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[♪Buzzing♪]

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So to introduce you to what I've got up on the screen;

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it's one of the tools we developed over the summer;

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it's called CS50 Spaces,

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and it's our collaborative coding application

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that we're going to use in sections.

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It's pretty cool.

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Have any of you seen CS50 run yet?

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Let me pull that up in another tab.

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Run.cs50.net

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So

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if you go to run.cs50.net,

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you can see this is an in your browser way to

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actually code--write C code,

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and then run it,

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so it both compiles it, runs it, then returns the output to you,

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so this is kind of a cool way to experiment

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with some of the code that David goes over in lectures

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and we give to on you on problem sets

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without actually having to fire up the appliance.

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>> So what Spaces does that's different from that

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if I switch back to this tab,

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Spaces is CS50 run, like you see right here,

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but if we scroll over and look at the other side of the window,

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you see the right I'm the only person in this space,

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but what I'm going to do is ask you guys to join this Space with me

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and we'll all get to code in the same little window,

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share code with each other,

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and talk about the differences between our code.

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So it's kind of a cool thing--

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so to join, if you would visit this link right here at the top

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that I've highlighted,

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you can just type in apps.cs50.net/spaces/vqcspSnO.

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>> All right, so to start off,

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I'm going to give you a little bit of a tour of this.

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We're gonna do some silly little coding with it at first,

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just so that everybody kinda gets a feel for what it's like,

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but what I've done--

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or what I'm about to do right here is--

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I'm going to take the code that I've written in here,

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which as you see right now is empty;

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it doesn't do anything.

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It's just boiler plate code, we'll call it.

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With a little to-do comment.

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I'm going to save it by hitting this Save button.

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And what that does is it

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highlighted it up here briefly--that revision save green box,

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and now, if you search for me in the Space,

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you click my name,

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you can see that I've got this revision right here,

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and if you click on that revision,

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it'll load that code into your Space, too.

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>> So has everybody loaded that code into their Space?

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All right, if you haven't yet, go ahead and do that,

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and then what I want you guys to do in this little to-do section

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is write a little bit using printf about yourself,

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so for example, I would write

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printf("My name is Nate");

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and then I'd write

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"I graduated in 2009

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from Stanford"

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and I would love to hear about

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what year you are in school,

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whether you're at the college or the business school

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or another Harvard school,

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what you're studying if it's known yet;

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if it's not known, that's totally cool.

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I didn't decide until I was late in my sophomore year.

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and then I would also love to know

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anything crazy you want to tell me or

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something about the class or just anything else.

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When you're done writing that with your little printf,

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if you'll save the revision or rather

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first make sure that you can run it

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and that it prints out exactly what you want it to print out,

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and then if you go ahead and save that revision

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using the Revision Save button, this first little guy over here,

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and we'll kinda go through and do a little introduction that way.

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So you should just have to hit the Play button,

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which is up here on the top left,

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so this Play button on the top left will

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send your code off to our server where it gets compiled and then run.

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So what's kinda crazy about this is

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this code actually isn't running on your computer;

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it's actually being sent across the Internet.

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>> How's it going, Charlotte?

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>>I--okay. I'm--I think I'm

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having difficulties saving it

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on the server such that it can--I can run the program.

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And then when I try to run it, it says that there's an error.

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Declared as a virus, which is not in there anymore.

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Oh, let's see; can you hit Save again?

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I see it on the screen.

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>>Right, okay, so that's what I have.

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And I don't know why it doesn't let me run it.

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Oh, okay, sorry.

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Can you explain why--what was wrong--what wasn't working with it?

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>>Yeah, because I didn't see that there was a last line where there's still a variable

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that I had not declared, which I did not intend for it being there.

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Yeah, perfect!

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>>Now if I save it, I think it should work.

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Now that you've saved it, it works; awesome.

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>> So yeah, as we go through--

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so what was going on,

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I'm just going to repeat this for the camera.

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What was happening was that this variable s

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was left in after--I assume you deleted some other stuff

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where you declared as,

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and so this was throwing things off and wasn't letting you run it.

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So this is exactly what we want to do in section

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is have people try writing different pieces of code,

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compiling it, see what works, see what doesn't,

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and then kinda talk about these things, because these are tripping points that

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I still encounter,

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I still--I do lots of silly stuff all the time,

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and I don't know why it doesn't work,

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so let's pull up Charlotte's revision 3,

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and we'll run it.

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>> So when I run, now that I've got her code in here,

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I can zoom in, hit the Run button,

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uploads it, builds it, runs it,

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says it ran successfully, and if I scroll down,

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see Charlotte is a junior at Adams House,

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psychology, secondary in economics; awesome.

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Great to meet you!

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Awesome.

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Okay, moving on, let's go to Daniel,

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go to revision 3,

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compile, run;

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why don't you tell us a little about yourself, Daniel?

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>>Call me Dan.

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I'm going to business school across from you guys,

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and I was in the Marines before here,

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and this is all new to me, so yeah.

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Don't feel bad; I'm 30 and I don't know any of this stuff.

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Awesome; thanks, Dan!

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All right, we got another Daniel.

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You go by Daniel--will you introduce yourself, Daniel?

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>> >>Sure, I'm Daniel.

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I'm a sophomore at Mather right now.

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I'm concentrating on government,

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but I wanted to take CS50 because I've heard great reviews about the class

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and I've never tried CS before.

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Scroll down here and show this--cool.

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Moving on, Jared.

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How's it going?

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>>Good, how are you?

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As you can see, I'm a junior philosophy concentrator in Eliot,

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originally from southern California,

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and I'm interested in taking CS50 because I like the logic of it,

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and it seems really cool to do things on the computer, so.

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Awesome; where in southern California are you from?

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>>About an hour from [inaudible], a small city [inaudible].

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Oh, yeah, my dad's from Redlands.

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Cool stuff.

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>> All right, Jimmy; how's it going, Jimmy?

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>>Good, how are you?

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Doing pretty well!

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Tell us about yourself.

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>>I'm a freshman, and I'm not entirely sure what I'm majoring in

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but I'm kinda leaning towards economics.

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Like Daniel before me, I've never done any CS, but it sounds kinda cool.

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I'm originally from Chicago.

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Awesome. So check out his printout here.

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I wanted to ask, did this print out as you expected?

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>>No, I forgot to print a new line.

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I actually forgot that when I was writing my thing

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for the first time, too.

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It's kind of a pain to remember to put those in manually.

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It'd be nice if it just know that's what you wanted.

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>> Okay, Kelly.

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Wow, we have 2 Kellys; 2 Daniels, 2 Kellys.

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So Kelly--do you go by Kelly Ren?

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Okay, awesome.

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Or Kelly, I mean.

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>>So yeah, so I'm Kelly.

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I'm a sophomore in Pfoho.

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I got an e-mail today that I still have 7 weeks to declare my concentration,

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so I'm trying not to think about it,

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and I like baking, running, and Craisins.

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I don't have experience with most of CS;

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I taught myself to build Web sites when I was really young,

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but I think I probably taught myself wrong a lot of times.

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Hopefully I'll learn a little bit more

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and--yeah.

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>> Awesome; well, thank you.

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Let's go to the other Kelly;

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I don't have anything--did you say something in here?

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>> [inaudible]

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There it is!

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Sweet!

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>>All right, so I'm Kelly.

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I'm a first-year graduate student.

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I'm majoring in Mind, Brain, and Education.

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I graduated in Franklin & Marshall College in 2008

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with majors in psychology and sociology

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and a minor in French.

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I'm taking CS50

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because I'm interested in getting a PhD in Neuroscience

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and I feel like this will help me understand the technologies being used.

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Awesome.

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Moving on, Mikaela.

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Is that correct pronunciation?

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Okay, awesome.

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>> >>I'm Mikhaila.

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I'm a sophomore in Kirkland House,

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and I'm concentrating in Mechanical Engineering.

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I really like cycling, built a bike this summer.

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I'm taking CS, actually,

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as a concentration requirement,

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but you know, I think it's still cool sort of engineering class.

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Awesome--you built a bike this summer?

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>>Yeah.

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Wow, like the frame and all that?

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>> Uh-hunh (affirmative).

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Wow, that's really cool.

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>>[inaudible off-camera chat]

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[laughter]

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Yeah, it's been so long.

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You never know, right?

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Well, cool; well thanks, Mikhaila.

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>> All right, Missy.

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>>My name's Missy; I'm a freshman,

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so that means I don't know anything!

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I want to concentrate in astrophysics.

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In CS50,

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I feel like will definitely help with that.

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An odd fact about me is I'm a nationally certified firefighter.

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Wow!

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How did that happen?

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>>I attended an academy

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and went through hell for a lot of weeks

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and got my certification.

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Wow, congrats!

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That is no small feat, for sure.

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Okay, so now we'll do me.

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I'm the next one in the row.

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I'm--run it.

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>> So my name is Nate.

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I was class of 2009 at Stanford.

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I was a Symbolic Systems major,

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which is similar to the Mind, Brain, Behavior concentration here,

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but it's kind of a mix of

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computer science, linguistics, psychology, and philosophy.

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Kind of--yeah, a lot of weird stuff, but it was great,

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and then I did my master's in Computer Science there, as well.

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So I'm new to Harvard this year.

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I really wanted to get into teaching.

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I spent the last 4 years at Stanford TA-ing Computer Science classes

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and so David let me come on,

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and I'm gonna be helping out with CS50,

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so I do this full time; I'm not in grad school.

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It's been fun.

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Hopefully it'll continue to be fun; I'm sure it will.

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>> All right, Sam.

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>>My name is Sam; I'm a sophomore at the college, in law specifically.

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My major will be history of science, probably, and VES secondary,

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and my favorite color is yellow.

259
00:13:18,790 --> 00:13:21,100
No need to pass judgment or anything, but it's a good color!

260
00:13:21,100 --> 00:13:24,780
And Stella.

261
00:13:24,780 --> 00:13:26,980
>>Hi, I'm Stella; I'm a freshman in Grays.

262
00:13:26,980 --> 00:13:30,070
I have no clue what I've concentrated in yet,

263
00:13:30,070 --> 00:13:32,500
so CS50 was just kind of random,

264
00:13:32,500 --> 00:13:34,220
which is interesting.

265
00:13:34,220 --> 00:13:36,080
Fun fact, Jimmy was the first Harvard friend of mine,

266
00:13:36,080 --> 00:13:38,360
because we're both from Chicago.

267
00:13:38,360 --> 00:13:42,450
Cool, and Grace--that's supposed to be the really nice one, right?

268
00:13:42,450 --> 00:13:45,220
>>I thought [inaudible].

269
00:13:45,220 --> 00:13:46,690
Oh, yeah.

270
00:13:46,690 --> 00:13:51,210
[inaudible/off-camera chat]

271
00:13:51,210 --> 00:13:54,420
Okay; I'm still learning, still learning.

272
00:13:54,420 --> 00:13:57,800
>> So we've got something else that's kind of interesting here.

273
00:13:59,600 --> 00:14:02,870
Yeah, I'm just wondering, do you have an idea that's going on here?

274
00:14:02,870 --> 00:14:05,890
You know how to interpret this?

275
00:14:05,890 --> 00:14:13,340
>>I was supposed to be put the semicolon after every single line.

276
00:14:13,340 --> 00:14:16,370
That's kind of a pain.

277
00:14:16,370 --> 00:14:19,100
Sometimes those semicolons can be a real pain to track down.

278
00:14:19,100 --> 00:14:21,420
So it turns out that

279
00:14:21,420 --> 00:14:22,820
the special program we're using this year to

280
00:14:22,820 --> 00:14:26,670
compile our C code is called Clang,

281
00:14:26,670 --> 00:14:29,460
and the reason we have started to use Clang this year--

282
00:14:29,460 --> 00:14:32,030
because we haven't used that program in the past;

283
00:14:32,030 --> 00:14:35,710
we've used another one called GCC that's really popular--

284
00:14:35,710 --> 00:14:38,420
Clang actually gives you error messages that make sense.

285
00:14:38,420 --> 00:14:41,420
This one says "expected ';' after expression",

286
00:14:41,420 --> 00:14:45,260
and it even tells you the expression after which you needed a semicolon.

287
00:14:45,260 --> 00:14:47,830
So are you fixing it?

288
00:14:47,830 --> 00:14:48,930
>>Yeah, right now.

289
00:14:48,930 --> 00:14:52,040
Awesome; let me know and we'll pull it up, and we'll run it.

290
00:14:52,040 --> 00:14:55,830
But yeah, putting these semicolons in is kind of a pain.

291
00:14:55,830 --> 00:14:59,580
>>I fixed it, so I think it should work.

292
00:14:59,580 --> 00:15:02,760
Awesome--revision 3.

293
00:15:02,760 --> 00:15:05,250
That looks like it.

294
00:15:05,250 --> 00:15:08,580
All right, so now if we run it,

295
00:15:08,580 --> 00:15:11,020
there it goes.

296
00:15:11,020 --> 00:15:13,830
Cool stuff.

297
00:15:13,830 --> 00:15:15,340
>> All right.

298
00:15:15,340 --> 00:15:18,230
So thanks for doing that.

299
00:15:18,230 --> 00:15:20,210
It's great to meet you guys.

300
00:15:20,210 --> 00:15:22,880
What we're gonna do now is spend the next little bit

301
00:15:22,880 --> 00:15:24,740
kinda diving into the section problems

302
00:15:24,740 --> 00:15:27,550
that come from the problem set from this week

303
00:15:27,550 --> 00:15:29,920
and we'll do that for most of the sections

304
00:15:29,920 --> 00:15:32,110
for the rest of the term,

305
00:15:32,110 --> 00:15:35,580
and then we'll save the last 15, 20 minutes or so

306
00:15:35,580 --> 00:15:38,490
to talk about the problem set itself

307
00:15:38,490 --> 00:15:41,420
and then just any other questions you have in general about the class.

308
00:15:43,380 --> 00:15:45,670
That said, what we really wanna do

309
00:15:45,670 --> 00:15:49,390
is have everybody participating

310
00:15:49,390 --> 00:15:51,500
using the Spaces stuff,

311
00:15:51,500 --> 00:15:53,500
throwing in code, throwing in suggestions,

312
00:15:53,500 --> 00:15:56,500
and talking about when things like this break

313
00:15:56,500 --> 00:15:58,360
or don't work, just because these are things that

314
00:15:58,360 --> 00:16:00,870
I know I came across

315
00:16:00,870 --> 00:16:03,180
when I started coding,

316
00:16:03,180 --> 00:16:05,490
and were really kinda frustrating at times

317
00:16:05,490 --> 00:16:08,690
because if it's like 2 am before your problem set is due,

318
00:16:08,690 --> 00:16:11,330
so--just for disclosure,

319
00:16:11,330 --> 00:16:14,590
I didn't start programming until I was a sophomore in college.

