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This presentation is delivered by the Stanford Center for Professional 

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Development. 

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Alrighty, 

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welcome back to CS106A. 

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If you're 

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stuck in the back, 

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just come on down, have a seat. 

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Originally, I thought maybe we would have slightly fewer people today than last 

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time, but that appears not to be the case. 

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So while we're waiting, 

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everyone loves babies, so 

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I decided to put - that's Karel, the Robot, the early days. 

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No, this actually - my son, and 

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yeah, I know. 

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He's a little bit older now but, like, 

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he's got these little robot pajamas and he runs around all the time. So I'm like, oh, 

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it's Karel, The Robot, and my wife looks at me like, 

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it's your son. But 

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that's a whole different issue. Anyway, a few administrative announcements before we 

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start. 

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A couple things, there are four more handouts today, just because we like consistency. 

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Four last time, there's four this time. They're all in the back. If you didn't already pick them up, 

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you can pick them up after class, but they have all the information about 

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downloading Eclipse, which is the environment that you're going to use for 

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programming in this class, both for Karel and for Java. There 

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is also a special handout in there just on using Karel, so it talks about how Karel 

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works in the Eclipse environment, and so you can get all set up with that. 

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And so after today, you'll know how Karel works and you'll have the environment to 

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use it. So surprisingly enough, you also get your first assignment today, 

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which is actually in two parts. So 

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the assignment, the real assignment is the programming part, 

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which is due 

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on Friday of next week, October 5th. 

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There is also an email part to it, where the email part is, we ask all of you kind 

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folks to send an email to Ben, the head TA, 

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myself, and also your section leader. 

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So Ben and I get a whole bunch of mail, your section leader will hopefully get a little bit 

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less mail. But you won't actually know your section leader until after you go 

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to your first section next week, 

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which is why that part of the assignment is not actually due until basically one 

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minute before midnight on Sunday. So I didn't make it midnight, just because then there's 

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always confusion, 

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midnight, which night? 

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So 11:59 p.m. on Sunday, October 7th is when you should send 

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that email. That's not really the critical part of the assignment. 

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The critical part of the assignment is the programming problems, which are actually 

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all due in class on Friday. Okay, 

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and then there's one last assignment, or one last handout, which is about how you actually 

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submit your work in this class. 

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You'll submit your work both electronically and in hard copy. The 

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electronic copy, so that your section leader can run it and verify it. 

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The hard copy is so that they can actually write comments on it, and then you'll do 

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interactive grading with them as well. A 

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couple other quick announcements. The website, in case you weren't here on 

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Monday and you don't know what handouts I'm talking about, and you don't know 

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where to get them, 

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go to the class website, cs106A.stanford.edu. There will be 

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electronic copies in PDF format of all the handouts there, so you can get caught 

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up if today is your first day. 

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You also need to sign up for a section. As we mentioned last time, section signups start 

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tomorrow, 5:00 p.m. Sign up early if you want to have the most flexibility in 

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terms of time, because 

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look around, there's a whole bunch of people in this class. 

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It's going to fill up quickly. So if you have constraints, sign up quickly, 

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and the place you sign up is 

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CS 198, not CS 106A, but CS198.stanford.edu/section. The 198 

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folks are the folks who lovingly run all of the sections and coordinate the whole program. Last, but not 

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least, just a word on the readings that I didn't mention last time. 

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On the syllabus, you'll notice that pretty much for every day, or most days in 

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the quarter, 

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there is some sort of reading associated with it. 

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That's the reading assignment that you should have done by that day's lecture, because that's what we will cover in that day's lecture. So for today 

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it 

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should be the first three chapters of the Karel book, or the course 

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reader. And if you're like, oh my God, I'm already three chapters behind, don't worry, it's like 

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20 pages or something. It's pretty lightweight, 

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but you should do the readings by that time in class. So any questions about anything 

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we covered last time, which is mostly logistics, or any 

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of the sort of administrivia? 

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All righty, 

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then let's get started on the real content. So 

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you remember from 

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the time before, we talked a little bit about Karel, the Robot, and here is Karel in that 

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world that I showed you before. And there were 

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avenues that run north-south, and streets that run east-west, and little beepers in 

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the world. And Karen can face different directions, and there's walls. And so now, we 

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want to think about how do we actually program Karel. How do we get this little guy 

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to do something interesting in the world? Okay, 

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and it turns out there are four commands that Karel understands, okay, 

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and those are pretty straightforward. So here they are. You're going to get all 

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of Karel's vocabulary in, like, one minute. There's a 

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command called move, and move 

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basically moves Karel one spot forward 

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in the direction he's facing. So if he's on corner one, one, and he moves one spot 

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forward, he's facing east. So he'll move over to two, one, basically. So 

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one corner forward in the direction he's facing. 

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Karel also started being a good democrat, knows how to turn left. 

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So he turns left. This is a lower case T, this is an upper case L. So turning left 

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changes his direction by 90 degrees in the left hand of whichever way he's 

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facing. So if he's facing east now, and he turns left, 

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he will be facing north. 

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And then he can turn left again and he'll be facing west. Okay, 

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now, question? [Inaudible]? 

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There is no space here. These are all one word. Good question, and 

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I love it when you make it easy. All right, 

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so besides turning left, 

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there's also B-person Karel's world, and if he couldn't do 

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anything with beepers, they wouldn't be interesting. So he can pick up beepers, 

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which interestingly enough is called pickBeeper, 

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again lower case P, upper case B, all one word, 

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and there is putBeeper. 

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And what these do is pickBeeper, if Karel happens to be on a corner that has 

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a Beeper on it, he picks up that Beeper, and stuffs it in his Beeper bag, in which case 

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the count in his Beeper bag goes up by one. Or if it's infinite, it remains infinite, 

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because 

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he's got a real big Beeper bag, and sometimes he can infinitely many Beepers in 

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there. It's just - 

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we can talk about the infinity of Karel's Beeper bags some other time, but if 

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you're interested, it's fascinating. And he 

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can also putBeeper, which means he can go to a corner and be like, "Hey, I'm just feeling 

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happy. 

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I'm going to put a Beeper down, say, on that corner right there." Except you can't 

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throw the beepers, right. Karel's kind of limited. He's got, you know, if you look at him 

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he's got no arms. He's got legs. 

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No arms, so basically all he can do is he sort of like drops the little Beeper 

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where he's at. So this puts it on the corner that he's at. 

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And these things are what we refer to as 

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methods. 

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Okay, 

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methods are basically some instruction that we can call, 

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okay, or use, like move, or turn left, or pickBeeper, or putBeeper. 

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And what we say is Karel responds to this method. 

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What we're doing is we're invoking, or we're calling a particular method on Karel, 

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and he takes some action, which is basically what that method specifies to do. Okay, 

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so 

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if we kind of think about this program, or if 

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we kind of think about this world, maybe we want Karel to do something. So here's the 

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initial configuration of the world. Maybe what we want Karel to do is pick up this 

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beeper, 

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drop it off at this corner, and end up at this corner facing to the east. 

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And you can think about how we might do that with some of the instructions we have. So 

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what we want the final configuration of the world to look like, and I'll just 

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put Karel at blinding speed. You can actually control Karel's speed here with this 

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little slider. 

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So I'm going to just show you the end configuration by running Karel 

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so fast you can't even see it. It's just - he's blindingly fast. 

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He's just that good, okay. 

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And so that's what we want to get to. Notice he's still facing east, but he's 

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picked up that beeper that was on that corner at two, one, 

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and moved it over to the corner at four, two. Okay, 

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and so how might we do this? Right, 

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so one thing we could consider is, what was the initial state of the world again? 

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So I'm just going to run this program again, and here I have a clip set up, which is 

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your handout number five explains how to get this. 

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But if I want to run a program, there is these two little running person icons, 

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or as you might notice, 

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we even have our own menu in Eclipse, because Stanford's just that much fun, okay. 

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So under the Stanford menu, we have these two options for run. Import project 

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is explained in the handout, is what you'll use. 

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We'll give you some initial stuff for Karel that you'll start with, 

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and you'll use import project to get it into Eclipse. But once it's there, we can run it, 

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and so what we're going to do 

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is say run 

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this particular Karel program. So it's get 

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- and we'll go through all this - it's get and go. And this is our first Karel program 

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that we want to run, 

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and it's sort of, here's the initial world again, 

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all big and happy. 

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So I'm going to shrink this down a little bit, so we can see it over here on 

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the side, 

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while we think about what commands Karel would need to execute to kind of get to that 

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final state we just talked about. 

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And over here, 

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happily enough, let me resize a little bit, 

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is our first file, which is empty right now, that we're going to write our first 

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Karel program. 

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So what commands might we consider Karel actually doing from the list you have 

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here, 

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to sort of affect what we want to happen in the world? [Inaudible]. 

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Yeah, so we want to move. Okay, 

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and then what do we want to do? [Inaudible]. 

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PickBeeper, then move. 

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Notice at that point we could have actually done something else. This is part of the art of 

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programming. There's actually many ways to solve the problem. We're just going to 

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happen to pick one. 

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So we've moved. We've picked the beeper. We've moved again, and now what do we do? Go to the left. 

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Turn left. What happens if we turn left? [Inaudible]. 

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Right, then what are we going to do? Move. Move. 

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And now what do we want? [Inaudible]. 

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Yeah, conceptually what we'd like to do is turn right, right? 

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At this point, Karel's like, 

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"No, man. 

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I'm just, you know, 

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I'm left wing. What can I do? I turn left. That's what I do." And you kind of think about it, and you 

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say, 

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"That's all right, because really the world is just one big spectrum, and if you go 

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around far enough to one side, you end up on the other." 

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So if we make you turn left three times, 

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that's equivalent to essentially turning 90 degrees to the right. 

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So here, we turn left, 

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turn left, 

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and turn left. 

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All right, now what do we do? Move. 

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Move. So we sort of go up the step. 

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PutBeeper and 

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move to the last spot. 

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Now, at this point we'd like to think, oh good times, we can just run this and it's 

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a Karel program, and life is happy. 

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In fact, this is not a valid Karel 

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program. What this is, is an algorithm. 

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This is a recipe for doing something, and we'll talk in a lot more detail about 

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different algorithms on Friday. 

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But the way you can think of distinguishing between an algorithm and a 

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program 

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is an algorithm is essentially the recipe for doing something. 

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The program is something that is valid syntactically according to the rules 

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of the language. And so 

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Karel actually has some specific rules to its language that we have to apply to 

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these statements to make them look a little bit different, so they follow the 

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valid syntax of Karel. 

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And that's what we're going to do now. We're just going to kind of go through 

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this step by step, okay. 

