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This presentation is delivered by the Stanford Center for Professional 

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Development. 

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So a little bit of additional background on Karel before we dive into the real meat 

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of things, okay? 

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One thing you may have noticed is all the Karel code that youfve either started writing, 

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if you're already working on assignment number one or all the code that we write 

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in class 

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is all in file that ends with dot Java. If you haven't noticed that before, 

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you'll - now you know, it ends with dot Java. 

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Because in fact, Karel is all implemented in Java. 

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So one of the things you might be wondering is hey, I know a little Java; can I use 

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some Java in conjunction with Karel? And the answer is no. 

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For the purpose of the Karel assignments, you should just use the constructs that 

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youfve been shown in class and in the Karel reader, 

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just keep it to those constructs. That still gives you plenty of stuff to do with Karel, 

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it's a good time, but keep it to that, and actually starting on Monday we're gonna, like, 

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sort of leave Karel behind and say bye-bye, Karel, and I'll be, like, bye-bye, I love you, 

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call 

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me. 

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But we'll get into Java, so if you know Java now, just use the Karel stuff in 

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the Karel 

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assignments that we actually do. All 

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right, so with that said, any questions to start off with before we dive into 

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something new? Um-hm? How do you 

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stop the program? Like, I had a problem, 

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it goes up into the corner and just kind of like it says an error message. And it kept looping there forever? Yeah, 

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and I tried making, like, an empty program called Stop, but that didnft really work. No, just go 

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up to the little icon - or the little thing in the top of the window that 

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allows you to close the window and just close the window. 

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Karel will be okay, he knows how to deal with that. 

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As a matter of fact, that's - I am so glad you asked that question. It's 

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just a wonderful thing, because it actually leads into the first topic that I want to talk about, 

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which is common errors. 

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And so there are some common errors that may come up, and these are good things to kind 

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of know about. And one of them is the thing you just ran into. 

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But before we actually talk about that at sort of the level of Karel, I wanna ask you a 

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question: [inaudible] another one 

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of these strange questions. How many people have actually ever 

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read the instructions on a bottle of shampoo? 

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A few folks - oh, wow. Man, that's more than - I was, like, 

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youfve really gotta get out more often, right? 

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But I'm surprised that many. And what do those instructions say? 

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Rinse, right, that's where you rinse your hair, and then you lather, 

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and then you repeat. 

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And if anyone actually followed these instructions, you would still be in the 

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shower now. 

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As a matter of fact, you'd be in the shower for the rest of your life. Why? Because you 

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rinse, you lather, and you repeat, 

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which means you rinse, you later, and you repeat. 

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And you just keep doing this, and it's like Karel just taking a shower, right? And he's 

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like you keep telling me to rinse 

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- to linse, rinse and lather together. 

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Rinse, 

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lather, and repeat, right? And this is what we refer to in program-speak 

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as an infinite loop. 

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Which as you can guess is a loop that keeps going forever. And this may come up in 

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your Karel programs. Here is an example of an infinite loop. You might be standing 

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somewhere and you might say while 

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front is clear. 

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Oh, well, if your front is clear, I want you to turn left, because, you know, I have 

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this idea that eventually you'll turn and you'll be facing something and your front won't be 

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clear anymore. 

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And that might be a great idea to have, 

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but what happens 

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is if here's our friend Karel, 

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happens to be standing in the world and guess what? There's no walls around him. 

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What does he do? He rinse, 

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lathers, and repeats, right? He just keeps sitting there, turning left 

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on this corner, and his front will never become blocked so he'll never get out 

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of that loop, okay? 

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That's one of the places where the syntax of the program is perfectly valid, but 

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in terms of our intention for what we wanted the program to do, it didnft work as 

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intended. This is the more common thing that you're actually going to need to deal with fixing in 

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your programs 

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as opposed to syntax-type problems. 

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As a matter of fact, I won't mention the name of the company but there was a 

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company years and years ago that put out this advertisement that was, like, 

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our processors are so fast they can execute an infinite loop in 2.5 

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seconds. 

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And you kind of looked at that and you were like ah, ha, ha, funny computer scientists. 

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But the thing that was actually funny about it was the company that put out 

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these chips, the chips actually had a flaw in them where if they executed things really fast they 

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would warm up 

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and then they would melt. 

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So in fact, they would execute an infinite loop in 2.5 seconds, 

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and then the processor would catch fire and it would stop. 

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So sometimes, that happens. Hopefully, that won't happen to your machine. Don't worry, 

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even if you get Karel going real fast, it's okay. If your processor melts, come 

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talk to me, it's never happened. But it'll make for a wonderful story in class, and it'll cost 

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you about $1,800.00. Fine. 

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So another kind of error that comes up fairly often 

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also has a little affectionate name to it. 

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And this is something that we refer to in programming speak as 

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an off-by-one bug. Or if you take the first letter of that, it becomes 

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an OBOB. And sometimes you'll hear people refer to OBOB, and if you're name is 

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Robert they're not saying, like, oh, Bob. No, that's 

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just referring to the off-by-one bug. 

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And what that means is 

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you forgot to do something one more time than you really needed to, even though 

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logically it didnft seem like you needed to. So the best way to understand 

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that is with a little concrete example. 

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So let me actually give you a concrete example by going over to the computer. 

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So here's Karel sitting in his world, 

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and one of the things we might wanna do with Karel - whoa - Karel in his world is to say 

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that 

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we actually want Karel to lay down a line of beepers, right? Just whatever 

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row he's in, just lay down a line of beepers till you hit the wall. And that's a fairly 

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common thing that you might wanna do, okay? 

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Now one way we might be inclined to actually do this if we were to write 

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some code 

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is to say something like 

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hey, I'm gonna, you know, write a new class, I'll call it fill row that extends 

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super Karel. I import all the Karel stuff, so I'm doing all the crazy stuff that 

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[inaudible] told me to do, and hey, I want to keep putting down beepers until I fill the 

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row. 

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So wouldnft it make sense to say while front is clear, right? There's no wall in 

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front of me. 

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Put down a beeper 

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and move to the next corner. 

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And as long as my front is clear I'll put down another beeper and keep taking 

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another step and doing this over and over, and this seems like - 

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hey, that seems like reasonable code to lay down a line of beepers. 

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What happens when I do this? 

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Yeah, so let's actually run it. Let's bring up our buddy Karel. 

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Where'd you go, Karel? Here you are. And run that program. 

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And so what Karel does, he puts down a beeper, his front is clear, so he moves, puts 

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down a beeper, front is clear, puts down a beeper, front is clear, puts down a 

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beeper, front is clear. He moves here, 

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front is no longer clear. 