320
00:16:14,590 --> 00:16:17,360
It was kind of this weird, unknown thing

321
00:16:17,360 --> 00:16:20,360
that I heard was kinda cool

322
00:16:20,360 --> 00:16:23,590
and I also heard it could be a real pain in the butt

323
00:16:23,590 --> 00:16:27,610
if you miss a semicolon and that's the only error keeping your program from working

324
00:16:27,610 --> 00:16:30,890
and keeping you from going to sleep,

325
00:16:30,890 --> 00:16:32,650
so definitely,

326
00:16:32,650 --> 00:16:34,020
if you have questions like that,

327
00:16:34,020 --> 00:16:37,060
if you have concerns like that, let me know,

328
00:16:37,060 --> 00:16:40,050
because I want to make this a good experience for you guys.

329
00:16:40,050 --> 00:16:44,620
>> So have you guys all downloaded the problem set yet?

330
00:16:44,620 --> 00:16:47,160
Kinda know where it is? Okay.

331
00:16:47,160 --> 00:16:51,490
So we'll go to cs50.net, because I don't have it on this account,

332
00:16:51,490 --> 00:16:54,110
pull up the problem sets,

333
00:16:54,110 --> 00:16:57,940
pull up problem set 2, and we'll look at the standard edition today.

334
00:16:57,940 --> 00:17:01,190
We'll pull up the specification here.

335
00:17:02,640 --> 00:17:04,420
Scroll down.

336
00:17:06,790 --> 00:17:11,109
So we're gonna start looking at this section of questions.

337
00:17:14,069 --> 00:17:16,710
Have you guys watched these shorts yet?

338
00:17:16,710 --> 00:17:18,329
Okay.

339
00:17:18,329 --> 00:17:21,329
So there are shorts on loops,

340
00:17:21,329 --> 00:17:24,700
which is done by Tommy, one of the head teaching fellows.

341
00:17:24,700 --> 00:17:27,230
There's one on scope,

342
00:17:27,230 --> 00:17:30,530
I believe done by another teaching fellow named David

343
00:17:30,530 --> 00:17:33,400
and then one on Caesar Cipher also done by David.

344
00:17:33,400 --> 00:17:35,550
They're pretty cool.

345
00:17:35,550 --> 00:17:37,770
Have you guys been watching their shorts so far?

346
00:17:37,770 --> 00:17:39,270
How've they been?

347
00:17:39,270 --> 00:17:41,490
Helpful? Okay, good to know.

348
00:17:41,490 --> 00:17:44,210
If they're not, you can say so.

349
00:17:46,290 --> 00:17:49,870
But before you do,

350
00:17:49,870 --> 00:17:52,600
have you guys looked at these questions

351
00:17:52,600 --> 00:17:55,430
at all?

352
00:17:55,430 --> 00:17:57,480
>> Let's talk about this a little bit.

353
00:17:57,480 --> 00:18:00,600
So we've introduced over the last couple of weeks

354
00:18:00,600 --> 00:18:02,480
in lecture

355
00:18:02,480 --> 00:18:04,910
a number of different loops.

356
00:18:04,910 --> 00:18:06,140
There's the for loop,

357
00:18:06,140 --> 00:18:09,570
the while loop, and then the do-while loop,

358
00:18:09,570 --> 00:18:13,100
and in lecture, we've been talking a lot about

359
00:18:13,100 --> 00:18:16,010
how those loops are all

360
00:18:16,010 --> 00:18:18,320
fairly similar in a lot of ways,

361
00:18:18,320 --> 00:18:21,210
and you can do much of the same things with a while loop

362
00:18:21,210 --> 00:18:23,260
that you can with a for loop.

363
00:18:23,260 --> 00:18:26,300
You just have to kind of use the right combinations of variables and such.

364
00:18:26,300 --> 00:18:28,680
and so really

365
00:18:28,680 --> 00:18:31,200
the real distinction between all of the loops is that

366
00:18:31,200 --> 00:18:33,620
sometimes they're more useful

367
00:18:33,620 --> 00:18:36,930
to use them at one time than at another time.

368
00:18:36,930 --> 00:18:39,340
>> So for example,

369
00:18:39,340 --> 00:18:42,260
the while loop differing from a do-while loop.

370
00:18:42,260 --> 00:18:45,680
Does everybody feel like they have a good handle on when

371
00:18:45,680 --> 00:18:48,650
a do-while loop is more

372
00:18:48,650 --> 00:18:50,990
useful or what the difference is?

373
00:18:50,990 --> 00:18:53,410
>>I think the difference is a do-while loop

374
00:18:53,410 --> 00:18:56,960
runs--I don't know quite what the terminology is,

375
00:18:56,960 --> 00:18:59,750
but the series of functions within the loop.

376
00:18:59,750 --> 00:19:02,020
They automatically--it does its runs at least

377
00:19:02,020 --> 00:19:05,390
before it checks the condition, or the while loop will only run it when

378
00:19:05,390 --> 00:19:08,290
it continuously runs--whether or not the condition is being satisfied.

379
00:19:08,290 --> 00:19:11,010
Yeah, does that make sense?

380
00:19:11,010 --> 00:19:14,200
>> Let's take a look at a little bit of code.

381
00:19:14,200 --> 00:19:16,930
So just to repeat the question,

382
00:19:16,930 --> 00:19:19,230
or the answer, rather;

383
00:19:19,230 --> 00:19:22,050
the answer was that

384
00:19:22,050 --> 00:19:25,850
a while loop will check to see if the condition

385
00:19:25,850 --> 00:19:27,440
that you specify is met

386
00:19:27,440 --> 00:19:31,320
before it runs the body of the loop for the first time,

387
00:19:31,320 --> 00:19:35,000
whereas the do-while loop will automatically run the body of the loop,

388
00:19:35,000 --> 00:19:38,450
is what we call it--it's weird terminology,

389
00:19:38,450 --> 00:19:41,460
but if I ever say anything that doesn't make sense, let me know--

390
00:19:41,460 --> 00:19:45,360
the loop body is that stuff that you put in between the curly braces.

391
00:19:45,360 --> 00:19:47,840
And in a do-while loop,

392
00:19:47,840 --> 00:19:50,220
that body get run at least once,

393
00:19:50,220 --> 00:19:54,460
and then the condition gets checked before it runs each successive time.

394
00:19:54,460 --> 00:19:56,580
>> So actually, Sam,

395
00:19:56,580 --> 00:20:00,820
do you mind writing a simple while loop

396
00:20:00,820 --> 00:20:02,840
in Spaces?

397
00:20:02,840 --> 00:20:05,600
>>How do you sort of like

398
00:20:05,600 --> 00:20:07,060
make a new...?

399
00:20:07,060 --> 00:20:09,220
I just delete whatever you have in there.

400
00:20:09,220 --> 00:20:11,300
Yeah, just clear it out.

401
00:20:11,300 --> 00:20:14,510
>>Well, I believe I'm having this--

402
00:20:14,510 --> 00:20:16,640
which one do you want, a while, or do you want...?

403
00:20:16,640 --> 00:20:19,350
Yeah, just do a while true, print something out.

404
00:20:21,050 --> 00:20:23,140
And let's see.

405
00:20:23,140 --> 00:20:25,850
Alexander, I realized we never introduced you.

406
00:20:25,850 --> 00:20:27,510
>>Oh, that's fine.

407
00:20:27,510 --> 00:20:30,760
Do you--can you code up a do-while loop for us?

408
00:20:30,760 --> 00:20:32,520
>>Yeah, I'll try.

409
00:20:32,520 --> 00:20:34,370
All right, we'll just pull these up on the screen,

410
00:20:34,370 --> 00:20:38,240
so that you kind of have a handle on what the differences are.

411
00:20:38,240 --> 00:20:41,280
>> So here's what Sam's done.

412
00:20:41,280 --> 00:20:44,460
He's got an integer called i

413
00:20:44,460 --> 00:20:47,480
that he's initialized to 17.

414
00:20:47,480 --> 00:20:51,570
Then he says while (int i = 17)

415
00:20:51,570 --> 00:20:54,230
printf("My name is Sam.");

416
00:20:54,230 --> 00:20:57,300
So what happens when we run this code?

417
00:20:57,300 --> 00:21:00,340
There's an error; what does the error say?

418
00:21:00,340 --> 00:21:02,890
Let's pull it up and talk about it.

419
00:21:02,890 --> 00:21:05,040
An error occurred, it says.

420
00:21:05,040 --> 00:21:07,400
Let's scroll down.

421
00:21:07,400 --> 00:21:09,460
So it gave us this error, and it said,

422
00:21:09,460 --> 00:21:12,690
"Hey, I expected an expression."

423
00:21:12,690 --> 00:21:16,180
That's kinda cryptic.

424
00:21:19,160 --> 00:21:23,140
Yeah, so exactly--and again,

425
00:21:23,140 --> 00:21:27,140
there will be another little bug that we will catch in just a second.

426
00:21:27,140 --> 00:21:31,800
But this is another one of those kind of weird terminology things,

427
00:21:31,800 --> 00:21:36,150
so this int i = 17

428
00:21:36,150 --> 00:21:39,290
isn't an expression.

429
00:21:39,290 --> 00:21:41,220
What it looks like it's doing is it's

430
00:21:41,220 --> 00:21:43,410
declaring an integer called i

431
00:21:43,410 --> 00:21:45,540
and setting it to equal to 17

432
00:21:45,540 --> 00:21:48,780
or, rather, putting 17 into that int called i.

433
00:21:50,560 --> 00:21:53,350
And that doesn't make sense

434
00:21:53,350 --> 00:21:55,790
in the context of this while loop.

435
00:21:55,790 --> 00:21:58,190
It does make sense, as we'll see,

436
00:21:58,190 --> 00:22:01,250
in the context of a for loop, because for loop declarations do start off

437
00:22:01,250 --> 00:22:05,800
with that for int i equals something

438
00:22:05,800 --> 00:22:07,830
and goes on.

439
00:22:07,830 --> 00:22:09,500
>> So did you--

440
00:22:09,500 --> 00:22:12,630
fix something, all right.

441
00:22:12,630 --> 00:22:15,270
Here it goes; awesome.

442
00:22:15,270 --> 00:22:17,920
So now things look a little different.

443
00:22:17,920 --> 00:22:19,030
A couple of things have changed.

444
00:22:19,030 --> 00:22:21,300
Can you tell us what things have changed?

445
00:22:21,300 --> 00:22:24,950
>>I took out the int, so there's no declaration [inaudible] instead of an assignment.

446
00:22:24,950 --> 00:22:28,770
The one equal sign, I put 2 equal signs, which means an equals.

447
00:22:28,770 --> 00:22:30,560
Awesome.

448
00:22:30,560 --> 00:22:33,320
Yeah, so what Sam did was he took out the int

449
00:22:33,320 --> 00:22:35,770
that was right here earlier,

450
00:22:35,770 --> 00:22:39,340
so if I just look at this--this is what it was earlier, it said int,

451
00:22:39,340 --> 00:22:41,970
which was declaring it--so we took out the type.

452
00:22:41,970 --> 00:22:44,710
There's int, char, string, short,

453
00:22:44,710 --> 00:22:47,380
those are called types.

454
00:22:47,380 --> 00:22:49,510
So you took out the type

455
00:22:49,510 --> 00:22:51,580
and then previously,

456
00:22:51,580 --> 00:22:54,190
he had just the single equals sign

457
00:22:54,190 --> 00:22:57,300
and what he said was problematic about this was

458
00:22:57,300 --> 00:22:59,580
when you have 1 equals sign,

459
00:22:59,580 --> 00:23:01,780
that's an assignment operator.

460
00:23:01,780 --> 00:23:04,930
That's not testing whether or not 2 things are equal.

461
00:23:04,930 --> 00:23:07,920
That's actually setting something equal to something.

462
00:23:07,920 --> 00:23:11,110
We're putting the number 17 into the variable i.

463
00:23:11,110 --> 00:23:13,850
So to do that kind of comparison,

464
00:23:13,850 --> 00:23:16,430
you have to use the double equals,

465
00:23:16,430 --> 00:23:19,640
which is kind of a weird thing

466
00:23:19,640 --> 00:23:22,760
but is a fairly common error.

467
00:23:23,980 --> 00:23:27,830
So one other thing I'll point out, if you don't mind,

468
00:23:27,830 --> 00:23:30,610
is that in

469
00:23:30,610 --> 00:23:33,210
our function here in int main,

470
00:23:33,210 --> 00:23:36,740
you see that the body of our function

471
00:23:36,740 --> 00:23:39,810
is offset from the left side

472
00:23:39,810 --> 00:23:42,400
by 4 spaces or a tab.

473
00:23:42,400 --> 00:23:44,730
The reason we do this

474
00:23:44,730 --> 00:23:47,000
is just for readability.

475
00:23:47,000 --> 00:23:51,620
As you can see in Sam's while loop here,

476
00:23:51,620 --> 00:23:53,120
he didn't indent at all,

477
00:23:53,120 --> 00:23:55,390
and that doesn't affect the functionality of the code.

478
00:23:55,390 --> 00:23:57,860
It will run exactly the same.

479
00:23:57,860 --> 00:24:00,950
>> So for example, if we come up to the top here

480
00:24:00,950 --> 00:24:02,680
and press our Play button, it will build it,

481
00:24:02,680 --> 00:24:04,470
it'll run it,

482
00:24:04,470 --> 00:24:06,700
and what's going to happen?

483
00:24:06,700 --> 00:24:08,360
Why does it still say running?

484
00:24:08,360 --> 00:24:10,560
>>Because it'll keep repeating hi.

485
00:24:10,560 --> 00:24:12,900
Exactly, and why does it keep repeating hi?

486
00:24:12,900 --> 00:24:15,850
>>Because the integer never changes, so it's always 17.

487
00:24:15,850 --> 00:24:18,030
Yeah, exactly, so

488
00:24:18,030 --> 00:24:20,970
let's see, after--remind me of your name again?

489
00:24:20,970 --> 00:24:24,020
Jimmy. So Jimmy said that

490
00:24:24,020 --> 00:24:27,340
the reason this keeps repeating hi over and over again

491
00:24:27,340 --> 00:24:30,510
is because this condition is always true.

492
00:24:30,510 --> 00:24:33,270
We never change the value of i.

493
00:24:33,270 --> 00:24:36,170
We never change the contents of what's stored in that variable.

494
00:24:36,170 --> 00:24:39,180
So at this point we got this error that

495
00:24:39,180 --> 00:24:42,170
your program generated too much output,

496
00:24:42,170 --> 00:24:45,100
and if we scroll down and kind of look at what's going on here,

497
00:24:45,100 --> 00:24:49,160
you can see that it is hi, hi, hi, hi, hi, hi, hi all the way through.