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So one of the things we'd like to be able to do 

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is, first of all, we want these to be valid commands. Right now, they're not valid 

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commands. To turn a command into a valid command, or what we would refer to as a 

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method call in Karel, 

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after the name of the command we put an open paren and a close 

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paren, 

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and then a semicolon. Okay, 

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so move, open paren, close paren, semicolon is actually the valid move 

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command, or move method invocation for Karel, 

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and we do that for all these things. So to save a little bit of time, I'm just 

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00:11:11,970 --> 00:11:13,269
going to copy this 

234
00:11:13,269 --> 00:11:16,470
and just go through and paste until the cows come home. 

235
00:11:16,470 --> 00:11:17,899


236
00:11:17,899 --> 00:11:18,549


237
00:11:18,549 --> 00:11:20,399
Let's do a little paste there. Oh, it 

238
00:11:20,399 --> 00:11:21,149
didn't copy. 

239
00:11:21,149 --> 00:11:24,580
All right, let's try that again. We do 

240
00:11:24,580 --> 00:11:26,290
a little paste, a little paste. 

241
00:11:26,289 --> 00:11:29,889
So we're going to go and turn all of these into valid 

242
00:11:29,889 --> 00:11:33,009
Karel commands. Okay, so now you might think, okay they're all valid Karel commands. 

243
00:11:33,009 --> 00:11:35,529
Are we ready to actually run this Karel program? 

244
00:11:35,529 --> 00:11:39,740
And it turns out no, we're not yet ready to run - this is not yet a valid Karel 

245
00:11:39,740 --> 00:11:40,310
program. 

246
00:11:40,309 --> 00:11:43,539
What makes it a valid Karel program is we need a few more things. 

247
00:11:43,539 --> 00:11:47,500
One of those things is, we need to tell Karel where to start running. 

248
00:11:47,500 --> 00:11:49,350
And you look at this and you're like, 

249
00:11:49,350 --> 00:11:52,149
what does that mean? Like, doesn't he just start at the top? Does Karel start somewhere in 

250
00:11:52,149 --> 00:11:53,009
the middle? 

251
00:11:53,009 --> 00:11:56,399
No, what it really means is that we're going to do is encapsulate a set of 

252
00:11:56,399 --> 00:11:59,379
instructions inside something that tells Karel, 

253
00:11:59,379 --> 00:12:02,639
hey, this is what you're going to execute. And the way we do that is we 

254
00:12:02,639 --> 00:12:03,480
create 

255
00:12:03,480 --> 00:12:06,800
something called a run method, and the syntax for that 

256
00:12:06,799 --> 00:12:09,409
looks a little bit archaic, but basically it says, 

257
00:12:09,409 --> 00:12:10,829
public, 

258
00:12:10,830 --> 00:12:11,689
void, 

259
00:12:11,688 --> 00:12:12,418
run, 

260
00:12:12,418 --> 00:12:16,000
open paren, close paren, and then we put a brace. And 

261
00:12:16,000 --> 00:12:20,570
that first brace says everything that you will now see until the closing brace 

262
00:12:20,570 --> 00:12:23,379
is all part of what we refer to as the body 

263
00:12:23,379 --> 00:12:24,958
for this thing called run. 

264
00:12:24,958 --> 00:12:28,328
So we come down here to the bottom and we put a closing brace, and everything 

265
00:12:28,328 --> 00:12:31,739
between the open brace and the close brace is referred to the body, 

266
00:12:31,740 --> 00:12:34,909
or what's actually executed when we do a run. Okay, 

267
00:12:34,909 --> 00:12:38,759
so get to know where the brace keys on your keyboard are. They will come in very 

268
00:12:38,759 --> 00:12:41,490
hand in programming, because you'll use them all the time. 

269
00:12:41,490 --> 00:12:43,649
One thing we also do with this is 

270
00:12:43,649 --> 00:12:45,169
we're going to actually 

271
00:12:45,169 --> 00:12:46,439
insert some tabs 

272
00:12:46,440 --> 00:12:48,660
to make it easier for the human to read 

273
00:12:48,659 --> 00:12:50,169
this program. Okay, 

274
00:12:50,169 --> 00:12:51,909
so when we put in tabs, 

275
00:12:51,909 --> 00:12:54,879
suddenly everything that's inside the body of run 

276
00:12:54,879 --> 00:12:57,919
gets tabbed in, and when I look at it visually, it's much more appealing. I can 

277
00:12:57,919 --> 00:13:01,419
just tell all the things that are actually inside run. Okay, 

278
00:13:01,419 --> 00:13:05,878
so we tab those in there and now we have the body of run is inside the braces. 

279
00:13:05,879 --> 00:13:09,269
What we've actually done is created a new method. 

280
00:13:09,269 --> 00:13:14,539
Well, run is here, is we defined for Karel a new method called run. 

281
00:13:14,539 --> 00:13:18,279
And run is a very special method because that's where Karel begins running. What 

282
00:13:18,279 --> 00:13:20,000
he says is when he starts up, 

283
00:13:20,000 --> 00:13:23,080
he comes into the world, he's like a brand new baby. He's like, "Hey, I'm here. I'm 

284
00:13:23,080 --> 00:13:24,910
in the world. What do I do?" And 

285
00:13:24,909 --> 00:13:29,209
he's preprogrammed to go find the method called run 

286
00:13:29,210 --> 00:13:32,730
and start executing from the beginning of it. So he does executed in a top down manner, 

287
00:13:32,730 --> 00:13:34,810
but he needs to have this run thing. 

288
00:13:34,809 --> 00:13:38,989
And so you might see them say, "All right, we're ready to go. Fire up Karel, right. I'm slipping a 

289
00:13:38,990 --> 00:13:42,120
shrimp on the barbie." I'm sitting here barbequing while Karel's going and moving the 

290
00:13:42,120 --> 00:13:43,169
beeper. It's 

291
00:13:43,169 --> 00:13:45,329
like, no, we're not quite there yet. You're 

292
00:13:45,330 --> 00:13:48,620
like, oh, what else do we need? Okay, so 

293
00:13:48,620 --> 00:13:51,500
one thing that we need is we need to take Karel. 

294
00:13:51,500 --> 00:13:55,289
Karel is actually implemented in Java. If you've done some Java programming before, 

295
00:13:55,289 --> 00:13:57,559
some of the syntax already looks familiar to you. 

296
00:13:57,559 --> 00:13:59,829
If you haven't done any Java before, 

297
00:13:59,830 --> 00:14:02,550
it's perfectly fine. You don't need to know anything 

298
00:14:02,549 --> 00:14:03,358
about Java. 

299
00:14:03,359 --> 00:14:06,778
What you do need to know about Karel is that Karel is defined in what we refer 

300
00:14:06,778 --> 00:14:08,629
to as a class. 

301
00:14:08,629 --> 00:14:12,299
Karel is sort of a class of robot. There's kind of like the Karel 1000, kind of 

302
00:14:12,299 --> 00:14:13,949
comes of the assembly line, 

303
00:14:13,950 --> 00:14:17,540
and what we'd like to do is take those Karel 1000s and sort of mold 

304
00:14:17,539 --> 00:14:19,230
them into our own version, 

305
00:14:19,230 --> 00:14:20,620
which does some 

306
00:14:20,620 --> 00:14:22,830
set of instructions that we'd like it to do. 

307
00:14:22,830 --> 00:14:24,530
So in order to sort of say, 

308
00:14:24,529 --> 00:14:28,529
"I want to create my own version of Karel," what we do is we create a class, and 

309
00:14:28,529 --> 00:14:29,899
again we say public, 

310
00:14:29,899 --> 00:14:33,438
and we say class now, instead of void. So far, all you need to worry about, the 

311
00:14:33,438 --> 00:14:35,908
stuff that's in purple, public void, and public class, 

312
00:14:35,908 --> 00:14:39,460
is just kind of the standard syntax of Karel. 

313
00:14:39,460 --> 00:14:43,240
And later on, all of this stuff will make sense what it actually means, but right 

314
00:14:43,240 --> 00:14:47,399
now just think of it as standard syntax, and just put it there, and life is good. 

315
00:14:47,399 --> 00:14:50,340
Public class, we now need to give this Karel 

316
00:14:50,340 --> 00:14:51,930
program, in some sense, a name, 

317
00:14:51,929 --> 00:14:55,049
and what we're going to name it, for lack of any better name, 

318
00:14:55,049 --> 00:14:58,609
is we could call it our Karel program, because we did it together. It's 

319
00:14:58,610 --> 00:15:02,409
kind of a collective process. So our Karel program, 

320
00:15:02,409 --> 00:15:05,569
and at this point we're not done. We need to actually say - you might have noticed after I type 

321
00:15:05,570 --> 00:15:08,510
this why suddenly all this red showed up on the side of the editor. And 

322
00:15:08,509 --> 00:15:12,990
it's like, oh, bad times, dogs and cats sleeping together, like what's going to happen? 

323
00:15:12,990 --> 00:15:14,330
Well, what happens is 

324
00:15:14,330 --> 00:15:18,700
we're creating our Karel program and it says, "Okay, you're creating Karel program," 

325
00:15:18,700 --> 00:15:22,129
but it doesn't even know anything about the basic Karel 1000. It 

326
00:15:22,129 --> 00:15:24,450
needs to somehow know that this Karel program 

327
00:15:24,450 --> 00:15:28,430
is creating a version of the Karel 1000. We'll call it the Karel 

328
00:15:28,429 --> 00:15:29,329
1001, 

329
00:15:29,330 --> 00:15:34,300
and the way we specify this is we say that our Karel program extends Karel, 

330
00:15:34,299 --> 00:15:38,599
which is sort of the basic Karel that actually exists and has these predefined 

331
00:15:38,600 --> 00:15:39,310
instructions in it. 

332
00:15:39,309 --> 00:15:42,569
So we say public Karel, our Karel program extends 

333
00:15:42,570 --> 00:15:43,190
Karel, 

334
00:15:43,190 --> 00:15:48,280
and now we need to, again, encompass what our Karel, our particular class Karel is 

335
00:15:48,279 --> 00:15:51,879
going to be running. So we put this inside braces and we say, "Yeah, this whole 

336
00:15:51,879 --> 00:15:56,220
run thing is part of our Karel program." So we put another brace at the end, 

337
00:15:56,220 --> 00:16:00,250
and to make it more readable, we go through one more time. Don't want 

338
00:16:00,250 --> 00:16:01,489
to put a tab there, 

339
00:16:01,489 --> 00:16:02,899
and tab everything 

340
00:16:02,899 --> 00:16:06,399
so that it's nicely readable by a person. Okay, 

341
00:16:06,399 --> 00:16:07,689
so all this stuff 

342
00:16:07,690 --> 00:16:10,420
now gets tabbed over one more spot, 

343
00:16:10,419 --> 00:16:14,399
and even this brace over here gets tabbed one more spot. So now you can see, 

344
00:16:14,399 --> 00:16:17,909
everything here is part of the run method, because it's nicely tabbed in. We 

345
00:16:17,909 --> 00:16:20,029
can tell that, and all this stuff 

346
00:16:20,029 --> 00:16:24,829
here is all part of the body of run, and all of it's encapsulated inside our Karel 

347
00:16:24,830 --> 00:16:29,270
program. [Inaudible]? 