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Before he puts down the beeper, he says hey, my front isnft clear anymore, I stop. 

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He's off by one because there's one more thing that he needed to do, which 

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was to put down another beeper in this last spot. 

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This is another very common logical error that comes up - you're off by one 

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in terms of what you're thinking, and so lo and behold if we go into the 

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program over here 

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and we look at this comment, and I'll tell you about comments in just a second, 

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it says this is buggy, it's off by one. You need to add a final put beeper. 

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And so what we do is we say hey, after you got out of your loop, your front is no 

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longer clear, but 

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you haven't put down a beeper on this corner yet. 

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Put down 

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a beeper, right? 

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And if we save that puppy off and we close this off and just so we know that it's 

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in fact running, 

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doo doo doo, we run our little program 

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and rock on, all right? 

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No more off-by-one bug, life is good. 

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So any questions about the OBOB or the off-by-one bug? 

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That's another common thing. 

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These are just a couple of common things I wanna show you, so, you if 

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they come up in your programs you don't feel like oh, you're out there adrift, all 

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alone. Like, these have been done millions of times by other very qualified 

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programmers. 

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Don't worry, it happens to everyone. All right, 

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so one of the things I just mentioned is this thing called the comment. This thing 

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that's up here in green. What's that all about? 

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So as I mentioned to you last time, one of the key software engineering 

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principles is to write programs that are understandable by people, not just by 

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machines. 

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And so what a comment is is a way of being able to put something in your 

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program that another human being reading your program can actually read and understand, and 

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it actually has no impact on the execution of the program at all. 

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And so the way the comment works is it starts off with a slash and a star, 

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and now everything you put - it can even span multiple lines - until you put a star 

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and a slash is a comment. It's there just for the person to read, and it doesnft 

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affect the program at all. 

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And so what I would encourage you to do as part of good programming style is all 

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of your programs up at the top should have a comment that says what the name of 

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the file is and has a little bit of an explanation about what your program 

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actually does. And if you're wondering how much of an explanation or how in-depth it 

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should be, be it as in-depth as you want, 

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but in the handout you got today you see examples of a bunch of programs, and they're 

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all commented, so you can see examples of comments in there. 

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There is also shorthand you can do. There is a comment that's just 

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slash, slash, and that just means everything on the remainder of this line 

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is a comment. So it actually has no close, in some sense, for the comment like this 

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does explicitly. Everything else to the rest of the line, when you hit a return, that's 

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how it knows it's done with the comment. So sometimes if you wanna have a 

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one-liner comment somewhere, 

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you can just put a slash, slash. [Inaudible.] 

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Yeah, when you get assigned to a different section leader, you'll actually 

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put your assignments in with your name and your section leader, and that's how 

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it'll get differentiated. So wait until you get your first section, and then you'll see 

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how submission stuff works. Um-hm? 

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What about when you write private programs [inaudible] public ones, do those need comments, too? Oh, when you write, like, other methods? 

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Yeah, you should comment those as well. So methods should also be commented as well, and I'll 

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show you an example of that today, actually, so you can see an example of how we 

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do that. 

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So that's actually - and I love it when everyone just, you know, 

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it's like ringers in the audience. You just lead me into the next topic, and that's a 

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perfect example to say 

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hey, let's actually look in, look at a program that we did last time, which was 

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the steeplechase program, 

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now with comments, okay? So up at the top - sometimes you need to click this little 

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plus sign to expand the comment, because Eclipse has a tendency to sort 

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of minimize the comments automatically. So just click the plus 

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sign and that'll stand them out. Steeplechase, so we have a nice little comment here at 

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the top of our program. 

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Notice this is a Java file. As I mentioned before, it ends with dot Java, 

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because Karel's actually implemented in Java. But you should use no features of 

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Java 

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other than what is in the Karel book. And if you're like hey, [inaudible] I don't 

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know Java, hey, nothing for you to unlearn. You're good to go, because you just know Karel, all right? 

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The other thing that's going on here is you'll notice that inside the program 

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we have comments as well. So this method, the run method, it says to run a race that's 

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nine avenues long. We need to forward or jump eight hurdles eight 

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times. 

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And that makes it explicit to the person, right? If we didnft have that comment, someone would 

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come along and say 

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hey, the race is nine avenues; why are you only iterating this eight times, right? And 

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that's a natural question to ask. And so you put in comments to clarify things in the 

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program, which are not obvious, okay? 

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Another thing related to that is what's referred to as pre-conditions and 

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post-conditions. What did you expect to be true before you called a particular 

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method or before a particular method is invoked, 

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and what's going to be true afterwards? 

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So our friend Jump Hurdle here - remember Jump Hurdle from last time? We ascend the hurdle, 

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which means we go up, 

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we move to cross over the top of the hurdle, and we descend hurdle to come back 

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down the other side? Well, 

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what we needed for that to be true for this to actually work right is 

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the precondition is you're facing east at the bottom of a hurdle. Which means 

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the hurdle was right in front of you and you were facing east in front of it. 

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That way, we know which way to turn and how to ascend the hurdle and how to get over it, 

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and when we're done you're going to be facing east again. So you can assume, 

227
00:11:54,059 --> 00:11:55,558
after this method is done, 

228
00:11:55,558 --> 00:11:58,129
that you're facing east again and you're at the bottom 

229
00:11:58,129 --> 00:11:59,420
of the world 

230
00:11:59,419 --> 00:12:01,428
in the next avenue after the hurdle. 

231
00:12:01,428 --> 00:12:05,808
So it makes clear, right, because we don't know what-all stuff is going on inside ascend hurdle 

232
00:12:05,808 --> 00:12:09,159
and descend hurdle, but a programmer now doesnft need to trace 

233
00:12:09,159 --> 00:12:11,459
through the execution of your program 

234
00:12:11,460 --> 00:12:13,019
to figure out what's going on. 

235
00:12:13,019 --> 00:12:15,889
This tells him what needs to be true before and what needs to be true 

236
00:12:15,889 --> 00:12:17,850
after, and it helps you with debugging. 

237
00:12:17,850 --> 00:12:20,480
Because if you're debugging your program and you call this 

238
00:12:20,480 --> 00:12:24,389
jump hurdle method and you haven't satisfied the precondition, it's probably 

239
00:12:24,389 --> 00:12:27,429
not gonna work right and that allows you to work backward to figure out what you 

240
00:12:27,429 --> 00:12:28,120
need to do 

241
00:12:28,120 --> 00:12:31,078
to fix things before you even call this method, 

242
00:12:31,078 --> 00:12:34,059
okay? So any questions about that? 