498
00:24:49,160 --> 00:24:51,200
Awesome.

499
00:24:51,200 --> 00:24:53,750
>> So kind of back to the original point

500
00:24:53,750 --> 00:24:57,020
that having the indentation here

501
00:24:57,020 --> 00:25:00,110
in the body of the while loop didn't affect anything.

502
00:25:00,110 --> 00:25:03,050
The program still ran just fine.

503
00:25:03,050 --> 00:25:06,790
Typically for good style--

504
00:25:06,790 --> 00:25:10,210
and this is something that we look for in your problem sets--

505
00:25:10,210 --> 00:25:13,930
we ask you to indent this by 4 spaces, which you can do

506
00:25:13,930 --> 00:25:16,440
by just pressing the tab key

507
00:25:16,440 --> 00:25:18,860
or by manually putting in 4 spaces yourself.

508
00:25:21,880 --> 00:25:25,050
Cool, okay so let's go back and look at

509
00:25:25,050 --> 00:25:30,460
Alexander's do-while.

510
00:25:30,460 --> 00:25:32,980
>>Just kinda having a little trouble.

511
00:25:32,980 --> 00:25:34,390
Okay, well let's work through it.

512
00:25:34,390 --> 00:25:36,400
Is that cool?

513
00:25:36,400 --> 00:25:37,430
>>Yeah.

514
00:25:37,430 --> 00:25:39,200
>> Okay, so here we go.

515
00:25:39,200 --> 00:25:41,010
This is the code that Alexander wrote.

516
00:25:41,010 --> 00:25:43,410
Do you go by Alex--okay.

517
00:25:43,410 --> 00:25:46,380
So here's the code that Alex has written.

518
00:25:46,380 --> 00:25:50,500
So, we've got our declaration of main right here,

519
00:25:50,500 --> 00:25:54,410
and then we've got this integer variable called i

520
00:25:54,410 --> 00:25:57,320
being declared right here.

521
00:25:57,320 --> 00:26:01,060
We've got a do statement

522
00:26:01,060 --> 00:26:04,600
and then the body again with these braces,

523
00:26:04,600 --> 00:26:06,520
so this is the body of the do-while--

524
00:26:06,520 --> 00:26:09,210
let me zoom in on this so it's more readable--

525
00:26:09,210 --> 00:26:12,730
and in the body, we're trying to print "this is the body",

526
00:26:12,730 --> 00:26:14,300
i++,

527
00:26:14,300 --> 00:26:17,710
so increment the contents of i and do this

528
00:26:17,710 --> 00:26:19,470
until i gets to be 3,

529
00:26:19,470 --> 00:26:22,440
and then our program ends.

530
00:26:22,440 --> 00:26:24,680
>> So what happens when we run this program?

531
00:26:24,680 --> 00:26:27,580
>>I expected an identifier.

532
00:26:27,580 --> 00:26:32,070
Okay, so build, run, error occurs.

533
00:26:32,070 --> 00:26:34,820
What happens?

534
00:26:34,820 --> 00:26:37,060
We scroll down here,

535
00:26:37,060 --> 00:26:40,260
we get this somewhat cryptic looking thing,

536
00:26:40,260 --> 00:26:43,930
expected semicolon after top level declaration.

537
00:26:43,930 --> 00:26:46,620
That's kinda weird.

538
00:26:46,620 --> 00:26:50,990
Does anybody have an idea of what might be going on?

539
00:26:53,080 --> 00:26:54,640
Kinda weird, yeah.

540
00:26:54,640 --> 00:26:57,640
>>[inaudible] be declared outside of the parentheses?

541
00:26:57,640 --> 00:27:03,480
So does that not apply then when you use parentheses?

542
00:27:03,480 --> 00:27:07,390
So he declared this guy outside of which parentheses?

543
00:27:09,670 --> 00:27:12,240
Oh, these guys--the curly braces?

544
00:27:12,240 --> 00:27:13,850
>>Yeah.

545
00:27:13,850 --> 00:27:15,310
Yeah, exactly.

546
00:27:15,310 --> 00:27:17,120
So it turns out that

547
00:27:17,120 --> 00:27:21,550
you can't actually declare a variable right here like this.

548
00:27:23,750 --> 00:27:26,690
What you might have been trying to do was--

549
00:27:26,690 --> 00:27:30,110
cut it out of there--

550
00:27:30,110 --> 00:27:34,330
you can either put it within the curly braces right here

551
00:27:34,330 --> 00:27:37,400
or another alternative

552
00:27:37,400 --> 00:27:40,350
is to put it completely outside.

553
00:27:42,480 --> 00:27:46,610
And so these--depending on where you actually put the declaration

554
00:27:46,610 --> 00:27:48,550
of that integer variable,

555
00:27:48,550 --> 00:27:53,540
you can get 2 very different kinds of variables;

556
00:27:53,540 --> 00:27:55,100
when it's outside of main,

557
00:27:55,100 --> 00:27:58,240
it's what we call a global variable.

558
00:27:58,240 --> 00:28:00,070
It's outside the scope of any of the functions--

559
00:28:00,070 --> 00:28:03,420
it's not within a function--

560
00:28:03,420 --> 00:28:07,050
whereas if we put it inside of main,

561
00:28:07,050 --> 00:28:10,000
like right here, then it's called a local variable,

562
00:28:10,000 --> 00:28:13,830
because it's local to the scope of main.

563
00:28:13,830 --> 00:28:16,720
And we'll talk a little bit more about what the differences are

564
00:28:16,720 --> 00:28:20,970
between the 2 types and why you use different kinds of variables,

565
00:28:20,970 --> 00:28:23,040
but for now,

566
00:28:23,040 --> 00:28:25,070
you can just keep all the variables local,

567
00:28:25,070 --> 00:28:28,320
because we don't have any need for global variables.

568
00:28:28,320 --> 00:28:31,390
>> Okay, so that's one thing right there.

569
00:28:31,390 --> 00:28:34,680
Let's try running this again, see what happens.

570
00:28:37,420 --> 00:28:40,270
Run, get another error.

571
00:28:40,270 --> 00:28:43,320
Zoom in a bit, what do we get?

572
00:28:45,110 --> 00:28:50,000
We get this expected parenthesis after do/while.

573
00:28:53,660 --> 00:28:55,260
Any ideas what's going on? Dan?

574
00:28:55,260 --> 00:28:58,880
>>Is the while then parenthesis and then--

575
00:28:58,880 --> 00:29:01,990
I don't know if you need to redeclare int,

576
00:29:01,990 --> 00:29:04,980
but the do while (i < 3).

577
00:29:04,980 --> 00:29:06,930
Exactly, yeah.

578
00:29:06,930 --> 00:29:08,830
So it turns out that Dan's exactly right.

579
00:29:08,830 --> 00:29:11,230
We don't need to put this type here again.

580
00:29:11,230 --> 00:29:14,440
We can instead kill the type

581
00:29:14,440 --> 00:29:17,220
and just do that.

582
00:29:19,270 --> 00:29:21,640
All right.

583
00:29:21,640 --> 00:29:24,950
I'll zoom back out, try it one more time.

584
00:29:26,360 --> 00:29:29,580
Look at that; it runs successful.

585
00:29:29,580 --> 00:29:33,260
If we scroll down, "this is the body" printed out 3 times.

586
00:29:33,260 --> 00:29:35,820
That's pretty sweet.

587
00:29:35,820 --> 00:29:38,670
Well done, well done, guys.

588
00:29:38,670 --> 00:29:41,080
Well done.

589
00:29:41,080 --> 00:29:43,250
How's this going so far?

590
00:29:43,250 --> 00:29:46,200
Decent pace?

591
00:29:46,200 --> 00:29:49,200
Okay, cool.

592
00:29:49,200 --> 00:29:52,590
>> So, as we kind of back to the original question,

593
00:29:52,590 --> 00:29:55,430
we--and now if we see the difference,

594
00:29:55,430 --> 00:29:58,240
you see that in this do-while loop,

595
00:29:58,240 --> 00:30:00,030
the condition comes last.

596
00:30:00,030 --> 00:30:02,530
So as the computer reads the program

597
00:30:02,530 --> 00:30:06,050
since C programs are executed kind of in this sequential order,

598
00:30:06,050 --> 00:30:09,450
where they literally start with the first line of main,

599
00:30:09,450 --> 00:30:13,450
and then execute line 6, line 7, line 8, line 9,

600
00:30:13,450 --> 00:30:16,150
line 10, line 11,

601
00:30:16,150 --> 00:30:18,440
all this stuff at the top right here

602
00:30:18,440 --> 00:30:20,020
is going to execute once

603
00:30:20,020 --> 00:30:23,350
before we ever test this condition to see

604
00:30:23,350 --> 00:30:26,240
whether or not i--or the contents of i, rather--

605
00:30:26,240 --> 00:30:29,750
are less than the number 3.

606
00:30:29,750 --> 00:30:33,300
And only if once we get to line 12,

607
00:30:33,300 --> 00:30:36,260
the contents of i are less than 3

608
00:30:36,260 --> 00:30:39,150
do we actually jump back up to line 7

609
00:30:39,150 --> 00:30:41,140
and then repeat the process,

610
00:30:41,140 --> 00:30:44,620
and then we test the condition again and again and again.

611
00:30:44,620 --> 00:30:47,440
>> So we can contrast this

612
00:30:47,440 --> 00:30:50,240
with--zooming back out,

613
00:30:50,240 --> 00:30:56,500
scroll down to Sam's version,

614
00:30:56,500 --> 00:31:00,760
we're gonna look at this, slightly changed so that it's no longer an infinite loop--

615
00:31:00,760 --> 00:31:03,570
where if we zoom in,

616
00:31:03,570 --> 00:31:06,300
we see that as this program executes,

617
00:31:06,300 --> 00:31:09,120
it's going to first declare int i,

618
00:31:09,120 --> 00:31:12,030
set the contents of i be to 0,

619
00:31:12,030 --> 00:31:15,820
and then it hits the condition of the while loop first,

620
00:31:15,820 --> 00:31:19,210
so in this case, this body, this printf("hi") and i++

621
00:31:19,210 --> 00:31:21,630
won't even execute

622
00:31:21,630 --> 00:31:24,310
unless i is actually less than or equal to 2.

623
00:31:24,310 --> 00:31:26,750
So if i is 3, for example,

624
00:31:26,750 --> 00:31:28,840
if we set this right here

625
00:31:28,840 --> 00:31:32,160
equal to 4--or I'll do 3, because I said 3--

626
00:31:32,160 --> 00:31:34,890
and then run it,

627
00:31:34,890 --> 00:31:38,320
it'll build, it'll run totally fine,

628
00:31:38,320 --> 00:31:40,530
but it won't actually print out anything,

629
00:31:40,530 --> 00:31:43,030
because it failed that test for the first case.

630
00:31:44,780 --> 00:31:47,530
So in class, we kind of talked about why these do-while loops

631
00:31:47,530 --> 00:31:49,800
are useful sometimes

632
00:31:49,800 --> 00:31:53,140
and why we would use it as opposed to a while loop.

633
00:31:53,140 --> 00:31:56,450
>> Are there--do you guys have any suggestions

634
00:31:56,450 --> 00:31:59,090
for why a do-while loop might be more useful?

635
00:31:59,090 --> 00:32:02,560
>>Well, I think do-while loops is if you want user input,

636
00:32:02,560 --> 00:32:05,020
because that way you can let them

637
00:32:05,020 --> 00:32:07,780
put something in once; if it's correct,

638
00:32:07,780 --> 00:32:09,300
we can move on to the rest of the program,

639
00:32:09,300 --> 00:32:11,250
but if it isn't correct, then you keep on prompting the user

640
00:32:11,250 --> 00:32:13,250
until you put the right input.

641
00:32:13,250 --> 00:32:14,260
Yeah, exactly--exactly,

642
00:32:14,260 --> 00:32:16,840
so a do-while loop is useful

643
00:32:16,840 --> 00:32:18,930
because when prompting for user input

644
00:32:18,930 --> 00:32:23,000
because you need to get the user's input before you can actually check it.

645
00:32:23,000 --> 00:32:27,330
So in pset 1, in pset 2,

646
00:32:27,330 --> 00:32:29,650
where you're prompting for user input a lot,

647
00:32:29,650 --> 00:32:31,200
that's a handy thing to use

648
00:32:31,200 --> 00:32:33,740
as opposed to just a simple while loop.

649
00:32:33,740 --> 00:32:36,780
If you use a while loop, you can do the same thing--

650
00:32:36,780 --> 00:32:38,750
you can do the same thing with a while loop

651
00:32:38,750 --> 00:32:40,580
as you do with a do-while loop.

652
00:32:40,580 --> 00:32:43,380
You just have to kind of have this duplicate process

653
00:32:43,380 --> 00:32:47,010
where you get the input first and then start your while loop.

654
00:32:47,010 --> 00:32:49,800
Whereas a do-while loop, you just have to write it once

655
00:32:49,800 --> 00:32:52,430
and it is a lot cleaner.

656
00:32:52,430 --> 00:32:54,650
>> All right.

657
00:32:55,970 --> 00:32:57,330
Any questions?

658
00:32:59,350 --> 00:33:01,380
Make sense?

659
00:33:01,380 --> 00:33:06,200
>>The return 0--that's just kind of a stall or is it--?

660
00:33:06,200 --> 00:33:08,960
Yeah, so we can--

661
00:33:08,960 --> 00:33:11,960
well, we'll cover this a little bit more later

662
00:33:11,960 --> 00:33:13,290
but it turns out that

663
00:33:13,290 --> 00:33:15,720
this return 0 that we put

664
00:33:15,720 --> 00:33:19,090
at the end of main all the time--

665
00:33:19,090 --> 00:33:21,980
you can kinda just take it as something we do

666
00:33:21,980 --> 00:33:24,930
for the time being.

667
00:33:24,930 --> 00:33:28,860
What we've started to talk about with functions

668
00:33:28,860 --> 00:33:31,600
kind of at the end of last week and then again this week

669
00:33:31,600 --> 00:33:33,550
is that a function

670
00:33:33,550 --> 00:33:37,680
takes inputs which are these things within the parentheses;

671
00:33:37,680 --> 00:33:41,690
these are 2 inputs, our arguments or parameters,

672
00:33:41,690 --> 00:33:44,540
what they're also called to our main function,

673
00:33:44,540 --> 00:33:48,510
and then a function produces an output

674
00:33:48,510 --> 00:33:51,290
which we call a return value.

