05:58,230 --> 01:06:00,950
It doesn't come with C. I have no idea what to do.

1599
01:06:00,950 --> 01:06:03,960
And so Clang just bails and quits with that error message.

1600
01:06:03,960 --> 01:06:05,850
So we can fix this in a couple of ways.

1601
01:06:05,850 --> 01:06:10,530
We can teach Clang what cube is by just moving where the

1602
01:06:10,530 --> 01:06:11,820
declaration is.

1603
01:06:11,820 --> 01:06:14,640
So I cut and pasted it atop main.

1604
01:06:14,640 --> 01:06:17,770
Now realize that just because main is no longer first, it's

1605
01:06:17,770 --> 01:06:19,150
still executed by default.

1606
01:06:19,150 --> 01:06:20,060
Main is main.

1607
01:06:20,060 --> 01:06:21,022
It's the default function name.

1608
01:06:21,022 --> 01:06:22,930
It doesn't matter where it is in a file.

1609
01:06:22,930 --> 01:06:26,910
But at least now Clang has seen cube before I use it.

1610
01:06:26,910 --> 01:06:28,500
So let's see if Clang is happier now.

1611
01:06:28,500 --> 01:06:31,410
Make increment, it did compile this time.

1612
01:06:31,410 --> 01:06:33,060
>> Let me run increment.

1613
01:06:33,060 --> 01:06:34,810
And indeed, it seems to be working.

1614
01:06:34,810 --> 01:06:36,810
Now, you can come up with scenarios eventually where

1615
01:06:36,810 --> 01:06:38,650
it's not feasible to put every function

1616
01:06:38,650 --> 01:06:39,740
above every other function.

1617
01:06:39,740 --> 01:06:42,140
You'll get stuck in this infinite loop in reality,

1618
01:06:42,140 --> 01:06:43,480
where this guy wants to be here but this

1619
01:06:43,480 --> 01:06:44,390
guy needs to be there.

1620
01:06:44,390 --> 01:06:45,830
So that doesn't always work.

1621
01:06:45,830 --> 01:06:49,020
So thankfully, C has a more elegant solution.

1622
01:06:49,020 --> 01:06:50,790
I'm going to put this back where it was, just because I

1623
01:06:50,790 --> 01:06:53,390
prefer, as a matter of principle, that main always be

1624
01:06:53,390 --> 01:06:55,550
at the top, because it's just nice to see what this program

1625
01:06:55,550 --> 01:06:56,920
does by default.

1626
01:06:56,920 --> 01:06:58,950
And what I'm going to do up here is declare what's called

1627
01:06:58,950 --> 01:07:00,250
a prototype.

1628
01:07:00,250 --> 01:07:05,730
I'm going to re-declare my cube function by literally

1629
01:07:05,730 --> 01:07:07,180
copying and pasting.

1630
01:07:07,180 --> 01:07:08,290
Actually, that's not literally.

1631
01:07:08,290 --> 01:07:13,060
So literally copying and pasting line 15

1632
01:07:13,060 --> 01:07:15,160
up above line 6.

1633
01:07:15,160 --> 01:07:17,010
It doesn't matter what line this ends up on.

1634
01:07:17,010 --> 01:07:18,380
It happens to be on line 4.

1635
01:07:18,380 --> 01:07:19,950
But it does have to be before main.

1636
01:07:19,950 --> 01:07:21,150
But notice the difference.

1637
01:07:21,150 --> 01:07:24,100
Line 4 ends with a semicolon, which means hey,

1638
01:07:24,100 --> 01:07:27,510
Clang, take my word for it that there exists a function

1639
01:07:27,510 --> 01:07:31,350
called cube that takes an int and returns an int.

1640
01:07:31,350 --> 01:07:33,020
But I'm not gonna tell you what it is yet.

1641
01:07:33,020 --> 01:07:35,180
Just know that I promise to tell you eventually.

1642
01:07:35,180 --> 01:07:38,490
And indeed, now it's okay that this is down below.