243
00:12:34,059 --> 00:12:37,500
So as we go further down, you'll notice - and this is all in your handout - all of the 

244
00:12:37,500 --> 00:12:40,289
methods have pre-conditions and post-conditions that are commented in here. This 

245
00:12:40,289 --> 00:12:43,838
is a good habit to get into, at least for your Karel programs, okay? 

246
00:12:43,839 --> 00:12:46,669
Even if you don't explicitly put pre- and post-conditions, you should at least 

247
00:12:46,668 --> 00:12:49,220
have some comment for every method, 

248
00:12:49,220 --> 00:12:52,210
explaining what that method does. So that's the level of commenting that we actually 

249
00:12:52,210 --> 00:12:53,290
want you to have. 

250
00:12:53,289 --> 00:12:56,879
Notice it's also perfectly fine to have very short, like you know, three-line 

251
00:12:56,879 --> 00:13:00,578
or in some cases even one-line methods is perfectly fine, 

252
00:13:00,578 --> 00:13:04,479
because it helps - like here's a two-line method over here - it just helps break the 

253
00:13:04,480 --> 00:13:05,909
program down, okay? 

254
00:13:05,909 --> 00:13:09,059
And that's a process we refer to as decomposition. 

255
00:13:09,059 --> 00:13:12,259
And so what's decomposition all about, right? We need to think of this in sort of more 

256
00:13:12,259 --> 00:13:16,799
concrete terms that you may understand from your daily life. So 

257
00:13:16,799 --> 00:13:20,109
besides reading the bottle of shampoo, another thing that we could talk about is 

258
00:13:20,110 --> 00:13:23,470
what you did this morning. So if I were to say 

259
00:13:23,470 --> 00:13:27,810
hey, what did you do before you got up for your - or before you went to your first 

260
00:13:27,809 --> 00:13:31,379
class this morning? How did you get yourself ready this morning? What did you do? 

261
00:13:31,379 --> 00:13:33,960
Anyone wanna volunteer what they did? I know it might be a little scary for 

262
00:13:33,960 --> 00:13:35,450
some of you. 

263
00:13:35,450 --> 00:13:42,450
Um-hm? What did you do this morning? Use the microphone, please. Pardon? 

264
00:13:43,100 --> 00:13:43,350
Basically I 

265
00:13:43,249 --> 00:13:48,980
basically rolled out of bed, and then - All right, so you woke up. Wow, did 

266
00:13:48,980 --> 00:13:52,330
you even hit the snooze bar at all? 

267
00:13:52,330 --> 00:13:57,620
Twice. Oh, good times. That's always good. That's why I set it early. She was good. I had a - my old roommate in college 

268
00:13:57,620 --> 00:14:01,509
actually would play the snooze bar game, no joke, for two and a half hours. 

269
00:14:01,509 --> 00:14:04,629
And I would be sitting there, I'd be in the room, like, I'd be awake, staring at the ceiling, and [inaudible] 

270
00:14:04,629 --> 00:14:07,808
I'd know his snooze bar, and I'd be like Joe, 

271
00:14:07,808 --> 00:14:12,649
why don't you just not set your alarm? And he'd be like no, man, I gotta set my alarm. And 

272
00:14:12,649 --> 00:14:13,889
every 

273
00:14:13,889 --> 00:14:16,740
day - anyway, I'm glad it's only twice for you. You can tell there's just, like, 

274
00:14:16,740 --> 00:14:20,610
you know, like I got this little twitch from, like, 15 years ago from the snooze bar. 

275
00:14:20,610 --> 00:14:23,159
But you woke up, and then what did you do? 

276
00:14:23,159 --> 00:14:23,889
Okay, and 

277
00:14:23,889 --> 00:14:27,299
then I brushed my teeth. Okay, you brushed teeth. 

278
00:14:27,299 --> 00:14:28,229
What else? 

279
00:14:28,230 --> 00:14:30,129
Go to the bathroom. 

280
00:14:30,129 --> 00:14:30,990
Bathroom. 

281
00:14:30,990 --> 00:14:33,620
I won't ask you for any more detail on that. Yeah. 

282
00:14:33,620 --> 00:14:35,350
What else. That'd be awkward. 

283
00:14:35,350 --> 00:14:39,360
Shower? Was shower in there, maybe? I shower later in the day. Oh, 

284
00:14:39,360 --> 00:14:44,539
okay, that's fine. After [inaudible]. I'm just gonna stick shower in here. 

285
00:14:44,539 --> 00:14:48,208
Okay. Just for good measure. But it's perfectly fine whenever you wanna take them. What 

286
00:14:48,208 --> 00:14:50,639
else? And then clothes, [inaudible] 

287
00:14:50,639 --> 00:14:53,610
clothes. Clothes. 

288
00:14:53,610 --> 00:14:56,389
And then presumably you probably went out of your room 

289
00:14:56,389 --> 00:14:59,590
or ate breakfast or - did you get up for breakfast? No. Yeah, 

290
00:14:59,590 --> 00:15:04,060
I know, it's brutally early, isnft it? Yeah. Like, I think, like, I ate 

291
00:15:04,059 --> 00:15:07,149
breakfast like once the whole time I was an undergrad, and it's because, like, I just 

292
00:15:07,149 --> 00:15:10,689
stayed up the night before and I was, like, oh my god, there's a thing called breakfast, and I can actually 

293
00:15:10,690 --> 00:15:12,950
go to it, so I went. 

294
00:15:12,950 --> 00:15:15,940
But so this is an interesting thing, you give us sort of a breakdown of what you do. Thank you 

295
00:15:15,940 --> 00:15:18,570
very much, what's your name, by the way? 

296
00:15:18,570 --> 00:15:24,240
Jasmine. Thanks, Jasmine. I'll try to see if I can get to you, but - 

297
00:15:24,240 --> 00:15:27,269
denied every time. So here's, you know, a standard set of things that we might 

298
00:15:27,269 --> 00:15:28,539
think about doing. Now 

299
00:15:28,539 --> 00:15:31,730
it turns out this is a very good list of things that you might want to consider 

300
00:15:31,730 --> 00:15:34,039
doing. Good hygiene for the morning. 

301
00:15:34,039 --> 00:15:35,169
But in fact, 

302
00:15:35,169 --> 00:15:38,209
when you think about something like brushing your teeth, 

303
00:15:38,210 --> 00:15:40,769
brushing your teeth - we all understand what brushing your teeth means. But if there's 

304
00:15:40,769 --> 00:15:44,250
someone, like when I, you know, try to get my 1.5-year-old son to brush his 

305
00:15:44,250 --> 00:15:44,919
teeth, 

306
00:15:44,919 --> 00:15:47,870
I say hey, William, you need to brush your teeth, he has no idea what I'm talking 

307
00:15:47,870 --> 00:15:48,990
about. 