675
00:33:51,290 --> 00:33:56,260
And the return value in the case of main is this int,

676
00:33:56,260 --> 00:33:58,320
this integer,

677
00:33:58,320 --> 00:34:01,160
and the way you tell a function, "Hey, this is what your output

678
00:34:01,160 --> 00:34:05,840
is going to be," is with this word "return",

679
00:34:05,840 --> 00:34:07,960
that keyword, rather,

680
00:34:07,960 --> 00:34:11,000
and so main returning 0

681
00:34:11,000 --> 00:34:13,389
or main outputting 0

682
00:34:13,389 --> 00:34:15,139
is standard in C programs

683
00:34:15,139 --> 00:34:18,110
because returning 0 from main means that

684
00:34:18,110 --> 00:34:21,600
"Hey, everything executed with my program just fine."

685
00:34:21,600 --> 00:34:24,639
If we were to put something other than 0,

686
00:34:24,639 --> 00:34:27,440
like 1, for example,

687
00:34:27,440 --> 00:34:31,080
that's used to tell whoever ran this program

688
00:34:31,080 --> 00:34:33,540
that something went wrong.

689
00:34:35,739 --> 00:34:39,989
So as we get more involved with stuff over the semester,

690
00:34:39,989 --> 00:34:42,670
if you're ever needing to say,

691
00:34:42,670 --> 00:34:46,650
"Oops, you know, something went wrong with this program,"

692
00:34:46,650 --> 00:34:49,409
rather than having my program crash, I want to notify

693
00:34:49,409 --> 00:34:52,310
the runner of my program that something went wrong,

694
00:34:52,310 --> 00:34:54,530
you can use return value ,

695
00:34:54,530 --> 00:34:57,230
as we call it, that's not 0.

696
00:35:00,180 --> 00:35:02,240
>> So we'll cover that a little bit more

697
00:35:02,240 --> 00:35:05,480
once we get into functions.

698
00:35:05,480 --> 00:35:09,030
That sort of makes sense?

699
00:35:09,030 --> 00:35:11,240
Okay.

700
00:35:11,240 --> 00:35:13,530
Functions are kind of bizarre--

701
00:35:13,530 --> 00:35:17,270
I think for people who have lots of math background,

702
00:35:17,270 --> 00:35:20,100
it comes a little more easily,

703
00:35:20,100 --> 00:35:22,540
especially if you have a little bit more abstract math background

704
00:35:22,540 --> 00:35:24,240
and you're used to talking about functions

705
00:35:24,240 --> 00:35:27,370
and all that kind of stuff.

706
00:35:28,590 --> 00:35:30,610
But we'll try and cover them

707
00:35:30,610 --> 00:35:33,280
and go nice and slow as they come up.

708
00:35:34,730 --> 00:35:37,870
Likewise, this whole int argc,

709
00:35:37,870 --> 00:35:41,090
char* argv[],

710
00:35:41,090 --> 00:35:44,960
for right now, you can also take that as boiler plate.

711
00:35:46,650 --> 00:35:49,050
We've started to cover those a little bit in lecture

712
00:35:49,050 --> 00:35:53,060
when we talk about arguments that get passed into programs on the command line,

713
00:35:53,060 --> 00:35:55,700
but in your problem sets and your programs,

714
00:35:55,700 --> 00:35:58,760
it's totally fine just to write void and keep it simple.

715
00:35:58,760 --> 00:36:01,490
And what this void means is that

716
00:36:01,490 --> 00:36:04,630
"Hey, main is not going to accept any inputs;

717
00:36:04,630 --> 00:36:07,440
this is just a program we're gonna just run.

718
00:36:07,440 --> 00:36:09,320
We're not gonna give it any extra information

719
00:36:09,320 --> 00:36:12,420
every time we start it up running."

720
00:36:14,730 --> 00:36:16,600
So for now, you can do this;

721
00:36:16,600 --> 00:36:19,710
you can also even omit the return 0

722
00:36:19,710 --> 00:36:22,650
at the end of your programs and they'll run just fine,

723
00:36:22,650 --> 00:36:26,150
but I recommend just keeping that in.

724
00:36:26,150 --> 00:36:28,320
In a couple of weeks, we'll start to kind of

725
00:36:28,320 --> 00:36:31,340
pull back the cover a little bit and understand more about

726
00:36:31,340 --> 00:36:34,380
all those different little moving parts.

727
00:36:34,380 --> 00:36:38,160
>> All right, now let's talk about our second question here.

728
00:36:38,160 --> 00:36:43,190
So what does "undeclared identifier" usually indicate

729
00:36:43,190 --> 00:36:46,040
if outputted by Clang?

730
00:36:46,040 --> 00:36:49,290
I think we might have seen that already today.

731
00:36:51,170 --> 00:36:54,370
Can somebody give a recap of what that means?

732
00:36:54,370 --> 00:36:57,680
Let's see, Kelly--one of the Kellys.

733
00:37:02,580 --> 00:37:05,300
>>So--

734
00:37:05,300 --> 00:37:07,080
Yeah, what does--

735
00:37:07,080 --> 00:37:10,140
>>I think when you, like, don't declare the variables and you try to use it.

736
00:37:10,140 --> 00:37:13,310
Yeah, exactly, exactly.

737
00:37:13,310 --> 00:37:15,250
So if you ever see this output

738
00:37:15,250 --> 00:37:17,690
when you try and compile your program,

739
00:37:17,690 --> 00:37:19,470
just like Kelly said,

740
00:37:19,470 --> 00:37:24,210
it means that you used a variable that you didn't declare,

741
00:37:24,210 --> 00:37:28,460
so if I go back to our Space,

742
00:37:28,460 --> 00:37:31,150
and I do something like

743
00:37:31,150 --> 00:37:34,800
printf(

744
00:37:34,800 --> 00:37:38,260
"hello %s")

745
00:37:40,070 --> 00:37:42,440
and try and fill in that %s

746
00:37:42,440 --> 00:37:45,640
with a variable s that I haven't declared anywhere

747
00:37:45,640 --> 00:37:48,330
and I try to run it,

748
00:37:48,330 --> 00:37:51,330
you'll see what I get down at this bottom right here:

749
00:37:51,330 --> 00:37:54,010
"use of undeclared identifier 's'";

750
00:37:54,010 --> 00:37:57,710
that just means you didn't declare your variable.

751
00:37:57,710 --> 00:38:00,750
Same thing happens if

752
00:38:00,750 --> 00:38:05,240
you have x and you try and store 7 in it

753
00:38:05,240 --> 00:38:08,720
and I didn't declare X.

754
00:38:08,720 --> 00:38:10,200
I get that same error:

755
00:38:10,200 --> 00:38:13,480
"use of undeclared identifier 'x'".

756
00:38:16,390 --> 00:38:18,250
>> All right,

757
00:38:18,250 --> 00:38:21,720
so the Caesar Cipher, not being very secure,

758
00:38:21,720 --> 00:38:24,590
I'll let you guys watch this short, kind of understand why,

759
00:38:24,590 --> 00:38:27,590
and hopefully after doing this week's problem set

760
00:38:27,590 --> 00:38:29,270
where you actually write a Caesar Cipher, you'll understand a little bit more

761
00:38:29,270 --> 00:38:32,600
about what it means to be a secure and insecure--

762
00:38:36,180 --> 00:38:38,960
or not secure, I guess--insecure is slightly different.

763
00:38:40,560 --> 00:38:43,140
The other thing was we have these other questions here

764
00:38:43,140 --> 00:38:45,470
about functions

765
00:38:45,470 --> 00:38:47,330
and what a function is,

766
00:38:47,330 --> 00:38:52,820
and we kind of talked about that briefly when we talked about return values;

767
00:38:52,820 --> 00:38:54,560
for somebody who's been to lecture,

768
00:38:54,560 --> 00:38:57,790
can you recap who's been to lecture today?

769
00:38:57,790 --> 00:38:59,670
All right.

770
00:39:01,210 --> 00:39:04,320
Let's see; let's have--can somebody tell us what a function is?

771
00:39:04,320 --> 00:39:06,060
Kelly?

772
00:39:07,800 --> 00:39:11,260
>> >>Basically something you draw upon something that's written in the library that's

773
00:39:11,260 --> 00:39:14,460
more complicated over there, and then you just write a short form of it,

774
00:39:14,460 --> 00:39:17,360
so like you're referring back to that library, using

775
00:39:17,360 --> 00:39:19,650
all that long code and [inaudible].

776
00:39:19,650 --> 00:39:22,680
Yeah, exactly, and then you wanna add something to that?

777
00:39:22,680 --> 00:39:27,050
>>Yeah, I think about it as similar to the concept of a formula in math

778
00:39:27,050 --> 00:39:30,130
where you just write a formula and then you can plug variables in

779
00:39:30,130 --> 00:39:32,660
and that's what a function is [inaudible].

780
00:39:32,660 --> 00:39:35,390
>> Yeah, so we've got a couple of answers here

781
00:39:35,390 --> 00:39:38,080
that are both really good and right on.

782
00:39:38,080 --> 00:39:43,040
There's referring to code that's been written in the library somewhere

783
00:39:43,040 --> 00:39:45,390
that is more complicated

784
00:39:45,390 --> 00:39:47,130
and then you can draw upon that code

785
00:39:47,130 --> 00:39:50,480
and actually use it by just writing the short form of it--

786
00:39:50,480 --> 00:39:54,060
or, rather, its name--

787
00:39:54,060 --> 00:39:57,590
and then there is also this comparison to a formula in math

788
00:39:57,590 --> 00:40:00,400
or physics, all that kind of stuff

789
00:40:00,400 --> 00:40:02,980
where you have a formula

790
00:40:02,980 --> 00:40:05,690
and you can then plug in

791
00:40:05,690 --> 00:40:08,100
the variables to that formula

792
00:40:08,100 --> 00:40:11,130
to then get the result of the formula,

793
00:40:11,130 --> 00:40:14,580
so you think about it as like the quadratic equation.

794
00:40:14,580 --> 00:40:17,690
Imagine if rather than having to learn about

795
00:40:17,690 --> 00:40:21,310
what the quadratic formula is in math class,

796
00:40:21,310 --> 00:40:24,610
you just had a little calculator program that did it for you.

797
00:40:24,610 --> 00:40:28,930
Did anybody use calculator programs in math classes in high school?

798
00:40:28,930 --> 00:40:31,020
Yeah, physics classes?

799
00:40:31,020 --> 00:40:34,670
Where you can just say, "Hey, quadratic formula.

800
00:40:34,670 --> 00:40:37,740
These are the variables that I want to input.

801
00:40:37,740 --> 00:40:40,250
Now give me the answer."

802
00:40:40,250 --> 00:40:42,170
That's kind of what a function is.

803
00:40:42,170 --> 00:40:44,850
It's a way for us to

804
00:40:44,850 --> 00:40:47,720
kind of write like a small little sub program

805
00:40:47,720 --> 00:40:51,700
within our larger program

806
00:40:51,700 --> 00:40:55,090
and use it over and over again

807
00:40:55,090 --> 00:40:58,380
so that we don't have to copy and paste code,

808
00:40:58,380 --> 00:41:04,220
we can give it different inputs and get different outputs,

809
00:41:04,220 --> 00:41:07,090
it's super handy

810
00:41:07,090 --> 00:41:11,420
when you're trying to write different programs at different times.

811
00:41:11,420 --> 00:41:14,580
>> So for example, a function that we've been using over and over and over again

812
00:41:14,580 --> 00:41:17,310
throughout the course is printf.

813
00:41:17,310 --> 00:41:19,680
Printf is itself a function

814
00:41:19,680 --> 00:41:23,360
and it turns out that it's actually a pretty long and complicated,

815
00:41:23,360 --> 00:41:26,420
nasty looking, function to write.

816
00:41:26,420 --> 00:41:29,590
So as simple as it seems to us

817
00:41:29,590 --> 00:41:31,810
to print stuff out to the screen,

818
00:41:31,810 --> 00:41:34,820
because we all just did that at the beginning of section today,

819
00:41:34,820 --> 00:41:36,870
it turns out that it's actually, you know,

820
00:41:36,870 --> 00:41:39,900
a good couple of hundred lines of code.

821
00:41:42,030 --> 00:41:45,620
It's not something that I would ever want to write, for sure,

822
00:41:45,620 --> 00:41:47,480
but we get to use it every day; that's kind of cool,

823
00:41:47,480 --> 00:41:49,020
so we get to share it with each other,

824
00:41:49,020 --> 00:41:51,790
with different programs,

825
00:41:51,790 --> 00:41:54,520
so we'll be diving more into functions;

826
00:41:54,520 --> 00:41:56,090
for the time being, we're not--

827
00:41:56,090 --> 00:41:59,150
the only function we're going to be writing is main.

828
00:41:59,150 --> 00:42:01,570
Main itself is also a function,

829
00:42:01,570 --> 00:42:04,270
and you can see it's got its output,

830
00:42:04,270 --> 00:42:06,610
its inputs like we talked about,

831
00:42:06,610 --> 00:42:08,930
and then its name, which is main.

832
00:42:08,930 --> 00:42:11,800
Main is kind of a--is a special function there.

833
00:42:11,800 --> 00:42:14,800
In your C program, there has to be a main function,

834
00:42:14,800 --> 00:42:17,900
and so for now, we've kind of omitted writing

835
00:42:17,900 --> 00:42:21,400
the prototype, or the declaration of, main.

836
00:42:21,400 --> 00:42:23,050
We've given that to you,

837
00:42:23,050 --> 00:42:25,650
and we've just had you fill in kind of the blanks.

838
00:42:25,650 --> 00:42:29,120
>> >>So for the second problem set, we don't need to

839
00:42:29,120 --> 00:42:31,350
write it in the second function?

840
00:42:31,350 --> 00:42:34,200
I don't believe so.

841
00:42:34,200 --> 00:42:37,600
I believe it's all in here.

842
00:42:37,600 --> 00:42:41,710
Yeah, so Caesar Cipher,

843
00:42:41,710 --> 00:42:45,000
you don't have to, it doesn't look like,

844
00:42:48,740 --> 00:42:53,150
and Vigenere, you don't either,

845
00:42:53,150 --> 00:42:56,400
so--and it turns out you don't ever actually have to write

846
00:42:56,400 --> 00:42:58,250
a function, right?

847
00:42:58,250 --> 00:43:01,950
You call always put all of your code within main,

848
00:43:01,950 --> 00:43:04,500
and many programmers do that.