1643
01:07:38,490 --> 01:07:41,275
>> So this is generally better, because then at the top of

1644
01:07:41,275 --> 01:07:44,240
your file, you can just rattle off, rapid-fire, one line

1645
01:07:44,240 --> 01:07:46,470
each, what the names of your functions are, what their

1646
01:07:46,470 --> 01:07:49,120
inputs are, what their outputs are.

1647
01:07:49,120 --> 01:07:52,210
And to be more clear, input generally means argument or

1648
01:07:52,210 --> 01:07:54,110
parameter, synonymous.

1649
01:07:54,110 --> 01:07:56,890
Output means return value, what does it

1650
01:07:56,890 --> 01:07:58,700
hand back to me.

1651
01:07:58,700 --> 01:08:03,420
So in this case here, cube has been declared at the top, but

1652
01:08:03,420 --> 01:08:06,940
defined, otherwise known as implemented, at the bottom.

1653
01:08:06,940 --> 01:08:09,620
So now let's go back here and re-run this.

1654
01:08:09,620 --> 01:08:13,430
So now let me go ahead and re-run make, re-run increment.

1655
01:08:13,430 --> 01:08:16,500
And it now seems to be working just fine.

1656
01:08:16,500 --> 01:08:19,450
So now we can go ahead and factor out something like the

1657
01:08:19,450 --> 01:08:23,720
beer example into this fourth version.

1658
01:08:23,720 --> 01:08:25,590
So let me scroll down here.

1659
01:08:25,590 --> 01:08:28,149
And notice that I kind of took this lesson to heart just now.

1660
01:08:28,149 --> 01:08:31,140
The fact that I was singing the same stanza again and

1661
01:08:31,140 --> 01:08:34,130
again and again, the same chorus line in the song, felt

1662
01:08:34,130 --> 01:08:36,439
like why don't I factor that out into a function?

1663
01:08:36,439 --> 01:08:38,470
And indeed, this should be one of the motivations.

1664
01:08:38,470 --> 01:08:40,960
Besides the fact that someone else in the world might want

1665
01:08:40,960 --> 01:08:42,390
to use a cube function--

1666
01:08:42,390 --> 01:08:44,560
that's a good reason to factor something out and write your

1667
01:08:44,560 --> 01:08:45,720
own custom function--

1668
01:08:45,720 --> 01:08:48,720
if there's a chunk of code in your program that just makes

1669
01:08:48,720 --> 01:08:51,370
conceptual sense, that you kind of want to give it a

1670
01:08:51,370 --> 01:08:53,740
name-- like in this case, chorus--

1671
01:08:53,740 --> 01:08:57,380
then you can similarly write that as a separate function.

1672
01:08:57,380 --> 01:08:59,560
You don't have to write everything in main if it just

1673
01:08:59,560 --> 01:09:02,609
feels cleaner to separate it out and give it a name.

1674
01:09:02,609 --> 01:09:05,529
>> So in this case here, notice that I have a comment atop

1675
01:09:05,529 --> 01:09:06,859
this function that just sings about the

1676
01:09:06,859 --> 01:09:08,630
specified numbers of bottles.

1677
01:09:08,630 --> 01:09:10,609
Notice here that I don't need to call these

1678
01:09:10,609 --> 01:09:12,520
things input and output.

1679
01:09:12,520 --> 01:09:16,090
In fact, this time I just called my input b for bottle.

1680
01:09:16,090 --> 01:09:19,960
And notice here, void suggests what?

1681
01:09:19,960 --> 01:09:21,309
That chorus--

1682
01:09:21,309 --> 01:09:22,660
AUDIENCE: Doesn't return it.

1683
01:09:22,660 --> 01:09:23,870
DAVID J. MALAN: Doesn't return a value.

1684
01:09:23,870 --> 01:09:26,800
And indeed, functions don't have to return values.

1685
01:09:26,800 --> 01:09:28,060
They can just do something.

1686
01:09:28,060 --> 01:09:30,270
They can have what are called side effects, which in this

1687
01:09:30,270 --> 01:09:33,109
case is just a whole bunch of printing on the screen.