308
00:15:48,990 --> 00:15:51,649
Because brushing teeth for him involves 

309
00:15:51,649 --> 00:15:55,079
getting the toothbrush - 

310
00:15:55,080 --> 00:15:56,009
brush - 

311
00:15:56,009 --> 00:15:58,500
putting toothpaste on it, 

312
00:15:58,500 --> 00:16:02,200
and sort of moving it on your teeth, 

313
00:16:02,200 --> 00:16:05,640
right? So there's a level at which I need to break this down. As a matter of fact, 

314
00:16:05,639 --> 00:16:08,710
moving it on your teeth, if you've ever talked to a dentist, 

315
00:16:08,710 --> 00:16:11,980
this is a whole involved process by itself. Like what angle you put it at and how 

316
00:16:11,980 --> 00:16:13,959
much you move it and how much time [inaudible]. It's like whoa, man, 

317
00:16:13,958 --> 00:16:17,979
I never thought there was that much to brushing your teeth, but evidently, there is. 

318
00:16:17,980 --> 00:16:21,379
And so what happens is we start with a high-level description, and then each one 

319
00:16:21,379 --> 00:16:24,409
of those steps in the high-level description we break down. 

320
00:16:24,409 --> 00:16:28,429
And we keep breaking it down until we get to a level of detail, which is 

321
00:16:28,429 --> 00:16:31,078
sufficient for us to be able to understand minutely. 

322
00:16:31,078 --> 00:16:33,949
Or if we're doing this on the computer we get to a level of detail 

323
00:16:33,950 --> 00:16:36,710
at which the computer understands it directly. 

324
00:16:36,710 --> 00:16:39,879
What that means is we eventually get to a level of detail where it turns into things 

325
00:16:39,879 --> 00:16:42,009
like put beeper or move 

326
00:16:42,009 --> 00:16:43,979
or while something is true, 

327
00:16:43,979 --> 00:16:47,359
right? So when we get to that level which we call primitives, that's when this process 

328
00:16:47,359 --> 00:16:50,949
eventually stops. But this is essentially a process that's called 

329
00:16:50,948 --> 00:16:53,919
stepwise refinement. 

330
00:16:53,919 --> 00:16:57,000
Because what we're doing at each step in the process 

331
00:16:57,000 --> 00:17:00,370
is we're coming up with some steps, and if those steps are not the primitives 

332
00:17:00,370 --> 00:17:03,830
that we understand we need to refine them into more steps and refine them into 

333
00:17:03,830 --> 00:17:06,480
more steps until we eventually get to the primitives, okay? 

334
00:17:06,480 --> 00:17:10,259
So we start at the high level and we break things down. And the notion of 

335
00:17:10,259 --> 00:17:13,779
breaking things down into their sub-pieces 

336
00:17:13,779 --> 00:17:17,399
is something that we refer to as decomposition. 

337
00:17:17,400 --> 00:17:22,659
Decomposition. Which just means taking something big and breaking it down into 

338
00:17:22,659 --> 00:17:25,110
smaller pieces, which is something you should do with your program. You wanna 

339
00:17:25,109 --> 00:17:27,449
break them down into smaller pieces, okay? 

340
00:17:27,450 --> 00:17:28,980
And this whole process, 

341
00:17:28,980 --> 00:17:32,620
the whole process itself is something we refer to as top-down 

342
00:17:32,619 --> 00:17:37,139
design. And you'll hear this phrase referred to a lot, and what top-down design 

343
00:17:37,140 --> 00:17:39,630
is is you start at the highest level, the top, 

344
00:17:39,630 --> 00:17:41,120
and you go down from there. 

345
00:17:41,119 --> 00:17:45,379
And that's to be contrasted with something that's referred to as bottom-up 

346
00:17:45,380 --> 00:17:46,630
design. 

347
00:17:46,630 --> 00:17:50,220
And what bottom-up design says is hey, I need to go do something in the 

348
00:17:50,220 --> 00:17:52,799
world. Well, let me start at the primitives. I need to make Karel, 

349
00:17:52,799 --> 00:17:55,960
you know, go to the end of the wall and put down beepers. What's the first thing I 

350
00:17:55,960 --> 00:17:59,850
need to do? I need to put a beeper, and then I need to move. And it starts with the 

351
00:17:59,849 --> 00:18:00,998
low-level stuff 

352
00:18:00,999 --> 00:18:04,298
and this is actually the place most programmers, when they start out, 

353
00:18:04,298 --> 00:18:07,119
this is what they begin doing is bottom-up design. People have actually done 

354
00:18:07,119 --> 00:18:11,019
psychological studies. It takes about 100 hours of programming proficiency. 

355
00:18:11,019 --> 00:18:14,288
In the average case, I think everyone at Stanford's above average so it'll take you 

356
00:18:14,288 --> 00:18:18,528
less, to go from thinking bottom-up to thinking top-down. Guess what? It's a 10-week 

357
00:18:18,528 --> 00:18:22,019
quarter. We might do about 100 hours of programming in here. Hopefully 

358
00:18:22,019 --> 00:18:25,038
by the end of this class, everyone's doing top-down design. But if you can 

359
00:18:25,038 --> 00:18:27,378
start from the very beginning doing top-down design, 

360
00:18:27,378 --> 00:18:28,199
you're golden. 

361
00:18:28,200 --> 00:18:30,259
In some cases this makes sense, 

362
00:18:30,259 --> 00:18:33,808
but a lot of times this just makes your life easier, to think about the most 

363
00:18:33,808 --> 00:18:36,048
abstract level and break down from there. 

364
00:18:36,048 --> 00:18:38,058
And so if you think about what we did 

365
00:18:38,058 --> 00:18:40,500
when we actually did the steeplechase program, 

366
00:18:40,500 --> 00:18:43,970
this was top-down design, right? When we actually wrote this program together, 

367
00:18:43,970 --> 00:18:45,440
when we went through it together, 

368
00:18:45,440 --> 00:18:48,940
we didnft talk about Karel doing individual moves to begin with. We said 

369
00:18:48,940 --> 00:18:50,710
hey, we need to run a steeplechase, 

370
00:18:50,710 --> 00:18:53,129
and that either involves doing the move 

371
00:18:53,128 --> 00:18:54,519
or jumping a hurdle. 