348
00:16:29,269 --> 00:16:32,370
Why is there a space, you mean like down here?] Yeah. 

349
00:16:32,370 --> 00:16:34,929
Just for readability. We don't actually need it. There doesn't need to be a space, 

350
00:16:34,929 --> 00:16:38,589
but that's a good question. 

351
00:16:38,590 --> 00:16:40,670
Oh, nice bank. I like that. 

352
00:16:40,669 --> 00:16:44,069
Feel free to bank. If someone - you're on the rebound and I'll set you up for the 

353
00:16:44,070 --> 00:16:47,680
rebound, just because you were the first one to do it. All right, so, 

354
00:16:47,679 --> 00:16:51,449
and you might say, "Okay, now we've got our Karel program. We've got our run method. Are we ready for 

355
00:16:51,450 --> 00:16:52,690
Karel?" Okay, 

356
00:16:52,690 --> 00:16:55,890
and you think we're ready for Karel, and someone's calling, saying, "You're ready 

357
00:16:55,889 --> 00:16:58,000
for Karel. Do it." 

358
00:16:58,000 --> 00:17:01,179
What I would actually encourage you to do is take a moment and turn off your cell phones, unless 

359
00:17:01,179 --> 00:17:04,720
your doctor. I actually would tell this to my students all the time. I had one student who was a 

360
00:17:04,720 --> 00:17:06,189
doctor once, and he says, "Actually, 

361
00:17:06,189 --> 00:17:08,200
your class is when I'm on call, 

362
00:17:08,200 --> 00:17:12,380
and if my phone rings, that means, like, someone's dying. So can I keep it on?" And I 

363
00:17:12,380 --> 00:17:13,970
was like, hmm, 

364
00:17:13,970 --> 00:17:14,870
yeah, okay. 

365
00:17:14,869 --> 00:17:18,509
So he got an exception, but other than that, I'd encourage you to turn your 

366
00:17:18,509 --> 00:17:22,828
cell phones, unless you have a really cool ring. Then you can leave it on. All right, 

367
00:17:22,828 --> 00:17:25,960
so at this point, you might say, "Oh, we're ready to run it," but 

368
00:17:25,960 --> 00:17:29,630
even now, we're not ready to run it. And the reason why we're not ready to run it is, 

369
00:17:29,630 --> 00:17:33,390
where does this Karel thing come from to begin with, right? Someone, somewhere 

370
00:17:33,390 --> 00:17:34,159
had to say, 

371
00:17:34,159 --> 00:17:37,090
"Hey, I'm going to build the original version of Karel that you're now 

372
00:17:37,089 --> 00:17:37,789
extending." 

373
00:17:37,789 --> 00:17:42,079
And that actually comes from a place called Stanford.Karel. So what we do 

374
00:17:42,079 --> 00:17:44,889
is we add a line called import, 

375
00:17:44,890 --> 00:17:50,250
Stanford.Karel.*, and we put a little semicolon at the end of it. 

376
00:17:50,250 --> 00:17:51,888
And what this tells us is, 

377
00:17:51,888 --> 00:17:56,488
hey, go and get everything that Stanford's previously defined about Karel. And when we get 

378
00:17:56,489 --> 00:17:59,389
to Java, this'll become much more clear, but for right now, all you can think 

379
00:17:59,388 --> 00:18:03,148
of that import's meaning is, go get me all the standard Karel stuff that 

380
00:18:03,148 --> 00:18:04,649
Stanford's already done for me. 

381
00:18:04,650 --> 00:18:08,600
And that defines this thing called Karel, which now we're extending in our Karel 

382
00:18:08,599 --> 00:18:11,009
program. Uh huh? What is 

383
00:18:11,009 --> 00:18:14,970
the semicolon for? The semicolons always come at the end of the line and Karel's syntax, what they mean 

384
00:18:14,970 --> 00:18:17,460
is this is the end of one statement. 

385
00:18:17,460 --> 00:18:20,460
So at the end of the import, for example, that says import the stuff, 

386
00:18:20,460 --> 00:18:21,730
that's the end of a statement, 

387
00:18:21,730 --> 00:18:28,160
or for move, or pickBeeper, or turn left, that's the end of a statement. So we put a semicolon. Uh huh? Do the stars 

388
00:18:28,160 --> 00:18:29,580
mean anything? Do stars mean anything? 

389
00:18:29,579 --> 00:18:33,699
Yeah, that star at the end of Stanford.Karel.* is actually important, 

390
00:18:33,700 --> 00:18:36,860
and in a couple weeks you'll understand why. For right now, it just means get 

391
00:18:36,859 --> 00:18:43,859
everything associated with Stanford.Karel. Uh huh? So then, is the run program [inaudible]? 

392
00:18:45,140 --> 00:18:48,929
It's all part of run. So when Karel starts up, and you'll see that in just a second, 

393
00:18:48,929 --> 00:18:50,019
we start - 

394
00:18:50,019 --> 00:18:53,240
we go to run and we just start executing the statements from run. 

395
00:18:53,240 --> 00:18:56,230
So let's actually run this and see what happens, and I'll take a couple more questions in a 

396
00:18:56,230 --> 00:18:59,829
second. So we go up to our little run icon here. Actually, before we do that, let me 

397
00:18:59,829 --> 00:19:02,369
cancel this, because we already have a running program over here. 

398
00:19:02,369 --> 00:19:06,239
So let's quit out of the running program and come here, and say run. 

399
00:19:06,240 --> 00:19:09,509
And sometimes if you actually happen to write multiple Karel 

400
00:19:09,509 --> 00:19:10,900
programs, and you hit the run, then 

401
00:19:10,900 --> 00:19:13,610
this guy sort of looks like he's running with smoke all around him, which means he's 

402
00:19:13,609 --> 00:19:14,659
running really fast. 

403
00:19:14,660 --> 00:19:17,170
That means run the last program I was running. 

404
00:19:17,170 --> 00:19:19,130
The guy without smoke around him means, 

405
00:19:19,130 --> 00:19:22,550
run one of the programs that I tell you to run, and it might give you a list if you 

406
00:19:22,549 --> 00:19:27,099
have multiple programs. Here we're going to pick first Karel program, okay. 

407
00:19:27,099 --> 00:19:30,649
So yes, we'll go ahead and save the resource because we forgot to save our file, 

408
00:19:30,650 --> 00:19:34,030
and here is life. And we'll run it slowly, and so we start the program, and 

409
00:19:34,029 --> 00:19:37,649
there goes Karel. Woo-hoo! 

410
00:19:37,650 --> 00:19:41,230
Congratulations. You are now all computer programs. Okay, because you got Karel to 

411
00:19:41,230 --> 00:19:42,860
do what you wanted to do, and you're like, "Oh, that's so 

412
00:19:42,859 --> 00:19:44,699
good. Let's do it again." 

413
00:19:44,700 --> 00:19:47,710
And so you're like, "Yeah, let me start the program again." 

414
00:19:47,710 --> 00:19:50,259
Wah-wah-wah, thanks for playing. 

415
00:19:50,259 --> 00:19:51,170
What happened? 

416
00:19:51,170 --> 00:19:55,410
It tried to execute the same program again, but the world was in a different 

417
00:19:55,410 --> 00:19:59,070
state, right? Karel was facing that wall, and the first thing we said is, "Hey, 

418
00:19:59,069 --> 00:20:00,929
Karel, move." And he's like, 

419
00:20:00,930 --> 00:20:04,200
"Okay." He's a little happy, scrappy robot. He doesn't know anything about the world, 

420
00:20:04,200 --> 00:20:07,590
and he just, wham, into the wall. Thanks for playing. 

421
00:20:07,589 --> 00:20:09,899
Dead Karel. All right, 

422
00:20:09,900 --> 00:20:14,290
no, because Karel's blocked, okay, and you get this little bug on top of Karel. It's like 

423
00:20:14,289 --> 00:20:18,389
Alfred Hitchcock, the birds, he's just being, like, harangued by this little bug. 

424
00:20:18,390 --> 00:20:21,270
But what that means is Karel tried to do something 

425
00:20:21,269 --> 00:20:24,259
in the world that the world would not allow him to do, and this is what happens 

426
00:20:24,259 --> 00:20:27,180
if you try to walk through a wall, or try to pick up a beeper from a corner where it 

427
00:20:27,180 --> 00:20:30,279
doesn't exist, or you try to put down a beeper and your beeper bag happens to be 

428
00:20:30,279 --> 00:20:30,950
empty, 

429
00:20:30,950 --> 00:20:32,379
that's the kind of error you'll get. 

430
00:20:32,378 --> 00:20:35,628
So if you want to start this program again, you need to load world - and we 

431
00:20:35,628 --> 00:20:37,329
happen to have first Karel program, 

432
00:20:37,329 --> 00:20:41,609
you need to reload your world and then you can start it again from that point. Okay, 

433
00:20:41,609 --> 00:20:48,609
so any questions about our first Karel program? Uh huh? [Inaudible]? 

434
00:20:49,640 --> 00:20:52,840
Yeah, he's moving basically from one grid point to the next grind point. 

435
00:20:52,839 --> 00:20:58,339
So he just moves one step at a time. Uh huh? 

436
00:20:58,339 --> 00:21:03,419
Is Karel case sensitive? Karel is case sensitive, yes. Everything you type will be case sensitive. My 

437
00:21:03,420 --> 00:21:06,730
throwing is case sensitive as well, and evidently, that was the wrong case. All 

438
00:21:06,730 --> 00:21:09,460
right, so something else we'd like to be able to do with Karel, and a few people 

439
00:21:09,460 --> 00:21:10,680
already pointed out is, 

440
00:21:10,680 --> 00:21:14,060
"Hey, we created this thing called run. That's kind of cool. Can I create other 

441
00:21:14,059 --> 00:21:14,690
stuff?" 

442
00:21:14,690 --> 00:21:18,140
And as a matter of fact, you can, because that's just how snazzy Karel is. 

443
00:21:18,140 --> 00:21:20,538
And one thing, you might look at this and be like, "Ooh, ooh, 

444
00:21:20,538 --> 00:21:23,148
ooh, I know what I want to create." What would you want to create if you could 

445
00:21:23,148 --> 00:21:25,250
create a new command? 