849
00:43:04,500 --> 00:43:07,130
As we move on,

850
00:43:07,130 --> 00:43:09,480
we start introducing functions as a way

851
00:43:09,480 --> 00:43:11,530
to clean up your code,

852
00:43:11,530 --> 00:43:13,380
to make it more elegant,

853
00:43:13,380 --> 00:43:16,890
in case you have tasks--kind of like when we talked about loops,

854
00:43:16,890 --> 00:43:20,400
how sometimes there are things that you want to do again and again and again,

855
00:43:20,400 --> 00:43:23,510
and you have kind of the naive way--

856
00:43:23,510 --> 00:43:26,360
or what we often call the "straw man" solution,

857
00:43:26,360 --> 00:43:29,390
is to just copy and paste over and over again--

858
00:43:29,390 --> 00:43:31,000
you can do the same thing with functions where you just copy and paste

859
00:43:31,000 --> 00:43:33,360
the same code over and over again,

860
00:43:33,360 --> 00:43:35,750
but that bloats your code--

861
00:43:35,750 --> 00:43:38,550
it ends up making your code really long--

862
00:43:38,550 --> 00:43:41,550
and it also gets a little confusing,

863
00:43:41,550 --> 00:43:43,470
and then it means that you aren't able to like--

864
00:43:43,470 --> 00:43:45,250
if you ever want to use that code again in another program,

865
00:43:45,250 --> 00:43:48,610
you're also stuck copying and pasting it again,

866
00:43:48,610 --> 00:43:51,170
so one of the things we'll talk about is

867
00:43:51,170 --> 00:43:54,960
kind of design and style of your programs,

868
00:43:54,960 --> 00:43:57,010
and so when it makes sense to kind of

869
00:43:57,010 --> 00:43:59,800
package something up and put it as a separate function,

870
00:43:59,800 --> 00:44:02,080
we encourage you to do that,

871
00:44:02,080 --> 00:44:04,700
and that is something that, again,

872
00:44:04,700 --> 00:44:07,790
there's no hard and fast rule.

873
00:44:07,790 --> 00:44:11,540
We rarely say that you must write a function called this,

874
00:44:12,870 --> 00:44:16,320
but it's more of one of those things you just develop over time

875
00:44:16,320 --> 00:44:19,470
as you write more and more programs.

876
00:44:21,940 --> 00:44:24,790
>> Any more questions?

877
00:44:26,340 --> 00:44:30,610
Okay, so let's now

878
00:44:30,610 --> 00:44:34,750
go back up to this guy right here,

879
00:44:34,750 --> 00:44:37,800
so I'm still in this section of questions.

880
00:44:37,800 --> 00:44:39,900
We've talked about all these different ones,

881
00:44:39,900 --> 00:44:42,050
what's a function, why to write functions,

882
00:44:42,050 --> 00:44:44,700
when you can just copy and paste?

883
00:44:44,700 --> 00:44:47,700
Hopefully we've convinced you that writing functions is a good thing.

884
00:44:47,700 --> 00:44:50,870
So now let's move on to this MySpace program,

885
00:44:50,870 --> 00:44:53,080
and we'll spend a little bit of time doing this.

886
00:44:54,600 --> 00:44:56,480
>> So, let's see,

887
00:44:56,480 --> 00:45:00,230
can I have somebody read this for me?

888
00:45:02,640 --> 00:45:04,660
Let's have Mikhaila--will you read this for us?

889
00:45:07,660 --> 00:45:08,890
>>You mean back when MySpace--

890
00:45:08,890 --> 00:45:10,920
Back when MySpace was cool.

891
00:45:10,920 --> 00:45:12,070
>>"Back when MySpace was cool,

892
00:45:12,070 --> 00:45:14,900
it was all the rage to TyPe LiKe ThIs.

893
00:45:14,900 --> 00:45:17,320
Maybe it still is? I'm not really sure.

894
00:45:17,320 --> 00:45:19,990
In any case, using the CS50 Appliance,

895
00:45:19,990 --> 00:45:22,600
CS50 Run, or CS50 Spaces,

896
00:45:22,600 --> 00:45:25,370
write a program that prompts the user for a message

897
00:45:25,370 --> 00:45:29,350
and then outputs the message with its first letter capitalized,

898
00:45:29,350 --> 00:45:31,800
with all the other letters in alternating case,

899
00:45:31,800 --> 00:45:34,010
as per the sample output below,

900
00:45:34,010 --> 00:45:37,060
wherein boldfaced text represents some user's input.

901
00:45:37,060 --> 00:45:39,800
For simplicity, you may assume that the user

902
00:45:39,800 --> 00:45:42,810
will only input lowercase letters and spaces.

903
00:45:42,810 --> 00:45:45,900
Consider this problem an opportunity to practice;

904
00:45:45,900 --> 00:45:48,640
you won't be asked to submit this program."

905
00:45:48,640 --> 00:45:51,640
>> Awesome, thank you.

906
00:45:51,640 --> 00:45:54,750
So as Mikhaila said,

907
00:45:54,750 --> 00:45:58,850
we are going to write a program that MySpaces your code.

908
00:45:58,850 --> 00:46:02,230
So did any of you guys use MySpace?

909
00:46:02,230 --> 00:46:04,100
Any of you have MySpace accounts?

910
00:46:04,100 --> 00:46:05,920
Oh, yeah, yeah!

911
00:46:05,920 --> 00:46:08,300
It was pretty cool.

912
00:46:08,300 --> 00:46:09,950
[laughter]

913
00:46:09,950 --> 00:46:12,060
I know.

914
00:46:12,060 --> 00:46:14,500
Dating myself.

915
00:46:15,490 --> 00:46:18,300
So what we're trying to do is write a program

916
00:46:18,300 --> 00:46:21,110
that goes through

917
00:46:21,110 --> 00:46:24,490
and capitalizes the first letter

918
00:46:24,490 --> 00:46:27,140
and then every letter thereafter--

919
00:46:27,140 --> 00:46:29,350
or every other letter thereafter, I'm sorry.

920
00:46:29,350 --> 00:46:32,910
>> So--and you can see in this example,

921
00:46:32,910 --> 00:46:35,260
this is how we run the program,

922
00:46:35,260 --> 00:46:38,780
and so this is just saying, "We're running a program called a.out.

923
00:46:41,490 --> 00:46:44,630
The input that we're going to giving it

924
00:46:44,630 --> 00:46:47,090
is going to be prompted.

925
00:46:47,090 --> 00:46:51,150
We're not gonna supply the input on the command line."

926
00:46:51,150 --> 00:46:54,100
If you've been following all the lectures

927
00:46:54,100 --> 00:46:56,710
and hearing about command line arguments,

928
00:46:57,810 --> 00:47:00,080
that's kind of where those--that funky syntax

929
00:47:00,080 --> 00:47:04,530
at main comes in with the int argc, all that stuff,

930
00:47:04,530 --> 00:47:07,630
so here we're not using that; we're not dealing with any of that.

931
00:47:09,400 --> 00:47:12,650
We're just gonna get this typed in by the user,

932
00:47:12,650 --> 00:47:15,700
and then this second line with the funky capitalization

933
00:47:15,700 --> 00:47:18,550
is what we want our program to output.

934
00:47:19,790 --> 00:47:22,860
>> So rather than using the Appliance or Run, we'll use Spaces,

935
00:47:22,860 --> 00:47:25,360
because we've got Spaces,

936
00:47:25,360 --> 00:47:28,640
and so if you guys would

937
00:47:28,640 --> 00:47:32,380
get your code to look something like this,

938
00:47:32,380 --> 00:47:35,930
so I'm going to save this revision

939
00:47:35,930 --> 00:47:38,390
just so you've got the boiler plate right here

940
00:47:38,390 --> 00:47:42,840
that looks like this, int main (void), nothing in it.

941
00:47:44,480 --> 00:47:48,460
Actually, what we'll do is we'll just delete everything all together.

942
00:47:48,460 --> 00:47:50,330
We want a blank slate,

943
00:47:50,330 --> 00:47:52,470
because we're not gonna start writing code;

944
00:47:52,470 --> 00:47:55,380
rather, we're gonna try writing some pseudocode

945
00:47:55,380 --> 00:47:58,580
which really just means we're going to write something that's not code

946
00:47:58,580 --> 00:48:01,010
that kind of describes what we want to do,

947
00:48:01,010 --> 00:48:05,180
kinda like David did on the first day of lecture,

948
00:48:05,180 --> 00:48:08,310
or I guess that was the second day of lecture when he did the whole socks demo.

949
00:48:08,310 --> 00:48:10,990
You guys remember that--seeing the socks?

950
00:48:10,990 --> 00:48:13,300
Joseph was up there kind of typing out in English?

951
00:48:13,300 --> 00:48:16,300
So we're gonna do the same thing.

952
00:48:16,300 --> 00:48:18,380
>> So when I start writing pseudocode,

953
00:48:18,380 --> 00:48:22,640
I like to start by looking at what the input is going to be.

954
00:48:22,640 --> 00:48:24,950
And I give myself a sample case,

955
00:48:24,950 --> 00:48:28,780
so like "thanks for the add!"

956
00:48:35,030 --> 00:48:38,010
And then I also write what I want the output to be,

957
00:48:38,010 --> 00:48:42,210
which in this case is something like this.

958
00:48:53,790 --> 00:48:56,050
And that kind of gives me a good, like, starting place to think about,

959
00:48:56,050 --> 00:48:59,730
"Okay, I want to take line 1

960
00:48:59,730 --> 00:49:03,560
and I want to transform it into line 2,"

961
00:49:03,560 --> 00:49:06,570
and that kind of helps guide my thinking.

962
00:49:06,570 --> 00:49:09,990
This is a good place to start.

963
00:49:09,990 --> 00:49:12,190
Another way to think about it, too, as we kinda move on

964
00:49:12,190 --> 00:49:15,020
and as you do this in your own programs, is to

965
00:49:15,020 --> 00:49:18,530
think about kind of like what are the edge cases to this?

966
00:49:18,530 --> 00:49:20,740
What are kind of the weird scenarios?

967
00:49:20,740 --> 00:49:23,660
What are the weird inputs that we might get?

968
00:49:23,660 --> 00:49:26,550
And what should those outputs look like?

969
00:49:26,550 --> 00:49:29,670
For example, one weird input

970
00:49:29,670 --> 00:49:33,200
would be this one,

971
00:49:33,200 --> 00:49:35,630
creating absolutely nothing,

972
00:49:35,630 --> 00:49:37,840
like what should we output in that case?

973
00:49:37,840 --> 00:49:40,530
Something we want to think about, too.

974
00:49:40,530 --> 00:49:44,450
>> So just as a question,

975
00:49:44,450 --> 00:49:46,870
what might we want to output?

976
00:49:46,870 --> 00:49:50,740
If we don't get any input at all, what should our program be outputting?

977
00:49:53,740 --> 00:49:54,860
>>Nothing.

978
00:49:54,860 --> 00:49:57,340
Nothing? Sounds great.

979
00:49:57,340 --> 00:49:59,280
So we want to make sure that we're not writing anything out

980
00:49:59,280 --> 00:50:01,500
if we don't get anything in.

981
00:50:01,500 --> 00:50:04,320
Cool. Okay.

982
00:50:04,320 --> 00:50:07,370
So

983
00:50:07,370 --> 00:50:09,380
I'm going to call on a couple of people as we go through

984
00:50:09,380 --> 00:50:12,790
and just figure out kind of like what the general algorithm is

985
00:50:12,790 --> 00:50:15,250
and we'll kind start working through this together.

986
00:50:16,240 --> 00:50:19,710
>> So, let's see. Missy.

987
00:50:19,710 --> 00:50:21,220
How do you think we'd start?

988
00:50:21,220 --> 00:50:24,830
What would be a good place to start here

989
00:50:24,830 --> 00:50:27,900
if we want to transform, say, line 1

990
00:50:27,900 --> 00:50:29,440
into line 2?

991
00:50:29,440 --> 00:50:32,650
Can you kind of just explain to me, in English, what you're thinking?

992
00:50:32,650 --> 00:50:34,770
Don't even worry about code.

993
00:50:34,770 --> 00:50:38,250
>>First thing I would do would be to

994
00:50:38,250 --> 00:50:41,170
kind of direct the user to tell them what you want.

995
00:50:41,170 --> 00:50:44,730
Okay, so, say,

996
00:50:44,730 --> 00:50:47,450
"Hey, user. I want

997
00:50:47,450 --> 00:50:50,560
a string"--awesome.

998
00:50:50,560 --> 00:50:52,990
So let's write this down.

999
00:50:52,990 --> 00:50:56,140
Prompt the user for a string.

1000
00:50:56,140 --> 00:50:59,270
That sounds like a good start.

1001
00:51:03,920 --> 00:51:06,460
All right.

1002
00:51:06,460 --> 00:51:09,750
Let's ask Kelly Ren this time.

1003
00:51:09,750 --> 00:51:11,580
What would we then want to do next?

1004
00:51:11,580 --> 00:51:14,220
What's kind of our next--?

1005
00:51:14,220 --> 00:51:18,080
>>Collect their input and store it as a variable.

1006
00:51:18,080 --> 00:51:20,470
Great.

1007
00:51:20,470 --> 00:51:24,280
So collect the string,

1008
00:51:25,450 --> 00:51:27,880
store it in a variable.

1009
00:51:34,840 --> 00:51:36,460
So is there anything special we want to do here?

1010
00:51:36,460 --> 00:51:39,700
Do we want to check their input

1011
00:51:39,700 --> 00:51:42,260
before--?

1012
00:51:42,260 --> 00:51:43,790
>>Yeah.

1013
00:51:43,790 --> 00:51:46,330
What might we want to check for?

1014
00:51:46,330 --> 00:51:48,980
>>Well, you said we don't have to check if it's a number, right?

1015
00:51:48,980 --> 00:51:52,400
Because it'll assume that it's lowercase?

1016
00:51:52,400 --> 00:51:55,650
Yeah.

1017
00:51:56,380 --> 00:51:58,920
>> So it turns out that--yeah.

1018
00:51:58,920 --> 00:52:01,850
So getString is guaranteed to return us a string.

1019
00:52:01,850 --> 00:52:05,050
It's not going to return a number to us or anything like that.

1020
00:52:05,050 --> 00:52:09,140
There might be digit characters within that string, though.

1021
00:52:09,140 --> 00:52:13,460
And those, we can ignore as we did the capitalization.

1022
00:52:13,460 --> 00:52:16,620
But an empty string is totally fine,

1023
00:52:16,620 --> 00:52:19,900
so the case right here on line 4 and 5,

1024
00:52:19,900 --> 00:52:22,020
right, where there's no input,

1025
00:52:22,020 --> 00:52:25,190
and we've put nothing as our output.

1026
00:52:25,190 --> 00:52:28,730
We don't have to worry about making sure that there's something in the string

1027
00:52:28,730 --> 00:52:31,840
or that the string is at least 4 characters long;

1028
00:52:31,840 --> 00:52:34,190
anything is fine.