372
00:18:54,519 --> 00:18:57,789
At this point, we've done enough step-wise refinement that we've gotten to the 

373
00:18:57,789 --> 00:19:01,200
level of a primitive. We don't need to go any further because Karel immediately 

374
00:19:01,200 --> 00:19:02,370
understands move. 

375
00:19:02,369 --> 00:19:04,278
Jump hurdle, Karel doesnft understand. 

376
00:19:04,278 --> 00:19:08,048
So once we've written this, we need to break it down into lower-level steps and say 

377
00:19:08,048 --> 00:19:09,750
what does jump hurdle involve? 

378
00:19:09,750 --> 00:19:12,558
And that's where we actually go down and define jump hurdle, 

379
00:19:12,558 --> 00:19:15,839
and guess what? When we define jump hurdle, there might be some other things 

380
00:19:15,839 --> 00:19:19,139
like ascending and descending a hurdle, which are the next steps down in the process 

381
00:19:19,140 --> 00:19:21,390
that we need to define, and we just keep doing this, 

382
00:19:21,390 --> 00:19:22,038
okay? 

383
00:19:22,038 --> 00:19:25,650
So with that said, let's actually do top-down design together. Let's start 

384
00:19:25,650 --> 00:19:29,300
another program from a blank slate and see if we can do top-down design together 

385
00:19:29,299 --> 00:19:32,759
to see how this process might actually go for writing code, okay? 

386
00:19:32,759 --> 00:19:36,059
So the problem that we wanna try to solve 

387
00:19:36,059 --> 00:19:39,049
is we wanna - oh, and don't look at the code - we wanna try to get - and this is kind 

388
00:19:39,049 --> 00:19:43,200
of a funky problem - it's Karel does math. If you didnft think Karel could do math, 

389
00:19:43,200 --> 00:19:46,920
in fact Karel can do math. Karel's a lot smarter than we sometimes give him credit 

390
00:19:46,920 --> 00:19:47,750
for. 

391
00:19:47,750 --> 00:19:50,480
So this is a little problem called double beepers. 

392
00:19:50,480 --> 00:19:53,089
So I don't know if you can see the number here, but there was a five here, 

393
00:19:53,089 --> 00:19:55,619
which means there was five beepers on this corner. 

394
00:19:55,619 --> 00:19:59,678
And when this program is done running, what we want to have happen is Karel 

395
00:19:59,679 --> 00:20:03,200
is guaranteed to start in position 1-1 and there's guaranteed to be a pile of beepers 

396
00:20:03,200 --> 00:20:05,720
in front of him and a space after him, okay? 

397
00:20:05,720 --> 00:20:09,150
When he's done, he should return back to the same position and the pile of beepers 

398
00:20:09,150 --> 00:20:13,190
should have exactly twice as many beepers on it as it had before, which 

399
00:20:13,190 --> 00:20:17,360
means Karel begins with a beeper bag that's full of as many beepers as he 

400
00:20:17,359 --> 00:20:20,250
needs, so it has infinite beepers in there so we can double the number of beepers. 

401
00:20:20,250 --> 00:20:24,299
So if I speed up the program and just run it, you can see the final state. So let me 

402
00:20:24,299 --> 00:20:26,529
just crank up the speed here 

403
00:20:26,529 --> 00:20:27,918
and start the program, 

404
00:20:27,919 --> 00:20:31,639
and it looks like nothing happened, but in fact this number changed from a five to 

405
00:20:31,638 --> 00:20:33,279
a 10. It doubled the number of beepers. 

406
00:20:33,279 --> 00:20:37,379
So two things should be going off in your head right now - one is thinking hey, 

407
00:20:37,380 --> 00:20:38,530
that's kind of slick. 

408
00:20:38,529 --> 00:20:41,648
How do I do that? What's the recipe for doing that? 

409
00:20:41,648 --> 00:20:45,319
And the recipe for doing that, the sort of general approach you wanna 

410
00:20:45,319 --> 00:20:48,269
take, is something that we call an algorithm, okay? 

411
00:20:48,269 --> 00:20:53,519
So the notion of an algorithm - and this is just a fancy computer science term 

412
00:20:53,519 --> 00:20:54,269
for 

413
00:20:54,269 --> 00:20:58,869
an approach to something - actually comes from - anywhere know where the word 

414
00:20:58,869 --> 00:21:02,629
algorithm comes from? 

415
00:21:02,630 --> 00:21:04,530


416
00:21:04,529 --> 00:21:08,299
Nineteenth century Persian mathematician named, if I can pronounce it right, 

417
00:21:08,299 --> 00:21:09,319
Al-Khwarizmi. 

418
00:21:09,319 --> 00:21:11,889


419
00:21:11,890 --> 00:21:14,970
Al-Khwarizmi, 

420
00:21:14,970 --> 00:21:18,569
which sounds like algorithm. There you go. 

421
00:21:18,569 --> 00:21:21,470


422
00:21:21,470 --> 00:21:25,309
Yeah, add 1100 years to that pronunciation and it sounds like algorithm, all right? Because this is 

423
00:21:25,309 --> 00:21:27,929
from the 19th century. It's your basic approach. As a matter of fact, there 

424
00:21:27,930 --> 00:21:31,259
was a band at Stanford years and years ago called the Algorithms, where 

425
00:21:31,259 --> 00:21:34,539
this was, like, rhythm, like hey, I got rhythm. Yeah. 

426
00:21:34,539 --> 00:21:36,639
I won't say how 

427
00:21:36,640 --> 00:21:39,220
good they were. Not very. All right. So 

428
00:21:39,220 --> 00:21:42,509
you wanna think about your general approach and then you turn your general 

429
00:21:42,509 --> 00:21:44,200
approach into 

430
00:21:44,200 --> 00:21:47,279
thinking in terms of step-wise refinement. How are you gonna solve this problem 

431
00:21:47,279 --> 00:21:51,410
from the top down? So let's actually start with a blank slate. Let's close this puppy out 

432
00:21:51,410 --> 00:21:53,179
and we'll start with a blank slate, 

433
00:21:53,179 --> 00:21:55,990
which we'll call, oh, Our Double Beepers. 

434
00:21:55,990 --> 00:21:59,120
So I just gave you the boilerplate stuff to begin with. There's a little comment 

435
00:21:59,119 --> 00:22:00,149
up at the top 

436
00:22:00,150 --> 00:22:03,840
that says file double beepers. That should be called Our Double Beepers, 

437
00:22:03,839 --> 00:22:06,230
actually, and Karel doubles the number of beepers on the corner directly in 

438
00:22:06,230 --> 00:22:09,669
front of him in the world, he returns to his original position and orientation. 