1688
01:09:33,109 --> 01:09:36,580
So notice that this code here, I literally just stole from

1689
01:09:36,580 --> 01:09:37,680
the previous example.

1690
01:09:37,680 --> 01:09:39,930
The only difference is instead of using i as my

1691
01:09:39,930 --> 01:09:42,890
variable, I'm now using b as my variable.

1692
01:09:42,890 --> 01:09:45,880
So I have b down here, I have b down here, I have b

1693
01:09:45,880 --> 01:09:47,109
minus 1 down here.

1694
01:09:47,109 --> 01:09:49,279
But the code is exactly the same.

1695
01:09:49,279 --> 01:09:52,529
But just to show you now how we can use this, let me go

1696
01:09:52,529 --> 01:09:56,780
ahead and actually change this to be a for loop.

1697
01:09:56,780 --> 01:10:03,850
for (int i = n; i > n; i--).

1698
01:10:03,850 --> 01:10:06,230
>> So I've stolen that from our previous example.

1699
01:10:06,230 --> 01:10:08,970
Previously, it's in line 37 that I would have started

1700
01:10:08,970 --> 01:10:10,640
singing this annoying song.

1701
01:10:10,640 --> 01:10:15,810
But instead, I'm just going to now call chorus of i.

1702
01:10:15,810 --> 01:10:16,870
Done.

1703
01:10:16,870 --> 01:10:20,260
So now in every iteration of this loop, I call this other

1704
01:10:20,260 --> 01:10:22,220
function, chorus, that I happened to write.

1705
01:10:22,220 --> 01:10:24,110
It wasn't written by someone else years ago.

1706
01:10:24,110 --> 01:10:27,930
But chorus, meanwhile, uses printf to print

1707
01:10:27,930 --> 01:10:29,840
out these four lines.

1708
01:10:29,840 --> 01:10:32,720
But the fact that I'm calling chorus again and again in a

1709
01:10:32,720 --> 01:10:35,900
loop means that I'm going to get, at the very end, the

1710
01:10:35,900 --> 01:10:39,310
exact same song as I have thus far.

1711
01:10:39,310 --> 01:10:42,130
So in short, now if I look back at my code, even though

1712
01:10:42,130 --> 01:10:44,240
functionally this is equivalent, notice that it's

1713
01:10:44,240 --> 01:10:46,020
starting to get even more readable.

1714
01:10:46,020 --> 01:10:48,410
I don't exactly know how GetInt

1715
01:10:48,410 --> 01:10:49,250
is implemented.

1716
01:10:49,250 --> 01:10:52,050
Frankly, I don't know how chorus is implemented.

1717
01:10:52,050 --> 01:10:52,970
But it doesn't matter to me.

1718
01:10:52,970 --> 01:10:55,620
I don't care, because now I can sort of, as a human, read

1719
01:10:55,620 --> 01:10:57,050
this from top to bottom.

1720
01:10:57,050 --> 01:10:59,950
And because the functions are named according to what they

1721
01:10:59,950 --> 01:11:02,910
do, my code is increasingly readable.

1722
01:11:02,910 --> 01:11:05,190
And as our programs get much more complex--

1723
01:11:05,190 --> 01:11:07,220
by the semester's end, you'll be writing hundreds of lines

1724
01:11:07,220 --> 01:11:10,970
of code in languages like PHP and JavaScript and the like--

1725
01:11:10,970 --> 01:11:13,550
you'll find that it's so much easier than to keep track of

1726
01:11:13,550 --> 01:11:14,080
what you've done.

1727
01:11:14,080 --> 01:11:15,810
And when you start collaborating with friends or

1728
01:11:15,810 --> 01:11:19,010
partners or colleagues, you'll be able to write much more

1729
01:11:19,010 --> 01:11:22,910
massive programs by starting to exercise these basic

1730
01:11:22,910 --> 01:11:23,990
building blocks.

1731
01:11:23,990 --> 01:11:25,550
>> So with that said, why don't we call it a day?

1732
01:11:25,550 --> 01:11:27,190
And we will see you on Wednesday.

1733
01:11:27,190 --> 01:11:33,611
[APPLAUSE]