446
00:21:25,250 --> 00:21:31,289
Turn right. Turn right. Right? That's a social, right? Everyone wants to turn right. All right, 

447
00:21:31,289 --> 00:21:34,000
except the person who actually said, "Turn right." So 

448
00:21:34,000 --> 00:21:38,259
what we can do is we can have a similar syntax that allows us to create new 

449
00:21:38,259 --> 00:21:40,859
methods in Karel, like the run method 

450
00:21:40,859 --> 00:21:43,428
actually is the place where Karel will always start. 

451
00:21:43,429 --> 00:21:46,690
But we can create new methods that we could use, and so maybe we want to create a 

452
00:21:46,690 --> 00:21:49,460
turn right. So we could say void, sorry, 

453
00:21:49,460 --> 00:21:52,329
here we'd say 

454
00:21:52,329 --> 00:21:53,699
private void. 

455
00:21:53,700 --> 00:21:56,900
Voice - sometimes typing is bad, especially when your hands are a little 

456
00:21:56,900 --> 00:22:01,130
chalky. So run - you'll notice run is always public void run. 

457
00:22:01,130 --> 00:22:03,830
For all the other commands that we're going to do as far as Karel's concerned, we're 

458
00:22:03,829 --> 00:22:07,579
going to use private void, and the name of the command. So we'll call this turn 

459
00:22:07,579 --> 00:22:09,049
right, 

460
00:22:09,049 --> 00:22:10,999
and this command will have a body, 

461
00:22:10,999 --> 00:22:14,610
and what is the body of the turn right command going to be? Turn left. 

462
00:22:14,609 --> 00:22:17,439
Turn left three times. So I'm just going to copy from here 

463
00:22:17,440 --> 00:22:21,880
and paste into - and the indenting even works out right. Rock on, right? 

464
00:22:21,880 --> 00:22:25,460
And what I've now done - I'll go ahead and save this - is I've created a new command 

465
00:22:25,460 --> 00:22:27,470
for Karel called turn right. 

466
00:22:27,470 --> 00:22:31,430
So anywhere else in my program, I can now say, "Turn right," like here are these three 

467
00:22:31,430 --> 00:22:32,009
turn lefts. 

468
00:22:32,009 --> 00:22:34,379
I can replace them. 

469
00:22:34,380 --> 00:22:35,820
Let me just get rid of them, 

470
00:22:35,819 --> 00:22:37,139
and I'll replace them 

471
00:22:37,140 --> 00:22:38,659
with a turn right, 

472
00:22:38,659 --> 00:22:40,429
and I'll appropriately indent that. 

473
00:22:40,429 --> 00:22:44,259
And what that means is, as you're executing the program, you move, you pickBeeper, you move, you 

474
00:22:44,259 --> 00:22:45,308
turn left, you move. 

475
00:22:45,308 --> 00:22:46,879
When you get to turn right, 

476
00:22:46,880 --> 00:22:49,220
it says, "Let me pause 

477
00:22:49,220 --> 00:22:51,089
where I saw that turn right. 

478
00:22:51,089 --> 00:22:56,119
I will go to the turn right method body and run all the instructions that are 

479
00:22:56,119 --> 00:22:59,969
in that body, and then I will return back to where I left off. 

480
00:22:59,970 --> 00:23:04,250
So this program is exactly equivalent to the program we wrote before, now in terms 

481
00:23:04,250 --> 00:23:09,460
of something called turn right, and just to prove that to you, we'll go ahead and run, 

482
00:23:09,460 --> 00:23:10,960
and there it is, 

483
00:23:10,960 --> 00:23:13,340
Karel does the equivalent of turn right and life is good. Okay, so 

484
00:23:13,339 --> 00:23:18,539
we've created a new command called turn right. Question? [Inaudible]? 

485
00:23:18,539 --> 00:23:20,259
The location's not important. 

486
00:23:20,259 --> 00:23:23,240
You can put it anywhere. I like to put it at the end, because that's convention in the 

487
00:23:23,240 --> 00:23:27,930
book, but you can put it anywhere. Uh huh? [Inaudible]? 

488
00:23:27,930 --> 00:23:31,509
Why is run always public? Because that's where things start, and for Karel to know where it is, 

489
00:23:31,509 --> 00:23:34,750
like private's all the stuff you're sort of like, eh, I'm keeping it to myself. 

490
00:23:34,750 --> 00:23:38,210
But Karel's like - he's, like, looking around. He's like, "Where do I run? What do I run? And 

491
00:23:38,210 --> 00:23:41,519
so you got to say, "Oh, it's public. It's available to the whole world." 

492
00:23:41,519 --> 00:23:44,579
And in a couple weeks when we get into Java, you'll see public and private rear their 

493
00:23:44,579 --> 00:23:51,579
ugly heads again and it'll all be clear. Uh huh? [Inaudible]? Pardon? 

494
00:23:53,579 --> 00:23:56,129
If you made that a public [inaudible]? It would still run. It would be fine. 

495
00:23:56,130 --> 00:23:59,450
It's just good programming style. See, there's all these things that we kind of 

496
00:23:59,450 --> 00:24:02,460
will eventually get to, but we got to start somewhere. So we sort of start right 

497
00:24:02,460 --> 00:24:03,419
in the middle 

498
00:24:03,419 --> 00:24:05,890
and we're going to build up from there, and eventually we'll go back and fill in 

499
00:24:05,890 --> 00:24:10,549
some of the pieces underneath. Uh huh? Do you have to [inaudible]? 

500
00:24:10,548 --> 00:24:14,450
Run is where - has to be called run. That's where Karel always knows where to 

501
00:24:14,450 --> 00:24:17,590
start, but everything else could be something else. Like, instead of turn right, I 

502
00:24:17,589 --> 00:24:20,109
don't know, what's the French word for turn right, 

503
00:24:20,109 --> 00:24:25,129
or the French two words for turn right? Anyone speak French? [Inaudible]. 

504
00:24:25,130 --> 00:24:25,640
Exactly. 

505
00:24:25,640 --> 00:24:29,880
We could have called it that if we wanted to, and then rather then turn right we would use 

506
00:24:29,880 --> 00:24:33,419
that, and it would be fine. So all the other things besides run, we can name 

507
00:24:33,419 --> 00:24:36,850
them whatever we want as long as we're consistent throughout. All right, 

508
00:24:36,849 --> 00:24:39,799
so we could also - you might say something else might be kind of fun to do is to 

509
00:24:39,799 --> 00:24:42,599
have a command called turn around, for example, 

510
00:24:42,599 --> 00:24:46,939
which turns Karel around 90 degrees. So 

511
00:24:46,940 --> 00:24:49,759
we'll just do a little copy and paste magic 

512
00:24:49,759 --> 00:24:53,579
and have turn around here. We might not actually use it in this program, but just to 

513
00:24:53,579 --> 00:24:54,148
have, 

514
00:24:54,148 --> 00:24:57,648
and turn around is actually just two turn lefts. It turns Karel around, and if you really 

515
00:24:57,648 --> 00:25:01,209
wanted to do it, you could do it as two turn rights, but that's kind of six turn lefts, 

516
00:25:01,210 --> 00:25:04,539
and that's not really a good idea. Okay, so 

517
00:25:04,539 --> 00:25:08,388
because turn right and turn around are such common things to do, 

518
00:25:08,388 --> 00:25:11,918
we actually have a souped up version of Karel. It's sort of like Karel with the training 

519
00:25:11,919 --> 00:25:13,320
wheels taken off, 

520
00:25:13,319 --> 00:25:17,279
and he's bad, and he's buff, and he knows everything Karel knows, all four 

521
00:25:17,279 --> 00:25:18,109
methods, 

522
00:25:18,109 --> 00:25:21,678
plus two more, which is turn right and turn around built in. 

523
00:25:21,679 --> 00:25:25,240
And that's super Karel. So if we get rid of these and we say, "Hey, our Karel program, 

524
00:25:25,240 --> 00:25:26,799
instead of extending Karel, 

525
00:25:26,799 --> 00:25:29,869
is going to extend the super Karel model." Okay, 

526
00:25:29,869 --> 00:25:33,019
oh, I know, it's dangerous, but it's okay. 

527
00:25:33,019 --> 00:25:35,190
Super Karel, we've gotten rid of 

528
00:25:35,190 --> 00:25:38,850
the turn right definition and the turn around definition, but if we run 

529
00:25:38,849 --> 00:25:41,869
this now, 

530
00:25:41,869 --> 00:25:43,899
start the program, 

531
00:25:43,900 --> 00:25:45,009
Karel 

532
00:25:45,009 --> 00:25:51,400
actually will do the right thing. 

533
00:25:51,400 --> 00:25:54,750
All right, let me quit out of this, unless my computer 

534
00:25:54,750 --> 00:25:59,079
decides to freeze, and sometimes it decides to freeze, and that would be bad. All right, 

535
00:25:59,079 --> 00:26:02,710
so at this point you're Karel programmers. You're rocking. You know all the Karel's 

536
00:26:02,710 --> 00:26:05,679
methods. That's all he knows, at least for the time being, 

537
00:26:05,679 --> 00:26:07,720
and you know how to actually make 

538
00:26:07,720 --> 00:26:09,960
new commands by 

539
00:26:09,960 --> 00:26:13,600
creating new definitions that have a body and the syntax around them to create new commands. 

540
00:26:13,599 --> 00:26:15,009
All right, 

541
00:26:15,009 --> 00:26:16,829
so now what we're going to do 

542
00:26:16,829 --> 00:26:19,339
is I want to ask you a philosophical question. I 

543
00:26:19,339 --> 00:26:22,538
know, I guess you're like, this is a computer science class, why do we have to 

544
00:26:22,538 --> 00:26:23,480
do philosophy in 

545
00:26:23,480 --> 00:26:24,980
here? Because it's good for you, 

546
00:26:24,980 --> 00:26:26,519
and the philosophical question is 

547
00:26:26,519 --> 00:26:30,129
what is the downfall of the modern college student? Anyone 

548
00:26:30,130 --> 00:26:33,050
know? You're 

549
00:26:33,049 --> 00:26:33,878
like, what does that 

550
00:26:33,878 --> 00:26:34,579
mean? 

551
00:26:34,579 --> 00:26:35,470
You're like, I 

552
00:26:35,470 --> 00:26:38,960
haven't fallen down yet. That's good. Hopefully it won't be that way. 

553
00:26:38,960 --> 00:26:42,419
But there is something, probably in your everyday life, that's just sitting there 

554
00:26:42,419 --> 00:26:45,050
niggling. Uh huh? 