439
00:22:09,669 --> 00:22:13,730
So we have the public class stuff for Our Double Beepers, we have an [inaudible] super 

440
00:22:13,730 --> 00:22:16,440
Karel. Now we're gonna run the run method, okay? 

441
00:22:16,440 --> 00:22:19,980
So what do we wanna do to get Karel to double the number of beepers at a high 

442
00:22:19,980 --> 00:22:26,980
level? What might we think about doing this? Hm. Um-hm? [Inaudible.] 

443
00:22:31,579 --> 00:22:34,509
We need to figure out how many beepers there are. In order to do that, maybe we 

444
00:22:34,509 --> 00:22:37,450
should move to the pile of beepers, right? It's one avenue in front of us. So we 

445
00:22:37,450 --> 00:22:38,690
do a move. 

446
00:22:38,690 --> 00:22:42,000
And now here's the part that almost seems like magic. I'm going to say move, hey, you're on this 

447
00:22:42,000 --> 00:22:43,490
pile of beepers now. Why 

448
00:22:43,490 --> 00:22:47,630
don't you, hey, double the beepers in the pile? 

449
00:22:47,630 --> 00:22:49,680
And you're like, uh, [inaudible] can I do that? Sure, 

450
00:22:49,680 --> 00:22:51,090
why not, 

451
00:22:51,089 --> 00:22:53,889
right? You're gonna write the program, you can do whatever you want. So 

452
00:22:53,890 --> 00:22:57,920
I would like to say move, double the beepers in the pile, and then actually 

453
00:22:57,920 --> 00:23:01,380
what I wanna do is not move forward again but I wanna move backward, 

454
00:23:01,380 --> 00:23:04,600
right? I wanna move - almost like Karel could go in reverse, that'd be kind 

455
00:23:04,599 --> 00:23:08,569
of cool, to keep the same orientation but goes back. And you're like but [inaudible] you 

456
00:23:08,569 --> 00:23:12,919
didnft tell me about move backward. Can super Karel do move backward? And I'm like, no. And you're 

457
00:23:12,920 --> 00:23:16,990
like oh, okay, well, all right, well, I guess I need to write some of these, right? Let's 

458
00:23:16,990 --> 00:23:19,420
actually do move backward, because that's the easier one. 

459
00:23:19,420 --> 00:23:22,220
If I'm sitting on a particular corner and I'm facing east, 

460
00:23:22,220 --> 00:23:25,150
what do I need to do to move backward one step 

461
00:23:25,150 --> 00:23:27,850
and face the same direction? 

462
00:23:27,849 --> 00:23:30,689
Yeah, I need - so let me define move backward here. 

463
00:23:30,690 --> 00:23:34,960
Move backward. I need to turn around, which is a primitive that I can use. Then 

464
00:23:34,960 --> 00:23:36,970
what do I do? Move. 

465
00:23:36,970 --> 00:23:38,230
And then what do I do? 

466
00:23:38,230 --> 00:23:41,960
I turn around again so I have the same orientation when I'm done as when I 

467
00:23:41,960 --> 00:23:42,569
started. 

468
00:23:42,569 --> 00:23:45,888
And now, guess what? Move backward is something I understand how to do, because 

469
00:23:45,888 --> 00:23:49,498
I've broken it all down into primitives. I don't need to go any further in terms of 

470
00:23:49,499 --> 00:23:52,470
the refinement for move backwards. 

471
00:23:52,470 --> 00:23:55,940
But now there's the magical part, right? There's double beepers in pile, 

472
00:23:55,940 --> 00:23:58,509
and that's something that might be a little bit more involved. So I come along and say 

473
00:23:58,509 --> 00:24:00,240
okay, I need to define 

474
00:24:00,240 --> 00:24:02,940
what double beepers in pile is. Double 

475
00:24:02,940 --> 00:24:04,730
beepers in pile. 

476
00:24:04,730 --> 00:24:07,579
At this point, I need to think about my algorithm. 

477
00:24:07,579 --> 00:24:10,189
What's going to be my approach for doubling the number of beepers? What's 

478
00:24:10,190 --> 00:24:13,528
a way that I could possibly think about that problem? Because Karel can't count, right? 

479
00:24:13,528 --> 00:24:14,909
There is no way for Karel to just say 

480
00:24:14,909 --> 00:24:17,260
hey, counting how many beepers are on this corner. 

481
00:24:17,259 --> 00:24:24,259
What's one way he might approach it? Um-hm? [Inaudible.] 

482
00:24:26,380 --> 00:24:31,200
That would be nice if we had a counter, but we don't have a counter, right? 

483
00:24:31,200 --> 00:24:33,088
So Karel can't count. 

484
00:24:33,088 --> 00:24:35,529
How is he potentially gonna do it? Um-hm, all the way in the back. 

485
00:24:35,529 --> 00:24:39,559
Use your microphone, 

486
00:24:39,559 --> 00:24:44,309
please. So he needs to pick up one beeper at a time and put it on the empty corner, and when he puts it on an empty corner, put two beepers for 

487
00:24:44,309 --> 00:24:48,919
that one he puts down? Yeah. 

488
00:24:48,920 --> 00:24:49,670
So why don't we - 

489
00:24:49,670 --> 00:24:51,560
I know it's not gonna happen. 

490
00:24:51,559 --> 00:24:54,509
Yeah, I know, everyone's just starting to cower now. I do realize that, I've been teaching for 

491
00:24:54,509 --> 00:24:58,430
about 15 years and my shot has never improved, so you're just, you know, at 

492
00:24:58,430 --> 00:25:00,670
the random candy mercy. 

493
00:25:00,670 --> 00:25:03,950
Since we can't count, we need to do something beeper by beeper. So wouldnft it 

494
00:25:03,950 --> 00:25:06,930
be interesting if we said hey, you're on that pile of beepers, 

495
00:25:06,930 --> 00:25:08,929
pick up one beeper off the pile 

496
00:25:08,929 --> 00:25:12,120
and take a step somewhere else and put two beepers down, 

497
00:25:12,119 --> 00:25:15,649
and keep doing that over and over. Pick up a beeper, put two down. And eventually, 

498
00:25:15,650 --> 00:25:18,109
you'll exhaust everything on this pile over here 

499
00:25:18,108 --> 00:25:21,269
but you'll have twice as many on the next pile beside you. 

500
00:25:21,269 --> 00:25:24,849
And then take that pile of twice as many beepers and move them all back to the 

501
00:25:24,849 --> 00:25:25,959
place where you started, 

502
00:25:25,960 --> 00:25:27,209
and then you're done. 