1029
00:52:34,190 --> 00:52:38,270
So this is contrasted, for example, with like the pennies programs that you wrote

1030
00:52:38,270 --> 00:52:42,340
where you had to make sure you were getting non-negative numbers;

1031
00:52:42,340 --> 00:52:46,430
you had to make sure that you were getting the correct number of days in a month.

1032
00:52:46,430 --> 00:52:49,630
And how are you guys doing that?

1033
00:52:49,630 --> 00:52:51,470
How would you do that test?

1034
00:52:51,470 --> 00:52:54,090
What did that require you to use?

1035
00:52:54,090 --> 00:52:55,650
>>Do-while loops.

1036
00:52:55,650 --> 00:53:00,840
Do-while loops; other kinds of loops, too, yeah.

1037
00:53:02,190 --> 00:53:04,520
So it turns out we don't have to use a loop in this case,

1038
00:53:04,520 --> 00:53:07,360
when we're prompting for a string, which is kind of nice.

1039
00:53:08,280 --> 00:53:10,620
>> All right, moving on.

1040
00:53:12,410 --> 00:53:14,270
So, Stella.

1041
00:53:14,270 --> 00:53:17,530
Can you tell us a little bit about what we should do next?

1042
00:53:17,530 --> 00:53:20,270
So now we've got the string, we've got it stored in a variable;

1043
00:53:20,270 --> 00:53:23,360
what do we need to do with the string?

1044
00:53:26,790 --> 00:53:33,530
>>We need to--

1045
00:53:33,530 --> 00:53:36,550
identify the 2--

1046
00:53:36,550 --> 00:53:39,290
Yeah, just say it in English, that's awesome.

1047
00:53:39,290 --> 00:53:42,670
>>Identify the [inaudible] capitals--or, uppercase.

1048
00:53:44,420 --> 00:53:48,130
Yeah, so you'd identify which ones are uppercase and lowercase.

1049
00:53:50,020 --> 00:53:54,180
We can't uppercase or lowercase them until we've identified which ones.

1050
00:54:03,790 --> 00:54:06,350
So we'll put that in here.

1051
00:54:06,350 --> 00:54:10,210
>> So, just thinking about this process,

1052
00:54:10,210 --> 00:54:13,200
how do we do this identification?

1053
00:54:13,200 --> 00:54:15,050
Do you know, Dan?

1054
00:54:15,050 --> 00:54:18,120
Kind of what we--what tool we'd have to use out of our toolbox?

1055
00:54:18,120 --> 00:54:22,500
>>I'd put it in somehow in like an array or each line

1056
00:54:22,500 --> 00:54:24,240
is its own little box,

1057
00:54:24,240 --> 00:54:27,240
and then uppercase it from there or not.

1058
00:54:27,240 --> 00:54:30,440
Okay, so one tool we can use is an array.

1059
00:54:30,440 --> 00:54:33,760
What do we have to look at in the string,

1060
00:54:33,760 --> 00:54:36,680
though, to figure out--we had to look at the characters.

1061
00:54:36,680 --> 00:54:40,890
>>So you've got to look at the length of the string?

1062
00:54:40,890 --> 00:54:43,880
Yeah, so part of that is getting the length of the string.

1063
00:54:50,450 --> 00:54:54,150
Just out of curiosity, what do we use to get the length of a string?

1064
00:54:54,150 --> 00:54:58,080
>>strlen?

1065
00:54:58,080 --> 00:55:02,870
Yeah, strlen; we call this "strlen"

1066
00:55:04,180 --> 00:55:08,620
All of these string functions, so strlen is a function,

1067
00:55:08,620 --> 00:55:10,670
and it's super handy, because we can use it to just say,

1068
00:55:10,670 --> 00:55:13,430
Hey, strlen: Here's a string, tell me how long it is.

1069
00:55:13,430 --> 00:55:17,350
In the lecture today, David talked a lot about how strlen is implemented,

1070
00:55:17,350 --> 00:55:20,370
what's actually going on behind the scenes.

1071
00:55:20,370 --> 00:55:25,450
But this is one of the very super handy ones.

1072
00:55:25,450 --> 00:55:29,170
There are other ones that are also in the string library,

1073
00:55:29,170 --> 00:55:34,690
such as strcmp, which is spelled "strcmp",

1074
00:55:34,690 --> 00:55:38,010
which is used to compare strings to each other.

1075
00:55:41,220 --> 00:55:45,020
Other ones you will use like strstr,

1076
00:55:45,020 --> 00:55:48,770
which is good for finding sub strings and so on.

1077
00:55:48,770 --> 00:55:53,380
For the most part, they all start with str and then go on to be something else.

1078
00:55:53,380 --> 00:55:57,080
The reason they don't actually spell them like string and so on

1079
00:55:57,080 --> 00:56:00,260
is because back in like the early days of C,

1080
00:56:00,260 --> 00:56:04,100
when people were first writing and all that,

1081
00:56:04,100 --> 00:56:07,580
they had limitations on the number of characters that you could use

1082
00:56:07,580 --> 00:56:09,950
for function names and variable names,

1083
00:56:09,950 --> 00:56:12,620
so everything is like super condensed.

1084
00:56:12,620 --> 00:56:15,750
But anytime you see something that begins with str,

1085
00:56:15,750 --> 00:56:18,600
that typically means this has something to do with a string.

1086
00:56:20,990 --> 00:56:23,350
>> Okay, so you've got to get the length of the string,

1087
00:56:23,350 --> 00:56:27,110
and then, what do we do with the characters?

1088
00:56:27,110 --> 00:56:29,020
Do you know, Sam?

1089
00:56:29,020 --> 00:56:32,410
>>I have no idea; I was trying to figure it out.

1090
00:56:32,410 --> 00:56:35,410
I really don't know--I wish there were some way to--

1091
00:56:35,410 --> 00:56:38,990
I mean, I would think maybe we could do something with odds and evens,

1092
00:56:38,990 --> 00:56:40,860
but I don't know of any function like that.

1093
00:56:40,860 --> 00:56:42,240
Yeah.

1094
00:56:43,160 --> 00:56:45,200
>>But that--it wouldn't necessarily work,

1095
00:56:45,200 --> 00:56:48,690
because then you would get the spaces to be--

1096
00:56:48,690 --> 00:56:51,130
you would have to have the space to get tips on how--

1097
00:56:51,130 --> 00:56:54,310
Okay.

1098
00:56:54,310 --> 00:56:56,490
>>I don't know--I'm kind of lost at this point.

1099
00:56:56,490 --> 00:56:58,120
Yeah--no, no, no, these are good points.

1100
00:56:58,120 --> 00:57:00,420
So what we need to do is, like Sam said,

1101
00:57:00,420 --> 00:57:02,970
we need to track which letters are the odd letters,

1102
00:57:02,970 --> 00:57:05,540
and which ones are the even letters.

1103
00:57:06,650 --> 00:57:09,090
But we want to make sure we're not just looking

1104
00:57:09,090 --> 00:57:12,110
at the odd and even

1105
00:57:12,110 --> 00:57:14,030
indices in the string,

1106
00:57:14,030 --> 00:57:17,660
but rather looking at the odd and even letters in the string,

1107
00:57:17,660 --> 00:57:20,190
so we need to kinda have this distinction between

1108
00:57:20,190 --> 00:57:25,190
letters--like what's actually A, B, C, D, or so on,

1109
00:57:25,190 --> 00:57:29,090
and what things are not letters but still in our string.

1110
00:57:29,090 --> 00:57:32,390
For example here, like this exclamation point is not a letter,

1111
00:57:32,390 --> 00:57:36,060
but it's still part of our string; it's still a char.

1112
00:57:36,060 --> 00:57:38,820
Likewise, all of these spaces--

1113
00:57:38,820 --> 00:57:41,710
it doesn't make sense to capitalize a space, right?

1114
00:57:41,710 --> 00:57:46,160
So we want to make sure that we somehow skip those

1115
00:57:46,160 --> 00:57:49,410
while still tracking odds and evens.

1116
00:57:50,900 --> 00:57:54,990
>> So kind of in our programming toolbox

1117
00:57:54,990 --> 00:57:57,270
so far, if you will,

1118
00:57:57,270 --> 00:58:01,310
we have a number of different tools that we've talked about.

1119
00:58:01,310 --> 00:58:04,640
So one, for example, are variables,

1120
00:58:04,640 --> 00:58:07,120
like we've talked about here.

1121
00:58:07,120 --> 00:58:10,730
Another tool we've talked about are functions.

1122
00:58:10,730 --> 00:58:15,800
So we've talked about variables in our pseudocode up here

1123
00:58:15,800 --> 00:58:19,760
as what we're going to use to store our string.

1124
00:58:19,760 --> 00:58:24,630
We've talked about functions to get the length of a string, like strlen.

1125
00:58:28,510 --> 00:58:31,170
We've been talking about something else, and we've talked about it

1126
00:58:31,170 --> 00:58:34,850
for a lot earlier in section today: loops.

1127
00:58:39,390 --> 00:58:42,030
>> What do you guys think about using a loop in this program?

1128
00:58:42,030 --> 00:58:44,110
Jared, what do you think?

1129
00:58:47,150 --> 00:58:50,030
Would we use a loop in this program?

1130
00:58:50,030 --> 00:58:52,500
>>Probably not, no, because you don't have to go through it more than once.

1131
00:58:52,500 --> 00:58:55,260
Okay, you don't have to go through what more than once?

1132
00:58:55,260 --> 00:58:57,190
>>The entire string.

1133
00:58:57,190 --> 00:59:00,660
Right, so you don't have to go through the string entire once.

1134
00:59:00,660 --> 00:59:03,330
Sorry that made absolutely no sense!

1135
00:59:03,330 --> 00:59:06,360
You don't have to go through the entire string more than once.

1136
00:59:06,360 --> 00:59:08,660
Okay.

1137
00:59:09,790 --> 00:59:11,920
What do you think?

1138
00:59:11,920 --> 00:59:14,110
>>I think that it's better to use them

1139
00:59:14,110 --> 00:59:16,640
so we can do like capitalize while

1140
00:59:16,640 --> 00:59:19,750
the location of it is [inaudible] number.

1141
00:59:19,750 --> 00:59:22,940
Great, and so capitalize what?

1142
00:59:22,940 --> 00:59:25,200
Okay, so we're saying that

1143
00:59:25,200 --> 00:59:28,130
we should use a loop because we want to

1144
00:59:28,130 --> 00:59:32,550
capitalize a letter while it's not a number.

1145
00:59:35,130 --> 00:59:37,140
So--sorry, did you want to add?

1146
00:59:37,140 --> 00:59:39,340
>>I was going to say that the way you treat the string

1147
00:59:39,340 --> 00:59:42,290
and it [inaudible] separate the [inaudible] on the characters?

1148
00:59:42,290 --> 00:59:43,710
Yeah.

1149
00:59:43,710 --> 00:59:45,850
>>And then we can assign the numbers to--

1150
00:59:45,850 --> 00:59:47,780
or based on the location of the letters in the string?

1151
00:59:47,780 --> 00:59:49,720
Yeah.

1152
00:59:49,720 --> 00:59:53,640
>>And identify whether it's odd or even and to capitalize the letter itself

1153
00:59:53,640 --> 00:59:57,750
while the location of the letter is [inaudible]--sorry.

1154
00:59:57,750 --> 01:00:00,150
Great.

1155
01:00:00,150 --> 01:00:03,430
>> So just a recap.

1156
01:00:03,430 --> 01:00:05,840
Can you remind me of your name again--I'm sorry.

1157
01:00:05,840 --> 01:00:06,900
>>Charlotte.

1158
01:00:06,900 --> 01:00:08,490
Charlotte, okay.

1159
01:00:08,490 --> 01:00:11,130
So what Charlotte is saying,

1160
01:00:11,130 --> 01:00:14,950
which is totally right--gosh, this likes to auto indent on me--

1161
01:00:14,950 --> 01:00:17,770
is that if we look at a string

1162
01:00:17,770 --> 01:00:19,160
such as this one at the top--

1163
01:00:19,160 --> 01:00:21,590
"ThAnKs FoR tHe AdD"--

1164
01:00:21,590 --> 01:00:24,790
as a sequence of characters

1165
01:00:24,790 --> 01:00:29,010
where the is at position 0,

1166
01:00:29,010 --> 01:00:32,260
the h is at position 1.

1167
01:00:32,260 --> 01:00:36,880
A is at 2, then we got 3, 4, 5;

1168
01:00:36,880 --> 01:00:40,830
the space is at position 6.

1169
01:00:42,380 --> 01:00:45,300
Whereas--what's the position of f?

1170
01:00:45,300 --> 01:00:47,610
7!

1171
01:00:47,610 --> 01:00:49,300
O? 8.

1172
01:00:49,300 --> 01:00:52,070
R? 9.

1173
01:00:52,070 --> 01:00:54,340
And then we've got 10, and then I can't really write it without it

1174
01:00:54,340 --> 01:00:57,430
kinda getting messy on the screen.

1175
01:00:57,430 --> 01:01:02,340
But this is actually how we can talk about strings in memory.

1176
01:01:02,340 --> 01:01:04,660
Is it a string

1177
01:01:04,660 --> 01:01:07,920
is a sequence of characters,

1178
01:01:07,920 --> 01:01:12,430
and when we refer to an individual character in a string,

1179
01:01:12,430 --> 01:01:15,280
we often do so by referring to that character's position

1180
01:01:15,280 --> 01:01:17,620
within the string,

1181
01:01:17,620 --> 01:01:19,910
probably the same way that we talk about September

1182
01:01:19,910 --> 01:01:22,870
being the ninth month of the year

1183
01:01:22,870 --> 01:01:27,730
or Monday being the first day of the week.

1184
01:01:29,280 --> 01:01:33,070
So this provides us a way to

1185
01:01:33,070 --> 01:01:37,030
figure out which ones are odd versus even

1186
01:01:37,030 --> 01:01:39,540
because you can see here,

1187
01:01:39,540 --> 01:01:43,640
if we match it up with this T,

1188
01:01:46,310 --> 01:01:49,360
you can see that all of the capitalized ones

1189
01:01:49,360 --> 01:01:53,040
are at even positions,

1190
01:01:53,040 --> 01:01:55,880
at least for the first word,

1191
01:01:55,880 --> 01:01:59,080
and then things get thrown off once we hit that space,

1192
01:01:59,080 --> 01:02:02,460
because we don't count this space as a letter,

1193
01:02:02,460 --> 01:02:05,150
so it's as if we started

1194
01:02:05,150 --> 01:02:08,930
and went 0, 1, 2, 3, 4, 5,

1195
01:02:08,930 --> 01:02:12,600
space, 6, 7, 8, space,

1196
01:02:12,600 --> 01:02:16,440
9, and then 10, 11, 12, and so on.