555
00:26:45,049 --> 00:26:50,038
Procrastination. Procrastination? It's related to that. What do you do to procrastinate in the morning? Sleep. Sleep. Oh, 

556
00:26:50,038 --> 00:26:57,038
sorry, yeah, and how do you prolong that? [Inaudible]. Snooze. 

557
00:26:58,019 --> 00:27:01,720
And the thing about snooze is, it's amazing how quickly you can do math in the 

558
00:27:01,720 --> 00:27:04,799
morning. I don't know about you, but my alarm has a nine-minute snooze on it. Why it's nine, 

559
00:27:04,799 --> 00:27:07,599
I don't know, but it came out of the factory with a nine-minute snooze. And so you're, 

560
00:27:07,599 --> 00:27:09,819
like, lying there. I remember when I was a student, like, 

561
00:27:09,819 --> 00:27:13,099
the dreaded, like, 10:00 a.m. class, 9:00 a.m. class. Heaven forbid you ever 

562
00:27:13,099 --> 00:27:15,319
have a 9:00 a.m. class, 

563
00:27:15,319 --> 00:27:18,519
and you're sitting there, and, like, the alarm goes off, and you're like, 

564
00:27:18,519 --> 00:27:21,410
yeah, not going to brush my teeth today. Snooze. 

565
00:27:21,410 --> 00:27:24,540
And then you think about it a little bit and nine minutes later, it goes off, and you're 

566
00:27:24,539 --> 00:27:27,920
like, okay, it must be, like, 8:09 a.m. now. And then the math kicks in. You're 

567
00:27:27,920 --> 00:27:31,539
like, if I don't take a shower, that's like two more whacks on the snooze bar, snooze, snooze. 

568
00:27:31,539 --> 00:27:34,579
And now you're up to like 8:27 a.m. You're just doing 

569
00:27:34,579 --> 00:27:37,759
this all while you're, like, have asleep, right. It's like calculus when you're fully awake, 

570
00:27:37,759 --> 00:27:39,509
yeah, impossible. Snooze bar, 

571
00:27:39,509 --> 00:27:43,430
you could have, like, your brain removed in a lobotomy and you could still figure it out. 

572
00:27:43,430 --> 00:27:46,650
But what the snooze bar actually gives you is a way of doing something a 

573
00:27:46,650 --> 00:27:49,480
certain number of times when you want to be able to do it a certain number of 

574
00:27:49,480 --> 00:27:52,288
times. And so you might say, "Hey, Karel, 

575
00:27:52,288 --> 00:27:54,269
sometimes rather than just, like, 

576
00:27:54,269 --> 00:27:57,349
telling you turn, left turn, left turn, left three times, can I just tell 

577
00:27:57,349 --> 00:28:00,469
you to turn left three times? Is there some way of doing that?" And in fact, in Karel 

578
00:28:00,470 --> 00:28:01,179
there is, and 

579
00:28:01,179 --> 00:28:04,030
it's something called a for-loop, and the syntax for 

580
00:28:04,029 --> 00:28:05,638
this 

581
00:28:05,638 --> 00:28:07,409
looks like this, 

582
00:28:07,410 --> 00:28:08,029
for, 

583
00:28:08,029 --> 00:28:11,069
just the word for, paren, 

584
00:28:11,069 --> 00:28:15,139
int I equals zero. And so this might look a little bit strange, and if you've 

585
00:28:15,140 --> 00:28:18,090
done some programming before, it won't look strange, and if you haven't done any programming, 

586
00:28:18,089 --> 00:28:21,278
this is just the syntax. Just use it and feel good about it. 

587
00:28:21,278 --> 00:28:26,559
This is int I equals zero, semicolon, 

588
00:28:26,559 --> 00:28:30,250
I is less than the number you want to count up to. So if I want to do something 

589
00:28:30,250 --> 00:28:34,490
three times, I'd have a three here, but in general this could be any number that 

590
00:28:34,490 --> 00:28:35,950
you want. 

591
00:28:35,950 --> 00:28:41,809
Then another semicolon, and then an I followed by a plus and a plus with no spaces, 

592
00:28:41,809 --> 00:28:44,079
and then a close paren. 

593
00:28:44,079 --> 00:28:47,349
And then after all that, you have a brace, an 

594
00:28:47,349 --> 00:28:51,889
open brace, and there's a close brace down here, and everything that goes here is 

595
00:28:51,890 --> 00:28:55,549
the body of what we refer to as the for-loop, 

596
00:28:55,548 --> 00:28:59,929
and that body gets executed however many times you say here. 

597
00:28:59,929 --> 00:29:03,519
It can be one instruction. It can be more than one instruction. So turn left 

598
00:29:03,519 --> 00:29:05,730
could actually be defined 

599
00:29:05,730 --> 00:29:07,980
- or turn right could be defined 

600
00:29:07,980 --> 00:29:12,230
as for I equals zero, I less than three, I 

601
00:29:12,230 --> 00:29:13,149
plus, plus, 

602
00:29:13,148 --> 00:29:16,979
turn left. And what this will do is it will execute this three times. I could have 

603
00:29:16,980 --> 00:29:20,450
multiple instructions in here if I want to, and it would execute that whole set 

604
00:29:20,450 --> 00:29:21,750
sequentially three times, 

605
00:29:21,750 --> 00:29:24,679
but this is the equivalent of turn right. Okay, 

606
00:29:24,679 --> 00:29:28,019
question? [Inaudible]? 

607
00:29:28,019 --> 00:29:30,049
So the spaces are, 

608
00:29:30,049 --> 00:29:30,569
sorry, 

609
00:29:30,569 --> 00:29:32,579
there is a space here. 

610
00:29:32,579 --> 00:29:34,829
Well, you don't actually need a space there, 

611
00:29:34,829 --> 00:29:38,449
but there are no spaces in here. So you need to have this space for sure. 

612
00:29:38,450 --> 00:29:41,509
The rest of these spaces are basically optional, but we like to kind of space 

613
00:29:41,509 --> 00:29:43,759
them out for it to make sense. 

614
00:29:43,759 --> 00:29:47,538
You definitely should not have any spaces in the I plus, plus. There's no 

615
00:29:47,538 --> 00:29:51,220
spaces in there. Uh huh? Is all of this syntax in the course reader? 

616
00:29:51,220 --> 00:29:54,190
It's all in the course reader, yeah. Yeah, 

617
00:29:54,190 --> 00:29:56,330
so you don't need to worry about - if you want to take notes, 

618
00:29:56,329 --> 00:29:59,549
that's great, but it's all kind of in the course reader if you want to follow along. So 

619
00:29:59,549 --> 00:30:02,909
you can do something any number of times that you want, right, and it'll just count up. So 

620
00:30:02,910 --> 00:30:04,019
this could be, like, 

621
00:30:04,019 --> 00:30:05,099
if you wanted to, 

622
00:30:05,099 --> 00:30:08,740
you could say, "Hey, turn left six times," and that's still the equivalent of turn right. Or 

623
00:30:08,740 --> 00:30:14,880
we can do the equivalent of snooze, which is that. Right, 

624
00:30:14,880 --> 00:30:17,179
so basically what does snooze do? 

625
00:30:17,179 --> 00:30:19,960
Nothing. You just sit there, and you spin around a few times, and you're kind of like 

626
00:30:19,960 --> 00:30:24,140
in the same spot, facing the same direction when you were done. Okay, but 

627
00:30:24,140 --> 00:30:27,369
anytime you want to do something some number of times, you can actually do it 

628
00:30:27,368 --> 00:30:30,608
using this for construct. And this is the way you should remember it. You're always just going 

629
00:30:30,608 --> 00:30:33,058
to use this syntax in 

630
00:30:33,058 --> 00:30:36,308
Karel, and the only thing that's going to change is this number of times that you 

631
00:30:36,308 --> 00:30:37,410
want to do something. 

632
00:30:37,410 --> 00:30:38,690
All right, 

633
00:30:38,690 --> 00:30:40,039
so besides that 

634
00:30:40,039 --> 00:30:43,750
there's also sometimes, though, where you don't know how many times beforehand 

635
00:30:43,750 --> 00:30:46,890
you want to do something. And you might say, "Huh, that's kind of weird. When would I not 

636
00:30:46,890 --> 00:30:49,120
know how many times I want to do something?" 

637
00:30:49,119 --> 00:30:53,009
Well, let's say I'm Karel, and I'm standing here, and there's a wall over there. 

638
00:30:53,009 --> 00:30:56,420
And I know there's a wall somewhere over there, because my world is finite. It has to 

639
00:30:56,420 --> 00:31:00,470
end at some point, but one told me how many steps it is to that wall. So if I say, 

640
00:31:00,470 --> 00:31:04,829
"Hey, I'm going to take ten steps to that wall using a for-loop." One, two, three, 

641
00:31:04,829 --> 00:31:06,349
four, five, six, 

642
00:31:06,349 --> 00:31:10,909
and then I'm like, oh, dead Karel, right, and that's a bad time. So 

643
00:31:10,910 --> 00:31:14,090
sometimes, we just don't know when the program starts, and what we want to say is, 

644
00:31:14,089 --> 00:31:15,809
"Do something 

645
00:31:15,809 --> 00:31:19,359
while some condition is true," like, keep going forward as long as there's no wall 

646
00:31:19,359 --> 00:31:23,459
in front of you. And when there's a wall in front of you, stop going forward. All right, 

647
00:31:23,460 --> 00:31:24,929
so the way we do that 

648
00:31:24,929 --> 00:31:28,710
is something called a while-loop. So 

649
00:31:28,710 --> 00:31:32,058
at this point, hopefully you're intimately familiar with all of Karel's 

650
00:31:32,058 --> 00:31:32,930
instructions. 

651
00:31:32,930 --> 00:31:35,590
And the way the while-loop works, 

652
00:31:35,589 --> 00:31:38,449
while, because that's what you want to do something. You want to do something while 

653
00:31:38,450 --> 00:31:40,620
there is some condition that's true, 

654
00:31:40,619 --> 00:31:43,668
is the syntax looks like this, while, 

655
00:31:43,669 --> 00:31:45,038
and then a paren, 

656
00:31:45,038 --> 00:31:48,849
and inside here we're going to have some condition. 

657
00:31:48,849 --> 00:31:51,539
Okay, so I'm going to put a squiggly line under it. So you don't put the word condition. We're 

658
00:31:51,539 --> 00:31:53,319
going to put a condition in there in just a second, 

659
00:31:53,319 --> 00:31:54,599
a close paren, 

660
00:31:54,599 --> 00:31:56,019
and then we're going to have 

661
00:31:56,019 --> 00:31:59,440
some body inside braces, open brace, close brace. 

662
00:31:59,440 --> 00:32:01,610
Now, where do these conditions come from? 

663
00:32:01,609 --> 00:32:05,439
So one thing we can think about is the notion of moving until we get to a wall. 