503
00:25:27,209 --> 00:25:31,308
That's the algorithm. Let's write the code to do that. And you're like okay, [inaudible] let's write the code 

504
00:25:31,308 --> 00:25:32,470
to do that. 

505
00:25:32,470 --> 00:25:34,610
And I'm like yeah, rock on, this is gonna be easy, right? 

506
00:25:34,609 --> 00:25:37,759
What we're gonna do is we're just gonna say you're on a pile of beepers. As 

507
00:25:37,759 --> 00:25:40,849
long as there are still beepers for you to pick up - so I'm gonna have a while loop 

508
00:25:40,849 --> 00:25:41,678
here - 

509
00:25:41,679 --> 00:25:43,590
while beepers 

510
00:25:43,589 --> 00:25:45,179
present. 

511
00:25:45,180 --> 00:25:48,450
Well, what I want you to do is pick up a beeper, yeah, you know how to do that - life 

512
00:25:48,450 --> 00:25:49,580
is good. 

513
00:25:49,579 --> 00:25:51,329
Beeper, I always misspell beeper, I don't know why. 

514
00:25:51,329 --> 00:25:54,169
I want you to put two beepers next door - 

515
00:25:54,170 --> 00:25:59,350
put two beepers next door - and you're like oh, this is always the part, right, where you're 

516
00:25:59,349 --> 00:26:02,740
thinking bottom-up design, you get uncomfortable 

517
00:26:02,740 --> 00:26:06,390
calling methods that don't exist, right? You're just like oh, but that doesnft exist, right? And 

518
00:26:06,390 --> 00:26:09,030
I'm like, it's cool, man, we're gonna write it. 

519
00:26:09,029 --> 00:26:13,009
And then we hold hands and we sing Kumbaya and everything's good because at the end, 

520
00:26:13,009 --> 00:26:15,990
Karel's gonna know what to do. So sometimes, you just write methods 

521
00:26:15,990 --> 00:26:18,929
and they don't exist, and it's fine because you're the one that's gonna be 

522
00:26:18,929 --> 00:26:21,809
writing them. So eventually, they'll exist, right? It's not like you're sitting there, 

523
00:26:21,808 --> 00:26:25,210
like, lighting candles, making some magic incantation, hoping the code just shows 

524
00:26:25,210 --> 00:26:27,890
up. And you're like, is it there yet? No, it's not there yet, 

525
00:26:27,890 --> 00:26:30,890
right? You're gonna write it. I had a friend once who actually thought that's how we did 

526
00:26:30,890 --> 00:26:34,259
computer science. I didnft bother to tell him that that wasn't true. 

527
00:26:34,259 --> 00:26:37,359
But, you know, think about it. So we pick a beeper - yeah, I know, it's kind of mean, 

528
00:26:37,359 --> 00:26:40,859
but that's all right. He does government now. 

529
00:26:40,859 --> 00:26:43,758
We pick up two beepers and guess what? After we do that, 

530
00:26:43,759 --> 00:26:47,389
when that's done we will have picked up all the beepers from that pile that 

531
00:26:47,388 --> 00:26:50,689
we're standing on, and we would have put two next door for every beeper we picked 

532
00:26:50,690 --> 00:26:51,370
up. 

533
00:26:51,369 --> 00:26:54,369
So when we're done with this while loop, we know that the thing that's true 

534
00:26:54,369 --> 00:26:57,859
is that there's no more beepers on that pile because beepers present is no 

535
00:26:57,859 --> 00:26:58,908
longer true. 

536
00:26:58,909 --> 00:27:02,549
So I have some pile next to me, say on the next corner, that has twice as many 

537
00:27:02,548 --> 00:27:05,710
beepers on it. And so what do I wanna do? I just wanna move 

538
00:27:05,710 --> 00:27:10,519
the pile next door back. 

539
00:27:10,519 --> 00:27:11,849
And that's my function. You're like, oh, really? 

540
00:27:11,849 --> 00:27:13,918
I can do that? Sure, why not? 

541
00:27:13,919 --> 00:27:16,210
So what do we need to write now? Well, we need to 

542
00:27:16,210 --> 00:27:20,200
know how to put two beepers next door. So let's go ahead and write that. 

543
00:27:20,200 --> 00:27:24,340
[Inaudible] private void, put two 

544
00:27:24,339 --> 00:27:25,509
beepers 

545
00:27:25,509 --> 00:27:27,548
next door, right? 

546
00:27:27,548 --> 00:27:30,750
Now if I'm standing on a particular corner and I'm facing east, let's say I wanna 

547
00:27:30,750 --> 00:27:33,929
put two beepers on the space in front of me and come back to where I am 

548
00:27:33,929 --> 00:27:35,340
still facing east, 

549
00:27:35,339 --> 00:27:39,689
so I'm left in the same position I was when I started, how do I do that? 

550
00:27:39,690 --> 00:27:42,539
I move. 

551
00:27:42,539 --> 00:27:46,279
I need to put two beepers so I could put beeper, put beeper. And if you really 

552
00:27:46,279 --> 00:27:49,490
wanted to you could put a four-loop here and say hey, [inaudible] you're doing something twice, ooh, 

553
00:27:49,490 --> 00:27:54,529
ooh, can I do a four-loop and do four I equal into I equals, zero, I less than two, I++, 

554
00:27:54,529 --> 00:27:56,369
put beeper? 

555
00:27:56,369 --> 00:27:59,429
Sure, right? There's multiple ways of writing a program. I'm just going to put two put beepers because it's 

556
00:27:59,430 --> 00:28:01,900
two lines. The four-loop would have been two lines too. 

557
00:28:01,900 --> 00:28:04,450
Now what do I do? 

558
00:28:04,450 --> 00:28:08,549
Ah, yeah, rock on. I move backward, right? Which allows me to move back one step 

559
00:28:08,549 --> 00:28:10,349
and keep the same orientation. 

560
00:28:10,349 --> 00:28:14,928
So I end up, my post-condition is essentially the same as my pre-condition, 

561
00:28:14,929 --> 00:28:20,350
except I've moved one beeper over to the - or I put two beepers onto the next spot. And 

562
00:28:20,349 --> 00:28:22,730
move backward, you're like hey, I already wrote that. 

563
00:28:22,730 --> 00:28:25,950
So everything now is in terms of instructions that Karel will understand, so at 

564
00:28:25,950 --> 00:28:27,630
this point I don't need to go any further. 

565
00:28:27,630 --> 00:28:31,000
So I know how to put two beepers next door, so the only thing left to do in this 

566
00:28:31,000 --> 00:28:31,369
program 

567
00:28:31,369 --> 00:28:35,428
is to move the pile next door back to where I am now, okay? 