1197
01:02:19,300 --> 01:02:22,910
>> Cool, so we definitely will want to use some sort of loop

1198
01:02:22,910 --> 01:02:27,760
to go over each of the characters in turn.

1199
01:02:31,330 --> 01:02:34,580
So at this point,

1200
01:02:36,490 --> 01:02:38,360
we'll come back to our old pseudocode here.

1201
01:02:40,970 --> 01:02:42,650
As part of this identification of which ones

1202
01:02:42,650 --> 01:02:45,350
are uppercase and which ones are lowercase,

1203
01:02:45,350 --> 01:02:48,340
we'll want to do this in some kind of loop.

1204
01:02:51,080 --> 01:02:53,390
Loop, colon.

1205
01:02:55,070 --> 01:03:01,220
What kind of loop, out of the types of loops that we have,

1206
01:03:01,220 --> 01:03:03,670
seems to be a good one to use in this case?

1207
01:03:03,670 --> 01:03:05,610
>>I was thinking the for loop.

1208
01:03:05,610 --> 01:03:07,500
And why are you thinking of for loop?

1209
01:03:07,500 --> 01:03:10,000
>>Because it starts at zero, and then

1210
01:03:10,000 --> 01:03:13,000
when we reach the length of the string,

1211
01:03:13,000 --> 01:03:16,170
and then each iteration you can [inaudible], too?

1212
01:03:16,170 --> 01:03:18,260
Yeah, exactly.

1213
01:03:18,260 --> 01:03:21,490
So, out of the different choices of loops,

1214
01:03:21,490 --> 01:03:25,350
we have for loops, while loops, and do-while loops.

1215
01:03:25,350 --> 01:03:28,550
Where a for loop is particularly useful

1216
01:03:28,550 --> 01:03:32,120
is when you want to know

1217
01:03:32,120 --> 01:03:34,940
kind of where you're at in the loop,

1218
01:03:34,940 --> 01:03:38,520
how many times the loop has been executed so far.

1219
01:03:38,520 --> 01:03:42,490
Which is exactly what we want to know in this situation,

1220
01:03:42,490 --> 01:03:44,640
like Kelly said,

1221
01:03:44,640 --> 01:03:47,400
based off of Charlotte's earlier comment that we want to know the position

1222
01:03:47,400 --> 01:03:49,900
of the character within the string,

1223
01:03:49,900 --> 01:03:51,750
and the for loop will give us just that,

1224
01:03:51,750 --> 01:03:54,680
because if we start kind of looking back at our string,

1225
01:03:54,680 --> 01:03:57,140
if we start our for loop at 0,

1226
01:03:57,140 --> 01:04:00,350
and we go up through the length of the entire string,

1227
01:04:00,350 --> 01:04:04,740
then we can associate the letter T

1228
01:04:04,740 --> 01:04:08,110
with the 0 in our for loop,

1229
01:04:09,950 --> 01:04:13,080
letter H with the 1 in our for loop, and so on,

1230
01:04:13,080 --> 01:04:16,600
going all the way through the end of the string.

1231
01:04:18,810 --> 01:04:21,890
>> So let's start coding this just a little bit

1232
01:04:21,890 --> 01:04:24,930
just so you guys get a feel for what this looks like.

1233
01:04:24,930 --> 01:04:27,980
I'm going to save this,

1234
01:04:27,980 --> 01:04:31,230
so we've got our pseudo code in here,

1235
01:04:33,490 --> 01:04:38,120
and I'm going to delete everything.

1236
01:04:38,120 --> 01:04:42,210
So I have a blank slate.

1237
01:04:42,210 --> 01:04:45,910
I'm gonna start by writing int main (void)

1238
01:04:45,910 --> 01:04:48,440
{

1239
01:04:48,440 --> 01:04:51,380
I'm gonna put the return 0 in

1240
01:04:51,380 --> 01:04:53,320
and then the }.

1241
01:04:53,320 --> 01:04:56,130
So usually when I start to program, this is what I start with.

1242
01:04:56,130 --> 01:04:59,370
I'll save it so that you guys have it.

1243
01:05:06,290 --> 01:05:08,440
So one of the first things we talked about was

1244
01:05:08,440 --> 01:05:10,920
getting input from the user

1245
01:05:10,920 --> 01:05:13,740
and storing it in a string.

1246
01:05:13,740 --> 01:05:16,420
How do we get input from a user?

1247
01:05:16,420 --> 01:05:19,640
>>GetString.

1248
01:05:19,640 --> 01:05:21,870
GetString, all right.

1249
01:05:23,150 --> 01:05:26,820
So if I type GetString()

1250
01:05:26,820 --> 01:05:32,150
and a semicolon.

1251
01:05:32,150 --> 01:05:36,630
Let's try running this; will this run, first of all?

1252
01:05:36,630 --> 01:05:38,430
And why won't this run?

1253
01:05:38,430 --> 01:05:40,090
>>You have to declare the variable.

1254
01:05:40,090 --> 01:05:43,040
I have to declare a variable. Okay.

1255
01:05:43,040 --> 01:05:45,700
>> So how do I do that?

1256
01:05:45,700 --> 01:05:48,400
Do you mind writing something, sharing it?

1257
01:05:48,400 --> 01:05:51,270
And then we can pull it up.

1258
01:05:51,270 --> 01:05:54,630
Let's see, Dan, right, okay.

1259
01:05:54,630 --> 01:05:59,100
Or Daniel. Copy that in, string input = GetString().

1260
01:05:59,100 --> 01:06:01,340
Awesome.

1261
01:06:01,340 --> 01:06:02,890
Will this run?

1262
01:06:02,890 --> 01:06:05,210
Let's try it.

1263
01:06:05,210 --> 01:06:08,890
Hit run, uploading source code, building, an error occurred.

1264
01:06:08,890 --> 01:06:11,040
Okay.

1265
01:06:11,040 --> 01:06:14,950
So whats going on here?

1266
01:06:14,950 --> 01:06:18,700
Use of undeclared identifier 'string'.

1267
01:06:18,700 --> 01:06:22,840
>>We have to include the libraries?

1268
01:06:22,840 --> 01:06:25,130
Yeah, so Dan's saying we have to include the libraries.

1269
01:06:25,130 --> 01:06:28,270
Is that what you were going to say, Stella? Awesome.

1270
01:06:29,450 --> 01:06:31,430
That's awesome, so Stella, can you

1271
01:06:31,430 --> 01:06:34,800
copy this code out of Daniel's Space?

1272
01:06:34,800 --> 01:06:37,580
If you click on Daniel Roland Ki, copy it,

1273
01:06:37,580 --> 01:06:41,360
and then will you put the include line in, and then save it out?

1274
01:06:46,580 --> 01:06:49,360
Let me know when you're ready.

1275
01:06:57,160 --> 01:06:59,490
So this include syntax is also kind of funky;

1276
01:06:59,490 --> 01:07:02,210
there's like that hash tag,

1277
01:07:02,210 --> 01:07:05,760
and there's the include, and then there are these angle brackets.

1278
01:07:12,490 --> 01:07:15,360
Do you remember how this works, or should I put it up on this code right here?

1279
01:07:15,360 --> 01:07:17,850
>>I'm good.

1280
01:07:17,850 --> 01:07:19,810
Okay.

1281
01:07:19,810 --> 01:07:23,260
>>I printed it--I wasn't sure which library, though.

1282
01:07:23,260 --> 01:07:25,570
Oh, okay--yeah, so it's the CS50 library.

1283
01:07:25,570 --> 01:07:27,660
>>Oh, I put stdio library.

1284
01:07:27,660 --> 01:07:30,950
>> Oh, got it--yeah, no, that's--

1285
01:07:30,950 --> 01:07:34,380
so it turns out that string is actually something that is special to CS50.

1286
01:07:36,590 --> 01:07:41,240
There actually isn't a true string type

1287
01:07:41,240 --> 01:07:45,000
because right here, string is in the type position,-

1288
01:07:45,000 --> 01:07:49,140
we're declaring a variable called input of type string--

1289
01:07:49,140 --> 01:07:53,320
and we're putting inside of input

1290
01:07:53,320 --> 01:07:58,440
the value that gets returned to us: the output of this GetString function call.

1291
01:07:58,440 --> 01:08:02,910
You can tell this is a function call because it's got the parentheses, just like printf does,

1292
01:08:02,910 --> 01:08:05,920
just like strlen does.

1293
01:08:05,920 --> 01:08:09,860
But string is actually not a real type

1294
01:08:09,860 --> 01:08:11,760
in the C language.

1295
01:08:11,760 --> 01:08:15,710
We've made it into a type, and we'll take about more what it actually is

1296
01:08:15,710 --> 01:08:19,000
in C, because there is a way to work with strings;

1297
01:08:19,000 --> 01:08:22,479
we've just done something to make it look a little nicer.

1298
01:08:22,479 --> 01:08:25,240
>> So here we go.

1299
01:08:25,240 --> 01:08:28,439
Here's Stella's code.

1300
01:08:28,439 --> 01:08:32,470
It's got the #include <cs50.h>.

1301
01:08:32,470 --> 01:08:36,390
Other people will call this like a hash include sometimes or a sharp include.

1302
01:08:36,390 --> 01:08:39,580
Now we can start it, run it,

1303
01:08:39,580 --> 01:08:43,649
and we still get an error.

1304
01:08:43,649 --> 01:08:46,290
Let's see what's happening.

1305
01:08:48,069 --> 01:08:50,569
"Unused variable 'input'."

1306
01:08:50,569 --> 01:08:53,350
Okay, so that's cool, that's cool.

1307
01:08:53,350 --> 01:08:55,859
I mean, it's not really cool,

1308
01:08:55,859 --> 01:08:57,930
because we're getting an error and it's not running,

1309
01:08:57,930 --> 01:09:00,410
but what this is saying is that

1310
01:09:00,410 --> 01:09:03,130
we haven't used our variable, which makes sense.

1311
01:09:03,130 --> 01:09:06,090
We haven't tried to do anything with it.

1312
01:09:07,819 --> 01:09:10,490
This is a way that

1313
01:09:10,490 --> 01:09:14,170
Clang is trying to help us make sure that we're not just using variables

1314
01:09:14,170 --> 01:09:18,819
and then inadvertently calling them something else later in our code,

1315
01:09:18,819 --> 01:09:21,649
helping us avoid errors that way.

1316
01:09:23,130 --> 01:09:28,810
>> All right, so we've got input from user,

1317
01:09:28,810 --> 01:09:31,910
we stored it in a variable.

1318
01:09:31,910 --> 01:09:34,569
The next step was to then

1319
01:09:34,569 --> 01:09:37,609
start looking at all of the characters in the string

1320
01:09:37,609 --> 01:09:41,830
and identifying which ones are to be uppercase

1321
01:09:41,830 --> 01:09:44,600
and which ones are to be lowercase.

1322
01:09:44,600 --> 01:09:49,439
And like Kelly and Charlotte brought up,

1323
01:09:49,439 --> 01:09:52,340
we do this using a for loop.

1324
01:09:52,340 --> 01:09:55,520
So does anybody remember the syntax of a for loop

1325
01:09:55,520 --> 01:09:58,670
and would like to take a stab at just writing

1326
01:09:58,670 --> 01:10:03,170
the first line of the for loop, just the for bit?

1327
01:10:05,740 --> 01:10:07,330
Kelly, you wanna give it a try?

1328
01:10:07,330 --> 01:10:10,300
>>Yeah, so you have to initialize first,

1329
01:10:10,300 --> 01:10:13,170
so I put i = 0,

1330
01:10:13,170 --> 01:10:15,390
and then I did i < strlen().

1331
01:10:15,390 --> 01:10:18,570
Is it in your Space--can I pull it up?

1332
01:10:18,570 --> 01:10:21,330
>>Yeah, I screwed up a little bit, but yeah.

1333
01:10:21,330 --> 01:10:24,180
>> Okay, let's pull it up.

1334
01:10:24,180 --> 01:10:26,490
Awesome.

1335
01:10:26,490 --> 01:10:29,510
>>But then I didn't know if you had that--

1336
01:10:29,510 --> 01:10:32,180
Yeah, okay.

1337
01:10:33,020 --> 01:10:36,030
So here's what Kelly's got: let's look at it quickly.

1338
01:10:36,030 --> 01:10:40,290
So she's included the cs50 library at the top.

1339
01:10:40,290 --> 01:10:44,790
She's included the standard IO library at the top,

1340
01:10:45,550 --> 01:10:48,990
and the standard IO has functions like printf,

1341
01:10:48,990 --> 01:10:51,690
so if you ever want to use printf, you need that line.

1342
01:10:53,060 --> 01:10:56,140
And then she included the string library,

1343
01:10:56,140 --> 01:10:58,200
which we will need,

1344
01:10:58,200 --> 01:11:01,010
because the string library has strlen function,

1345
01:11:01,010 --> 01:11:04,320
which we'll get to in just a bit.

1346
01:11:04,320 --> 01:11:07,560
So if we look at what Kelly's done here,

1347
01:11:07,560 --> 01:11:10,260
can you walk us through what you did?

1348
01:11:10,260 --> 01:11:13,600
>>Yeah, so--I mean, I got errors, so--

1349
01:11:13,600 --> 01:11:15,380
Yeah, no, it's--

1350
01:11:15,380 --> 01:11:18,580
>>So I wanted to call the string that I got just "string".

1351
01:11:18,580 --> 01:11:22,230
I wanted to print out the string

1352
01:11:22,230 --> 01:11:25,640
and then I--

1353
01:11:27,450 --> 01:11:30,930
I wanted to like set the string as like an array of each character

1354
01:11:30,930 --> 01:11:33,170
being different [inaudible], right?

1355
01:11:33,170 --> 01:11:38,080
And then I wanted to start at position 0

1356
01:11:38,080 --> 01:11:40,630
and then every 2 spaces,

1357
01:11:40,630 --> 01:11:42,580
I was going to capitalize,

1358
01:11:42,580 --> 01:11:45,680
so I just described it as 23, because I was looking at the ASCII table.

1359
01:11:45,680 --> 01:11:48,730
So that's what I was thinking--I know it's not right.

1360
01:11:48,730 --> 01:11:50,920
>> Got it, so this is a great start.

1361
01:11:50,920 --> 01:11:54,120
I think a lot of the framework is in place here.

1362
01:11:54,120 --> 01:11:57,500
Let's go through kind of line by line and see what's going on.

1363
01:11:57,500 --> 01:12:01,040
So the way I like to do this is just start by compiling the code

1364
01:12:01,040 --> 01:12:04,300
and letting the compiler tell me like what's wrong with my code,

1365
01:12:04,300 --> 01:12:07,450
because the compiler is really good at telling you what is wrong with your code.