664
00:32:05,440 --> 00:32:09,269
And so Karel, it turns out, has some things that he can sense about his 

665
00:32:09,269 --> 00:32:12,139
world, which are conditions you can check to see if there's something going on in 

666
00:32:12,140 --> 00:32:13,009
this world. 

667
00:32:13,009 --> 00:32:15,369
One of the things you can check is 

668
00:32:15,369 --> 00:32:17,629
front is 

669
00:32:17,630 --> 00:32:19,540
clear, 

670
00:32:19,539 --> 00:32:22,700
and that's an open paren and a close paren, and so 

671
00:32:22,700 --> 00:32:24,528
there's actually two parens. 

672
00:32:24,528 --> 00:32:27,869
Lower case F, capital I, capital C, all one word. 

673
00:32:27,869 --> 00:32:30,558
And so what that asks Karel to do is say, 

674
00:32:30,558 --> 00:32:34,109
"Where you're standing right now is your front clear," which means is there a wall 

675
00:32:34,109 --> 00:32:37,089
right in front of the direction you're facing. If 

676
00:32:37,089 --> 00:32:38,869
your front is clear, 

677
00:32:38,869 --> 00:32:42,119
so while your front is clear, what it does, it basically checks if the front is clear, 

678
00:32:42,119 --> 00:32:44,769
and that's true, then it executes the body of 

679
00:32:44,769 --> 00:32:45,690
the while loop. 

680
00:32:45,690 --> 00:32:48,538
So we could say, for example, move 

681
00:32:48,538 --> 00:32:51,750
here. And so what it's going to do is, while my front is clear I move and it's still 

682
00:32:51,750 --> 00:32:54,659
clear, and it's still clear, and I just keep doing this. 

683
00:32:54,659 --> 00:32:56,499
And at some point, I move along, 

684
00:32:56,499 --> 00:33:00,068
and my front is no longer clear at this point. Front is clear is no longer 

685
00:33:00,068 --> 00:33:02,750
true, and so I stop. Okay, 

686
00:33:02,750 --> 00:33:04,349
and basically at 

687
00:33:04,349 --> 00:33:07,889
that point I stop executing this loop and I just pick up from 

688
00:33:07,890 --> 00:33:09,700
right after the close brace. So I'm 

689
00:33:09,700 --> 00:33:12,659
after I'm done executing a loop, whether it's a for-loop or a while-loop, I just 

690
00:33:12,659 --> 00:33:15,280
pick up executing after that close brace. Okay, 

691
00:33:15,279 --> 00:33:19,759
so I could actually turn this, for example, into a new method, 

692
00:33:19,759 --> 00:33:21,349
private, void, 

693
00:33:21,349 --> 00:33:24,759
walk to wall, or we'll call it move to wall like that. 

694
00:33:24,759 --> 00:33:28,869


695
00:33:28,869 --> 00:33:32,398
And then anytime I basically want to tell Karel, "Just keep moving forward until you hit 

696
00:33:32,398 --> 00:33:34,738
a wall," I just essentially 

697
00:33:34,739 --> 00:33:38,479
invoke, or call the move to wall method, and it will just get me up until I get to a 

698
00:33:38,479 --> 00:33:39,269
wall. 

699
00:33:39,269 --> 00:33:42,048
Now you might wonder, hey, where are all these conditions and what are all the 

700
00:33:42,048 --> 00:33:44,869
conditions? So if we can get the overhead for just a second, 

701
00:33:44,869 --> 00:33:45,869
I'll show you. 

702
00:33:45,869 --> 00:33:49,009
Page 18 of your reader. 

703
00:33:49,009 --> 00:33:51,509


704
00:33:51,509 --> 00:33:56,289
So on page 18, there's the lovely table one. 

705
00:33:56,289 --> 00:33:59,409
Oh, lovely table one, and these are all the conditions you can check in Karel. 

706
00:33:59,410 --> 00:34:02,620
There's front is clear, left is clear, right is clear. So you can sort of check to your 

707
00:34:02,619 --> 00:34:03,448
left and right. 

708
00:34:03,449 --> 00:34:05,740
There's no behind is clear. 

709
00:34:05,740 --> 00:34:07,980
Important for babies, not for Karel. 

710
00:34:07,980 --> 00:34:11,760
There's beepers present, which means is there a beeper, at least one on the corner 

711
00:34:11,760 --> 00:34:13,129
that you're at. Are 

712
00:34:13,128 --> 00:34:14,548
there beepers in your bag? 

713
00:34:14,548 --> 00:34:18,298
Is your bag not empty? Are you facing north, east, south, or west? So you can 

714
00:34:18,298 --> 00:34:19,679
check all those conditions. 

715
00:34:19,679 --> 00:34:23,459
You can also check the opposite of all the conditions, like front is blocked. 

716
00:34:23,458 --> 00:34:25,739
So you can actually check, like while your front 

717
00:34:25,739 --> 00:34:28,969
is blocked you might want to turn left, because you want 

718
00:34:28,969 --> 00:34:32,588
to keep turning until eventually your front gets cleared up, and then maybe you go 

719
00:34:32,588 --> 00:34:33,668
do something else. 

720
00:34:33,668 --> 00:34:37,099
Or your left is blocked, or your right is blocked. So these are all - I'm not going 

721
00:34:37,099 --> 00:34:39,869
to go through them all, but they're all sort of obvious. They sort of give you 

722
00:34:39,869 --> 00:34:43,009
the obvious information that you would expect about Karel's world, and they're all in your 

723
00:34:43,010 --> 00:34:45,659
course reader on page 18. All right, 

724
00:34:45,659 --> 00:34:47,818
so given that you have these 

725
00:34:47,818 --> 00:34:49,628
conditions you can check. 

726
00:34:49,628 --> 00:34:52,498
Sometimes you want to do something while some condition is true, and as soon as 

727
00:34:52,498 --> 00:34:55,578
this condition is no longer true, you stop executing. 

728
00:34:55,579 --> 00:34:59,209
There's also sometimes that rather than doing something over and over, you just 

729
00:34:59,208 --> 00:35:01,349
want to say, let me check some condition 

730
00:35:01,349 --> 00:35:05,449
and if it's true, then do this one thing. So I'm not going to do it as a loop, but 

731
00:35:05,449 --> 00:35:08,028
I'm just going to done thing. Okay, 

732
00:35:08,028 --> 00:35:11,699
and that's something called an if statement, and the way an 

733
00:35:11,699 --> 00:35:13,188
if statement works 

734
00:35:13,188 --> 00:35:16,078
is it's if, 

735
00:35:16,079 --> 00:35:20,329
space, open paren, some condition, again I'll put this condition in squigglies, 

736
00:35:20,329 --> 00:35:21,429


737
00:35:21,429 --> 00:35:26,329
open brace, close brace, and this is the body. And it says, if some condition is true, 

738
00:35:26,329 --> 00:35:30,179
then do what it's in the braces. And if it's not true, just completely skip over this 

739
00:35:30,179 --> 00:35:33,478
and keep executing from right after the braces. 

740
00:35:33,478 --> 00:35:36,668
So you might want to say, "Hey, I only want to pick up a beeper if I know a beeper exists in 

741
00:35:36,668 --> 00:35:39,969
the world." So I sort of want to have a notion of having a safe 

742
00:35:39,969 --> 00:35:43,889
pickup of a beeper. So I could say if 

743
00:35:43,889 --> 00:35:44,990
beeper 

744
00:35:44,989 --> 00:35:45,778


745
00:35:45,778 --> 00:35:49,668
is present, which means there is a beeper, at least one on the corner that I'm at, then it's 

746
00:35:49,668 --> 00:35:50,719
safe 

747
00:35:50,719 --> 00:35:52,509
to pick a beeper. 

748
00:35:52,509 --> 00:35:55,369
Because if I didn�ft necessarily check before, 

749
00:35:55,369 --> 00:35:57,969
and I didn't know there was a beeper there for sure, I might try to pick up 

750
00:35:57,969 --> 00:36:01,208
a beeper on some corner, and there isn't a beeper, and Karel blows up, and life 

751
00:36:01,208 --> 00:36:02,509
is unhappy, and 

752
00:36:02,510 --> 00:36:06,229
tears well in your eye, and the whole deal. All right, so 

753
00:36:06,228 --> 00:36:07,578
if statement, 

754
00:36:07,579 --> 00:36:10,778
that's just what we refer to it as kind of as an if statement. It's all 

755
00:36:10,778 --> 00:36:11,670
lower case. 

756
00:36:11,670 --> 00:36:16,130
Now, sometimes what happens in life is you don't want to just say, "If 

757
00:36:16,130 --> 00:36:18,140
something is true, then 

758
00:36:18,139 --> 00:36:21,868
essentially what you want to do is this." You want to say, "If that thing is false, 

759
00:36:21,869 --> 00:36:24,640
then I want to do something different." Okay, 

760
00:36:24,639 --> 00:36:28,208
so you might want to say, "Hey, if there's a beeper present, I'm going to pick I up. 

761
00:36:28,208 --> 00:36:31,928
But if there isn't one there, oh, Karel feels a little sad and he's going to put a 

762
00:36:31,929 --> 00:36:36,329
beeper down to, like, plant a new tree because he wants to fight global warming." 

763
00:36:36,329 --> 00:36:38,180
So else 

764
00:36:38,179 --> 00:36:41,179
and then we're going to have another body here, 

765
00:36:41,179 --> 00:36:42,288
putBeeper, 

766
00:36:42,289 --> 00:36:45,839
assuming that Karel has beepers in his bag. So what this says is, if 

767
00:36:45,838 --> 00:36:47,190
there's a beeper present, 

768
00:36:47,190 --> 00:36:48,809
then pick it up. 

769
00:36:48,809 --> 00:36:51,340
If there is not a beeper present, it doesn't 

770
00:36:51,340 --> 00:36:55,130
try to pick a beeper, it skips this part and does the part that's referred 

771
00:36:55,130 --> 00:36:56,380
to by the else. 

772
00:36:56,380 --> 00:36:57,590
So we'll put a beeper. 

773
00:36:57,590 --> 00:37:02,700
So it will only execute one of these bodies, and which body it does depends on whether or 

774
00:37:02,699 --> 00:37:05,599
not there are beepers present. If it's true, it's this one, if it's false, if that's one. Question? Is that a 

775
00:37:05,599 --> 00:37:10,818
space 

776
00:37:10,818 --> 00:37:14,380
or a tab [inaudible]? These are spaces here. So these are required to be spaces, and the syntax is also 

777
00:37:14,380 --> 00:37:21,380
all in book too. So you can see where the spaces are there as well. Uh huh? Can you make multiple [inaudible]? 

778
00:37:22,679 --> 00:37:25,470
For the time being, you should just check one condition at a time. 

779
00:37:25,469 --> 00:37:29,368
Now, you can actually sort of, what we refer to as nest, 

780
00:37:29,369 --> 00:37:30,650
these kind of statements. 

781
00:37:30,650 --> 00:37:34,280
So you could say, "If beeper is present," you might say, "Hey, if there's a beeper 

782
00:37:34,280 --> 00:37:37,420
present there, that's some marker that I should move forward if there is not a wall in front 

783
00:37:37,420 --> 00:37:38,389
of me." 

784
00:37:38,389 --> 00:37:42,359
So what I want to do is, if there is a beeper present, what I now want to do 

785
00:37:42,360 --> 00:37:46,509
is check to see if I can move forward. So if 

786
00:37:46,509 --> 00:37:47,548
front 

787
00:37:47,548 --> 00:37:52,528
is clear, 

788
00:37:52,528 --> 00:37:55,478
move. 

789
00:37:55,478 --> 00:37:58,798
And this is what we refer to as a nested expression. Notice how I sort of tabbed it 

790
00:37:58,798 --> 00:38:02,858
in a little bit, so that it's kind of nested inside this other guy. 

791
00:38:02,858 --> 00:38:05,288
What it means if check to see if beepers are present. 

792
00:38:05,289 --> 00:38:07,410
If they're not present, then it skips 

793
00:38:07,409 --> 00:38:08,518
this whole body 

794
00:38:08,518 --> 00:38:10,189
and it just puts a beeper down. 

795
00:38:10,190 --> 00:38:13,400
If there is a beeper present, it's going to executed this body. Well, what is the 

796
00:38:13,400 --> 00:38:16,749
body? It's another statement that says, "Is your front clear?" 

797
00:38:16,748 --> 00:38:19,608
If your front is clear, you say, "Hey, I was on a beeper, and my front is clear. 

798
00:38:19,608 --> 00:38:22,088
Now, I'll take a step forward." But 

799
00:38:22,088 --> 00:38:25,748
if there was a beeper present, and my front was clear, or was not clear, I'm 

800
00:38:25,748 --> 00:38:27,448
not going to take a step forward, 

801
00:38:27,449 --> 00:38:30,858
in which case it doesn't do the if part. It finishes up 

802
00:38:30,858 --> 00:38:34,130
and there's nothing else for it to do here. So it's done with this whole part and 

803
00:38:34,130 --> 00:38:37,670
it's done with the whole if-else statement. Okay, 

804
00:38:37,670 --> 00:38:39,289
any questions about that, 

805
00:38:39,289 --> 00:38:41,079
what's actually going on there, right? 

806
00:38:41,079 --> 00:38:43,900
If beepers are present, you know you're only going to execute this part. This 

807
00:38:43,900 --> 00:38:46,528
part is now a goner, because that's the else part, and 

808
00:38:46,528 --> 00:38:47,909
beepers present was true. 

809
00:38:47,909 --> 00:38:51,059
So to do this part, you come in here and you say, "Is front clear?" 

810
00:38:51,059 --> 00:38:54,629
No, front's not clear. I have some wall in front of me. Oh, okay, well I'm not going to do the 

811
00:38:54,628 --> 00:38:56,719
move. Is there an else over here? No. 

812
00:38:56,719 --> 00:39:00,219
This else is not attached here. This else is attached to this if that says, oh, there's no 

813
00:39:00,219 --> 00:39:02,889
else. Okay, there's nothing else 

814
00:39:02,889 --> 00:39:05,608
I should do. Are there any other statements over here that I should 

815
00:39:05,608 --> 00:39:06,248
execute? 

816
00:39:06,248 --> 00:39:09,209
No, I'm done with the whole body for this part. So it's done 

817
00:39:09,210 --> 00:39:14,659
with the entire statement and it continues executing from down there. Okay, 

818
00:39:14,659 --> 00:39:19,588
so any questions about that? 

819
00:39:19,588 --> 00:39:22,849
Uh huh? So is it every brace always comes in pairs? Every brace comes in pairs. You always have an open brace and there's going to 

820
00:39:22,849 --> 00:39:27,318
be some close brace that's going to define the body. Oh, that was the double. All right, 

821
00:39:27,318 --> 00:39:28,639
so what we're going to do 

822
00:39:28,639 --> 00:39:31,998
is we're going to put this all together in a big program. 

823
00:39:31,998 --> 00:39:35,408
Okay, so we're going to use all these things in a big program, and this program, 

824
00:39:35,409 --> 00:39:38,890
I'll show you, is basically, Karel is going to run a little steeplechase. You're 

825
00:39:38,889 --> 00:39:41,558
basically sort of like running hurdles, okay. 

826
00:39:41,559 --> 00:39:45,869
So we're going to open up another program. Don't worry about all the details right now. 

827
00:39:45,869 --> 00:39:49,440
I'm just going to run this so I can show you what it looks like. 

828
00:39:49,440 --> 00:39:53,679
So you can see what the world looks like and what we want Karel to do. So 

829
00:39:53,679 --> 00:39:55,930
we're running, we're running, 

830
00:39:55,929 --> 00:39:58,538
steeplechase. 

831
00:39:58,539 --> 00:40:01,910
So what Karel's going to do here 

832
00:40:01,909 --> 00:40:05,949
is they're the world that is always nine avenues long, and he knows that. It's 

833
00:40:05,949 --> 00:40:07,930
always nine avenues long, 

834
00:40:07,929 --> 00:40:11,699
which means there are, at most, eight steeples in the world, or eight hurdles. 

835
00:40:11,699 --> 00:40:16,018
The reason why we call them steeples as opposed to hurdles is because some of them are big. 

836
00:40:16,018 --> 00:40:20,138
Some of them are just ginormous, and so what Karel needs to do is he needs to start 

837
00:40:20,139 --> 00:40:20,690
here 

838
00:40:20,690 --> 00:40:25,079
and end up over here, and run around every hurdle he encounters. 

839
00:40:25,079 --> 00:40:28,089
So how are we going to do that using the stuff that we've learned, and putting it 

840
00:40:28,088 --> 00:40:29,909
together into a larger program? 

841
00:40:29,909 --> 00:40:32,899
Well, the first thing we're going to do is we need to start with our friend, the 

842
00:40:32,900 --> 00:40:33,769
run method. 

843
00:40:33,769 --> 00:40:36,608
Right, so in the run method, 

844
00:40:36,608 --> 00:40:39,960
all, Eclipse doesn't jump out in front of me. Right, so notice we have all the kind of other 

845
00:40:39,960 --> 00:40:41,300
boilerplate that we want to have. 

846
00:40:41,300 --> 00:40:44,690
We import Stanford Karel. We're going to have some public class we'll call 

847
00:40:44,690 --> 00:40:45,340
steeplechase that 

848
00:40:45,340 --> 00:40:49,269
extends super Karel. Right, all the stuff you've seen before as boilerplate. 

849
00:40:49,268 --> 00:40:51,379
We're going to define the run method 

850
00:40:51,380 --> 00:40:55,090
and what the run method is going to do is, if you have nine avenues you need 

851
00:40:55,090 --> 00:40:58,269
to go through, that means intervening the avenues, there's at most 

852
00:40:58,269 --> 00:40:59,329
eight steeples. 

853
00:40:59,329 --> 00:41:02,969
So I want you to do something eight times. So we're going to have a for-loop that 

854
00:41:02,969 --> 00:41:04,628
executes eight times. 

855
00:41:04,628 --> 00:41:06,299
What are you going to do inside there 

856
00:41:06,300 --> 00:41:09,289
Well, you have a choice. When you look at the world, 

857
00:41:09,289 --> 00:41:13,950
either you're facing a steeple or you're not. So if we look over here, right, here 

858
00:41:13,949 --> 00:41:17,558
Karel starts out facing a steeple, but after he jumps over it, hey it's clear. So he 

859
00:41:17,559 --> 00:41:20,298
could actually move forward before jumping the next steeple. 

860
00:41:20,298 --> 00:41:25,449
So that's essentially what we check for in the program. At every step, what we say is, 

861
00:41:25,449 --> 00:41:28,539
"If your front is clear, there is no steeple for you to jump. 

862
00:41:28,539 --> 00:41:31,049
So just move ahead. But 

863
00:41:31,048 --> 00:41:34,070
if your front is not clear, you got some work to do buddy. 

864
00:41:34,070 --> 00:41:36,179
So jump hurdle." "And you're like, 

865
00:41:36,179 --> 00:41:39,298
jump hurdle. I've been sitting here for 45 minutes. You never told me about jump hurdle. 

866
00:41:39,298 --> 00:41:40,608
Where is jump hurdle?" And I'm like, 

867
00:41:40,608 --> 00:41:43,389
"Yeah, I didn't tell you about jump hurdle because we're going to write jump hurdle," and you're 

868
00:41:43,389 --> 00:41:47,098
like, "Oh, yeah, new instruction that we create." So we're going to break our 

869
00:41:47,099 --> 00:41:48,659
program down into smaller pieces. 

870
00:41:48,659 --> 00:41:49,980
So what's jump hurdle about? 

871
00:41:49,980 --> 00:41:52,059
How do we jump a hurdle? 

872
00:41:52,059 --> 00:41:55,369
Well, think about what you want to do to jump a hurdle, and 

873
00:41:55,369 --> 00:41:58,329
I'll put it in the simplest possible terms. You want to ascend the hurdle. You want to go 

874
00:41:58,329 --> 00:41:58,870
up. 

875
00:41:58,869 --> 00:42:02,029
You want to move past the hurdle, and you want to descend the hurdle to come 

876
00:42:02,030 --> 00:42:03,159
back down. You're like, 

877
00:42:03,159 --> 00:42:04,949
"Yeah, thanks, Ron, that really helped a lot. 

878
00:42:04,949 --> 00:42:08,829
How do I ascend and descend the hurdle, right?" So what we're doing is we're breaking the 

879
00:42:08,829 --> 00:42:10,470
program down into more and more steps, 

880
00:42:10,469 --> 00:42:12,750
and notice this is reading like English. 

881
00:42:12,750 --> 00:42:16,449
And the important thing to realize in programming is programming is not just 

882
00:42:16,449 --> 00:42:20,989
about writing a program that the computer understands. Programming is about writing 

883
00:42:20,989 --> 00:42:22,868
programs that people understand. 

884
00:42:22,869 --> 00:42:27,789
I can't stress that enough. That's huge. That's what programming style is all about. 

885
00:42:27,789 --> 00:42:30,330
It's what good software engineering is all about. 