1366
01:12:09,790 --> 01:12:12,780
So if we play it, building, and it doesn't build.

1367
01:12:12,780 --> 01:12:14,860
It says there was an error,

1368
01:12:14,860 --> 01:12:17,320
so if we scroll down and look at the first error,

1369
01:12:17,320 --> 01:12:20,960
it said--

1370
01:12:20,960 --> 01:12:22,040
we scroll back up to the top,

1371
01:12:22,040 --> 01:12:26,070
we can see there are 5 errors generated down here at the bottom.

1372
01:12:26,070 --> 01:12:30,180
Oops, that's really cut off.

1373
01:12:31,810 --> 01:12:35,790
I guess that's just not going to get any better.

1374
01:12:36,970 --> 01:12:38,500
>> Here we go.

1375
01:12:39,790 --> 01:12:42,530
So there are 5 total errors generated.

1376
01:12:42,530 --> 01:12:45,140
Here I can only see the first 2,

1377
01:12:45,140 --> 01:12:49,100
so I can move this up a little bit and then kind of scroll up.

1378
01:12:49,100 --> 01:12:53,220
This is the command that was run to compile the code,

1379
01:12:53,220 --> 01:12:59,030
and then the first error was in file.c right here:

1380
01:12:59,030 --> 01:13:03,300
"expected identify or '('", and it was on this line

1381
01:13:03,300 --> 01:13:06,680
where we said string = GetString().

1382
01:13:07,660 --> 01:13:12,920
You can see right here that it was on line 8 of file.c,

1383
01:13:12,920 --> 01:13:17,230
and then I believe this is character 12--that's the error.

1384
01:13:17,230 --> 01:13:20,390
So what's going wrong on line 8--Kelly?

1385
01:13:20,390 --> 01:13:22,810
>>You just need to call it, like, a letter or something,

1386
01:13:22,810 --> 01:13:24,400
like string s or something?

1387
01:13:24,400 --> 01:13:28,250
Yeah, so it turns out that these special type names,

1388
01:13:28,250 --> 01:13:31,770
so string, int, char, short,

1389
01:13:31,770 --> 01:13:35,740
they're all special keywords,

1390
01:13:35,740 --> 01:13:38,940
and you can't actually use them as names of your variables,

1391
01:13:38,940 --> 01:13:41,430
because the compiler looks at that and says,

1392
01:13:41,430 --> 01:13:43,680
"Oh, I know that's a type,"

1393
01:13:43,680 --> 01:13:46,530
so you can't actually have a string variable called string,

1394
01:13:46,530 --> 01:13:50,800
like I'm trying to do here; this won't work.

1395
01:13:52,610 --> 01:13:58,250
But instead, we can call it something like s or st or str.

1396
01:13:58,250 --> 01:14:01,540
We also couldn't call it int.

1397
01:14:01,540 --> 01:14:03,740
We can't have a string called int.

1398
01:14:03,740 --> 01:14:06,740
We can't have a string called char; none of those will work.

1399
01:14:06,740 --> 01:14:09,420
>> So we'll just call it s for the time being.

1400
01:14:10,630 --> 01:14:13,170
Awesome.

1401
01:14:13,170 --> 01:14:17,500
So what next is going on in this line?

1402
01:14:21,390 --> 01:14:24,350
String--this line looks pretty good?

1403
01:14:24,350 --> 01:14:27,830
Let's give it another try; let's run it.

1404
01:14:27,830 --> 01:14:30,360
Okay, so we still get errors.

1405
01:14:30,360 --> 01:14:32,080
This time, we're down to 4 errors.

1406
01:14:32,080 --> 01:14:34,350
So we got one of them.

1407
01:14:34,350 --> 01:14:36,130
Let's move up.

1408
01:14:36,130 --> 01:14:38,890
"Unexpected type name 'string': expected expression," yes?

1409
01:14:38,890 --> 01:14:41,780
>>Yeah, that's because I tried to name the string.

1410
01:14:41,780 --> 01:14:44,270
Yeah, so it looks like we now also have to go fix

1411
01:14:44,270 --> 01:14:46,560
all of these references to string.

1412
01:14:46,560 --> 01:14:50,070
You can see there's a reference to string here, one here, one here,

1413
01:14:50,070 --> 01:14:53,440
so we can move all these to s.

1414
01:14:56,040 --> 01:15:00,930
Great. Okay, run it again.

1415
01:15:00,930 --> 01:15:03,260
Building--another error occurred.

1416
01:15:03,260 --> 01:15:06,900
>> Okay, now we're down to one error,

1417
01:15:06,900 --> 01:15:11,090
so int i = s and then the square brackets.

1418
01:15:11,090 --> 01:15:14,880
So can you tell us a little bit about what you were doing here, Kelly?

1419
01:15:14,880 --> 01:15:17,120
>>Yes.

1420
01:15:17,120 --> 01:15:19,410
It was on line 10.

1421
01:15:19,410 --> 01:15:22,760
>>I don't know if we needed it all,

1422
01:15:22,760 --> 01:15:25,560
but I think I was trying to--

1423
01:15:25,560 --> 01:15:27,580
I'll save it so you can follow along.

1424
01:15:27,580 --> 01:15:30,470
>>I think I was

1425
01:15:30,470 --> 01:15:33,150
trying to tell it to start at position 0

1426
01:15:33,150 --> 01:15:35,030
at the beginning of my for loop,

1427
01:15:35,030 --> 01:15:38,370
so I think maybe that saying was unnecessary.

1428
01:15:39,690 --> 01:15:41,350
So just to recap,

1429
01:15:41,350 --> 01:15:44,850
you were saying that this--you were trying to start this at position 0

1430
01:15:44,850 --> 01:15:47,030
and get this--yeah,

1431
01:15:47,030 --> 01:15:50,460
so I think earlier, too, when you were walking through the code,

1432
01:15:50,460 --> 01:15:54,070
you were talking about how you wanted to treat the string as an array

1433
01:15:54,070 --> 01:15:57,440
so we can access each character individually.

1434
01:15:57,440 --> 01:16:00,660
I think Dan mentioned arrays, too, trying to convert strings into arrays.

1435
01:16:01,400 --> 01:16:03,300
>> It turns out that kind of under the hood,

1436
01:16:03,300 --> 01:16:06,060
a string is actually stored as an array already,

1437
01:16:06,060 --> 01:16:08,060
and you can access it like an array;

1438
01:16:08,060 --> 01:16:12,390
there's no conversion that you have to do.

1439
01:16:12,390 --> 01:16:15,770
A string isn't--so--

1440
01:16:15,770 --> 01:16:18,200
a string isn't really an array,

1441
01:16:18,200 --> 01:16:21,940
but you can access it as such, because all of the characters are stored

1442
01:16:21,940 --> 01:16:25,160
one right next to each other in memory,

1443
01:16:25,160 --> 01:16:28,310
so if you want to get the first character of the string,

1444
01:16:28,310 --> 01:16:31,910
it's the same as getting the first integer in an integer array.

1445
01:16:31,910 --> 01:16:35,010
So we often--we'll

1446
01:16:35,010 --> 01:16:36,360
refer to the 2 like

1447
01:16:36,360 --> 01:16:38,590
oh, as being a string, as being an array,

1448
01:16:38,590 --> 01:16:40,830
even though it really--they're very different things,

1449
01:16:40,830 --> 01:16:43,410
but you can treat them very much the same.

1450
01:16:44,700 --> 01:16:47,400
So it turns out that this line actually--

1451
01:16:47,400 --> 01:16:51,270
we don't need to do any conversion,

1452
01:16:51,270 --> 01:16:53,500
and in fact,

1453
01:16:53,500 --> 01:16:56,790
this actually isn't a valid expression,

1454
01:16:56,790 --> 01:16:59,040
which is what our compiler is telling us.

1455
01:16:59,040 --> 01:17:01,290
It's saying, "Hey, it expected an expression,

1456
01:17:01,290 --> 01:17:04,610
and what I've got here is something that's not an expression."

1457
01:17:04,610 --> 01:17:07,310
To declare an array,

1458
01:17:07,310 --> 01:17:11,780
all you need to do is something like this with a semicolon,

1459
01:17:11,780 --> 01:17:15,080
and then you tell the array how long you want it to be,

1460
01:17:15,080 --> 01:17:18,370
such as 5, so this line 10

1461
01:17:18,370 --> 01:17:23,880
will declare an array called i with 5 integers in it.

1462
01:17:23,880 --> 01:17:26,540
>> This is an aside; this isn't necessary for the code,

1463
01:17:26,540 --> 01:17:30,970
but this is--this is kind of what the declaration looks like.

1464
01:17:30,970 --> 01:17:35,310
>>How would you not use char i[strlen(s)]?

1465
01:17:35,310 --> 01:17:38,500
Char i[strlen(s)]?

1466
01:17:38,500 --> 01:17:41,500
>>char i, and then inside the brackets, strlen(s).

1467
01:17:43,900 --> 01:17:47,710
Yeah, so we could--

1468
01:17:47,710 --> 01:17:49,460
we could do this,

1469
01:17:49,460 --> 01:17:52,160
and this would declare--what would this do?

1470
01:17:52,160 --> 01:17:55,900
>>I think it would declare an array

1471
01:17:55,900 --> 01:18:01,780
of, like--of--

1472
01:18:01,780 --> 01:18:04,540
oh, you don't need the i--oh,

1473
01:18:04,540 --> 01:18:06,420
yeah, you definitely don't need the i.

1474
01:18:06,420 --> 01:18:10,250
You declare an array of characters

1475
01:18:10,250 --> 01:18:13,150
of length s.

1476
01:18:14,680 --> 01:18:16,770
And so what Sam's saying is that

1477
01:18:16,770 --> 01:18:20,660
this line 10--let me put some space in between it and the rest so we can isolate it--

1478
01:18:20,660 --> 01:18:23,440
this line 10 would declare an array

1479
01:18:23,440 --> 01:18:28,520
of chars, strlen(s) worth of them,

1480
01:18:28,520 --> 01:18:32,270
but we actually do need the i,

1481
01:18:32,270 --> 01:18:35,710
and the reason we need the i is because we need to give the array a name;

1482
01:18:35,710 --> 01:18:38,900
otherwise, we wouldn't have a way to refer to the array later in the code.

1483
01:18:38,900 --> 01:18:41,180
>> So this right here is the array name

1484
01:18:41,180 --> 01:18:43,420
and inside is the length.

1485
01:18:43,420 --> 01:18:45,010
This will give us an array,

1486
01:18:45,010 --> 01:18:47,430
but this is--again, this is a declaration.

1487
01:18:47,430 --> 01:18:50,380
It's not an assignment; it's not an initialization.

1488
01:18:50,380 --> 01:18:52,330
If we declare this array like this,

1489
01:18:52,330 --> 01:18:54,490
there's actually nothing in the contents of the array.

1490
01:18:54,490 --> 01:18:57,490
It's just junk--whatever is in there.

1491
01:18:57,490 --> 01:18:59,780
And like we said,

1492
01:18:59,780 --> 01:19:01,340
we actually don't even need to do this conversion,

1493
01:19:01,340 --> 01:19:04,810
because we can already treat string s as an array.

1494
01:19:06,750 --> 01:19:09,170
I know we're running a bit long on time,

1495
01:19:09,170 --> 01:19:12,330
but I want to just really quickly get to this for loop

1496
01:19:12,330 --> 01:19:14,850
and show you how to access the individual characters in the array,

1497
01:19:14,850 --> 01:19:17,170
because that's key for our assignment.

1498
01:19:17,170 --> 01:19:20,790
And then we'll let you go from there.

1499
01:19:20,790 --> 01:19:24,610
>> So here, we've got a for loop

1500
01:19:24,610 --> 01:19:28,310
that looks almost good.

1501
01:19:28,310 --> 01:19:30,960
What's not quite almost there?

1502
01:19:30,960 --> 01:19:33,240
What's--what's not quite right about this for loop?

1503
01:19:33,240 --> 01:19:36,010
>>No curly braces?

1504
01:19:36,010 --> 01:19:38,420
No curly braces, for one.

1505
01:19:38,420 --> 01:19:42,580
Wanna make sure that we have the body of the loop with curly braces.

1506
01:19:42,580 --> 01:19:45,240
Oops, not the square brace!

1507
01:19:45,240 --> 01:19:48,660
And what is the next thing that we need to do with this loop?

1508
01:19:48,660 --> 01:19:52,640
If I try and run it,

1509
01:19:52,640 --> 01:19:56,740
whats this error that I get?

1510
01:19:56,740 --> 01:20:00,990
I get all these errors--I have to initialize i.

1511
01:20:00,990 --> 01:20:03,610
Great.

1512
01:20:03,610 --> 01:20:05,770
So this will be a loop--this isn't quite there,

1513
01:20:05,770 --> 01:20:08,680
but it's getting really close.

1514
01:20:08,680 --> 01:20:11,260
This code we can now run,

1515
01:20:11,260 --> 01:20:14,620
build, an error still occurs.

1516
01:20:17,960 --> 01:20:19,800
Why is it still complaining,

1517
01:20:19,800 --> 01:20:22,030
saying the expression is unused,

1518
01:20:22,030 --> 01:20:24,600
and that's because this line doesn't do anything.

1519
01:20:24,600 --> 01:20:27,620
We didn't store it anywhere; we didn't save the result.

1520
01:20:27,620 --> 01:20:30,210
Where should we be saving this result?

1521
01:20:30,210 --> 01:20:32,920
>>s or i?

1522
01:20:32,920 --> 01:20:35,990
Exactly, and this funky syntax,

1523
01:20:35,990 --> 01:20:38,550
this s with the square brackets

1524
01:20:38,550 --> 01:20:41,500
means access the i-th character of s.

1525
01:20:41,500 --> 01:20:45,650
That's going to be something that's really important for the assignment.

1526
01:20:45,650 --> 01:20:48,070
Unfortunately, we're going to have to stop here,

1527
01:20:48,070 --> 01:20:50,550
so that Sebastian section can come in,

1528
01:20:50,550 --> 01:20:53,980
but I'm happy to stay after outside and will take any questions.

1529
01:20:53,980 --> 01:20:56,680
So thank you guys for coming.

1530
01:20:56,680 --> 01:20:59,730
This should now upload, build, run.

1531
01:21:01,570 --> 01:21:04,320
We can enter a string like "hello"

1532
01:21:06,570 --> 01:21:08,730
and it prints "QOX".

1533
01:21:08,730 --> 01:21:11,030
[laughter]

1534
01:21:11,030 --> 01:21:14,640
>> So not quite there, but moving, getting towards those loops!

1535
01:21:14,640 --> 01:21:18,640
Thanks for coming, and I'll be outside.