886
00:42:30,329 --> 00:42:34,248
Writing a program that just works, that someone else can't read and understand, 

887
00:42:34,248 --> 00:42:37,768
is actually really terrible software engineering. And I've seen software companies do this where 

888
00:42:37,768 --> 00:42:41,458
some hotshot programmer comes along and they're like, "Oh, I'm great programmer." And they go and write some program to do 

889
00:42:41,458 --> 00:42:43,128
something, 

890
00:42:43,128 --> 00:42:47,018
and then it breaks. And then they come along and they say, "Hey, we need to actually get 

891
00:42:47,018 --> 00:42:50,098
that program to do something slightly different, or we need to upgrade it. And it's 

892
00:42:50,099 --> 00:42:53,440
written in terms of the code so badly that they can't do anything with it, 

893
00:42:53,440 --> 00:42:56,130
and they throw it out completely, fire the programmer, 

894
00:42:56,130 --> 00:42:59,289
and get a team of good software engineers to actually do it. I've actually 

895
00:42:59,289 --> 00:43:02,429
seen that happen multiple times. Okay, so 

896
00:43:02,429 --> 00:43:04,769
write programs for people to read, 

897
00:43:04,768 --> 00:43:07,588
not just for computers to read. Both of them need to be able to read it, 

898
00:43:07,588 --> 00:43:10,969
but it's far more important that a person reads it and understands it, and that 

899
00:43:10,969 --> 00:43:14,488
the computer still executes it correctly. But that's the first major software 

900
00:43:14,489 --> 00:43:16,039
engineering principle to think about. 

901
00:43:16,039 --> 00:43:19,650
So how do we ascend and descend the hurdle? Move we knew about. Well, 

902
00:43:19,650 --> 00:43:21,619
how do we ascend the hurdle? 

903
00:43:21,619 --> 00:43:23,920
If you think about ascending the hurdle. 

904
00:43:23,920 --> 00:43:28,048
Right, what that means is you're basically facing a wall, some wall that goes up 

905
00:43:28,048 --> 00:43:30,028
some amount that you don't know. 

906
00:43:30,028 --> 00:43:34,199
Well, how am I going to get up that wall? I'm going to turn left, so now I'm looking up 

907
00:43:34,199 --> 00:43:38,489
the wall. And guess what? If I put my right hand out against that wall and 

908
00:43:38,489 --> 00:43:42,068
keep moving up along the wall, I can eventually get to a point where I've gotten 

909
00:43:42,068 --> 00:43:46,208
to the top of that wall. So I turn left to face north, 

910
00:43:46,208 --> 00:43:48,548
while my right is blocked, which is why that 

911
00:43:48,548 --> 00:43:49,298
wall is there. 

912
00:43:49,298 --> 00:43:53,579
I keep moving all the way up until my right is no longer blocked. That means I've 

913
00:43:53,579 --> 00:43:55,479
extended all the way up the hurdle. 

914
00:43:55,478 --> 00:43:57,728
At this point, I'm still facing north. 

915
00:43:57,728 --> 00:44:00,379
This guy's expecting me to face east, so I can 

916
00:44:00,380 --> 00:44:03,898
move over the hurdle, right. If I'm facing north, I'm just going to step one more up. So I 

917
00:44:03,898 --> 00:44:04,989
turn right at the end 

918
00:44:04,989 --> 00:44:09,099
to now be way above the hurdle and be facing an open place where I can actually 

919
00:44:09,099 --> 00:44:10,318
do that move. 

920
00:44:10,318 --> 00:44:12,639
So now, I do the move. I'm on the other side of the hurdle. 

921
00:44:12,639 --> 00:44:14,578
I need to descend the hurdle. 

922
00:44:14,579 --> 00:44:17,829
Well, how do I descend the hurdle? 

923
00:44:17,829 --> 00:44:21,149
Ah, this is where our friend, move to wall, that we just wrote, comes in handy. 

924
00:44:21,148 --> 00:44:23,958
So I'll show you both of these at the same time. If I want to descend, 

925
00:44:23,958 --> 00:44:28,219
I'm at the top of the hurdle, I turn right. So I'm looking south now, I'm looking down, 

926
00:44:28,219 --> 00:44:31,670
and I just move all the way down to the wall. It hits the floor, 

927
00:44:31,670 --> 00:44:34,969
and this is move to wall that we just wrote, right, while front is clear just 

928
00:44:34,969 --> 00:44:36,949
moves. I move all the way down, 

929
00:44:36,949 --> 00:44:41,139
and so when I move to wall and I get to the wall, I'm facing south. I want to turn left one 

930
00:44:41,139 --> 00:44:44,239
last time so I'm facing forward to go again. Okay, 

931
00:44:44,239 --> 00:44:47,389
so any questions about that? 

932
00:44:47,389 --> 00:44:50,368
Let me run Karel, so we can see 

933
00:44:50,369 --> 00:44:53,440
if he's feeling up to the challenge. So here he is. We'll start the program. 

934
00:44:53,440 --> 00:44:54,719
There he goes. 

935
00:44:54,719 --> 00:44:57,240
Notice what he's doing. He goes over every hurdle. 

936
00:44:57,239 --> 00:44:59,419
If his front is blocked, he goes up 

937
00:44:59,420 --> 00:45:02,480
and over. Here's one where you can see it. He goes all the way up until his right 

938
00:45:02,480 --> 00:45:05,760
is clear. Then he moves and he comes all the way down until he hits the wall, 

939
00:45:05,760 --> 00:45:08,950
and he keeps doing this. And if he does this eight times, he always ends up at 

940
00:45:08,949 --> 00:45:15,949
the end because we know it's guaranteed to be nine avenues long. Uh huh? [Inaudible]? Yeah, 

941
00:45:17,338 --> 00:45:21,168
good eye. So you might say, "Hey, before when Karel turned right and he was making 

942
00:45:21,168 --> 00:45:23,958
three right and left turns, what's going on now?" 

943
00:45:23,958 --> 00:45:25,928
It's super Karel, baby. 

944
00:45:25,929 --> 00:45:29,309
We're loving it. Super Karel not only knows the three turn lefts, just turn right, 

945
00:45:29,309 --> 00:45:31,079
but he's like, "Hey, 

946
00:45:31,079 --> 00:45:33,250
forget turn left, man. I'm just turning right." 

947
00:45:33,250 --> 00:45:37,389
So he knows, or she knows, or it knows, or whatever, that amorphous form in the sky 

948
00:45:37,389 --> 00:45:44,389
knows. Uh huh? 

949
00:45:46,929 --> 00:45:50,999
[Inaudible]? Good question, because that's not the specification of the program, right. 

950
00:45:50,998 --> 00:45:54,268
So one question you could ask is, like, "Hey when I bought my word processor, I bought 

951
00:45:54,268 --> 00:45:57,778
my word processor to, like, write essay with. So how come when I got my word 

952
00:45:57,778 --> 00:45:59,298
processor," and this is a valid question to ask, 

953
00:45:59,298 --> 00:46:02,989
"Why didn't it come with all the essays I needed to write at Stanford?" Right? I mean, 

954
00:46:02,989 --> 00:46:06,499
it would be nice if it did, right. I'd go pay the $400. I'd be like, woo hoo, 

955
00:46:06,498 --> 00:46:07,469
super Karel, baby. 

956
00:46:07,469 --> 00:46:10,598
I got super word processor. Right, 

957
00:46:10,599 --> 00:46:13,479
because that's not the specification of the program. The specification of the 

958
00:46:13,478 --> 00:46:17,348
program is to go over the hurdles one by one. Now, 

959
00:46:17,349 --> 00:46:20,219
just to see if - and you might say, "Oh, that�fs kind of interesting, but does it 

960
00:46:20,219 --> 00:46:23,880
just work on this one world?" So let's consider another world, 

961
00:46:23,880 --> 00:46:26,670
super steeplechase. So we can load a world, 

962
00:46:26,670 --> 00:46:29,380
say you can just click load world, and there are some worlds that we'll provide to you in 

963
00:46:29,380 --> 00:46:32,480
your starter project, which is all explained in the handout. Super 

964
00:46:32,480 --> 00:46:34,528
steeplechase. So here's super steeplechase. 

965
00:46:34,528 --> 00:46:36,548
Yeah, that might look familiar. 

966
00:46:36,548 --> 00:46:39,318
More bars in more places, right. So 

967
00:46:39,318 --> 00:46:43,418
here's Karel, right. This is like a world that 50 high. That's Karel down there and you're 

968
00:46:43,418 --> 00:46:45,279
like, "Oh, man, you're really making," yeah, 

969
00:46:45,280 --> 00:46:47,329
we're making him work. All right, so 

970
00:46:47,329 --> 00:46:50,160
go little Karel. 

971
00:46:50,159 --> 00:46:54,250
There he goes. Oh, oh, come on, give him a little encouragement. Go Karel, 

972
00:46:54,250 --> 00:46:55,268
go Karel, 

973
00:46:55,268 --> 00:46:58,218
go Karel. Aw, yeah, because you 

974
00:46:58,219 --> 00:47:02,420
can just speed 

975
00:47:02,420 --> 00:47:04,559
him up if your program's running slow, 

976
00:47:04,559 --> 00:47:09,130
but there he goes, right. Huge steeples, he's done, same program works. 

977
00:47:09,130 --> 00:47:12,980
So the important lesson that comes from that is to think about the generality of 

978
00:47:12,980 --> 00:47:16,679
the program, right. You don't want to just write a program that works in 

979
00:47:16,679 --> 00:47:20,359
one particular world, unless that's the only world you need to worry about. 

980
00:47:20,360 --> 00:47:22,679
You want to generalize your program in a way 

981
00:47:22,679 --> 00:47:26,209
so that it's using, in some sense, general principles that will apply to any world 

982
00:47:26,208 --> 00:47:29,538
that meets your specifications. Right, it still 

983
00:47:29,539 --> 00:47:33,180
needs to meet the specification. Our world is still nine avenues long and we know what the specification of the 

984
00:47:33,179 --> 00:47:34,838
steeples are. All right, 

985
00:47:34,838 --> 00:47:41,838
so any quick questions before we go? Uh huh? So if you had the [inaudible]? 

986
00:47:42,829 --> 00:47:46,700
No, because it's still just nine avenues. The for-loop counter stays the same, right. 

987
00:47:46,699 --> 00:47:50,308
It's that while-loop that's actually going up and down the hurdles, because it 

988
00:47:50,309 --> 00:47:52,769
doesn't know how big the hurdles are. 

989
00:47:52,768 --> 00:47:58,938
Any other questions? [Inaudible]? 

990
00:47:58,938 --> 00:48:05,938
That it's nine avenues and the hurdles are just pools, basically, of any length. [Inaudible]. Yeah, 

991
00:48:07,079 --> 00:48:09,849
all right, I will see you on Friday. If you have any more questions, come on down.