568
00:28:35,429 --> 00:28:39,580
So what that means is how do I move that pile back to where I am? 

569
00:28:39,579 --> 00:28:40,978
So I'll call this, 

570
00:28:40,979 --> 00:28:45,649
for lack of - well, I know what the function name is, I'll just - or the method name. I'll 

571
00:28:45,648 --> 00:28:48,899
just steal it from up here, because sometimes you just get tired of typing. All 

572
00:28:48,900 --> 00:28:51,680
right, so I'll move the pile next door back. Well, 

573
00:28:51,680 --> 00:28:56,190
the place that I call this, right, I call this up here. I'm not actually on the 

574
00:28:56,190 --> 00:29:00,009
pile next door when I called it, right? My pre-condition is I am on the avenue before 

575
00:29:00,009 --> 00:29:01,788
the pile. I'm facing the pile, 

576
00:29:01,788 --> 00:29:04,759
but I'm not on the pile. So the first thing I need to do 

577
00:29:04,759 --> 00:29:06,058
in terms of moving the 

578
00:29:06,058 --> 00:29:07,389
pile next door back 

579
00:29:07,390 --> 00:29:10,009
is I need to move over to where that pile is. 

580
00:29:10,009 --> 00:29:13,279
So now I'm standing on that pile of twice as many beepers, and 

581
00:29:13,279 --> 00:29:16,349
I wanna move all the beepers back one. So 

582
00:29:16,349 --> 00:29:18,609
what could I do? 

583
00:29:18,609 --> 00:29:22,750
I could pick beeper, but there's something I wanna do multiple times. 

584
00:29:22,750 --> 00:29:26,049
I need to have a wild loop, right? And I'm gonna do something - what condition 

585
00:29:26,049 --> 00:29:28,319
do I wanna test, right? 

586
00:29:28,319 --> 00:29:31,779
If beeper is present, right? So as long as there's beepers present there's still more 

587
00:29:31,779 --> 00:29:34,908
work for me to do because there's still more beepers I need to move back. 

588
00:29:34,909 --> 00:29:36,790
So while there's beepers present, 

589
00:29:36,789 --> 00:29:40,649
hey, why don't I move one beeper back? Move one 

590
00:29:40,650 --> 00:29:43,809
beeper back. 

591
00:29:43,808 --> 00:29:47,539
All right? Again, I haven't wrote that yet, that's fine. But when this loop is done, 

592
00:29:47,539 --> 00:29:50,950
what I know is I've exhausted all of the beepers that are on this pile because every 

593
00:29:50,950 --> 00:29:54,130
time there was beeper present I picked it up, I moved it back one spot, 

594
00:29:54,130 --> 00:29:57,400
and hopefully that method brought me back, should bring me back to the place that 

595
00:29:57,400 --> 00:29:59,269
I'm standing right now, and I just keep doing this 

596
00:29:59,269 --> 00:30:01,079
until I've transferred all the beepers. 

597
00:30:01,079 --> 00:30:03,809
So after I've transferred all the beepers, what do I need to do to satisfy 

598
00:30:03,809 --> 00:30:07,289
my post-condition, which is I wanna go back to the place I just transferred all 

599
00:30:07,289 --> 00:30:08,898
the beepers? 

600
00:30:08,898 --> 00:30:10,719
I need to move 

601
00:30:10,720 --> 00:30:11,240
backward. 

602
00:30:11,240 --> 00:30:14,630
Because all the beepers were transferred one avenue back. 

603
00:30:14,630 --> 00:30:18,049
So I just move backward to go back one avenue, okay? 

604
00:30:18,048 --> 00:30:19,160
Now 

605
00:30:19,160 --> 00:30:24,470
I'm almost done. I'm almost feeling good, but not quite. I need to do the move 

606
00:30:24,470 --> 00:30:25,360
one 

607
00:30:25,359 --> 00:30:26,799
beeper back. 

608
00:30:26,799 --> 00:30:29,869
And at this point, I've gotten to such a simple level that maybe I could just do 

609
00:30:29,869 --> 00:30:33,339
this. So if I'm standing on a pile of beepers facing east, how do I 

610
00:30:33,339 --> 00:30:36,129
move one beeper back behind me, 

611
00:30:36,130 --> 00:30:40,470
put it down, and come back to the spot that I'm at? 

612
00:30:40,470 --> 00:30:43,950
Well, first let me pick a beeper, right? Because I need to actually move the beeper. I 

613
00:30:43,950 --> 00:30:46,830
could turn around first, that's perfectly fine. There's multiple ways I can do things, 

614
00:30:46,829 --> 00:30:50,058
but I'm gonna pick the beeper first. Then what do I 

615
00:30:50,058 --> 00:30:53,250
do? Yeah, rather than turning around, let me just use move backward. That'll take me 

616
00:30:53,250 --> 00:30:54,859
back one spot. 

617
00:30:54,859 --> 00:30:56,449
Then I 

618
00:30:56,450 --> 00:30:58,120
put beeper. 

619
00:30:58,119 --> 00:30:59,589
And I 

620
00:30:59,589 --> 00:31:02,038
move to move back to the spot that I'm at, 

621
00:31:02,038 --> 00:31:05,759
right? Now I'm all in terms of things, what I refer to as primitives, 

622
00:31:05,759 --> 00:31:06,220
right? 

623
00:31:06,220 --> 00:31:08,740
Or instructions that I've previously defined. 

624
00:31:08,740 --> 00:31:11,569
And there's nothing more for me to define, because every time I define 

625
00:31:11,569 --> 00:31:13,490
some new method and I said hey, I'm just gonna call that, 

626
00:31:13,490 --> 00:31:16,950
I went and wrote it and I just kept doing that over and over until I'd written 

627
00:31:16,950 --> 00:31:23,950
everything I needed to do. Um-hm? Wouldn't you have to move backward twice [inaudible]? 

628
00:31:28,039 --> 00:31:30,128
On move pile next door back? 

629
00:31:30,128 --> 00:31:32,230
No, because move one beeper back, 

630
00:31:32,230 --> 00:31:36,009
I pick up a beeper, I move backward, I drop it off, and I move forward. So I'm on 

631
00:31:36,009 --> 00:31:40,879
the pile that has all the beepers. [Inaudible.] 

632
00:31:40,880 --> 00:31:44,669
Yeah, and that's what I did up here, right? So after I doubled the beeper pile, then I 

633
00:31:44,669 --> 00:31:48,059
moved backwards. So yeah, that's a good point, but you always wanna remember what 

634
00:31:48,058 --> 00:31:51,148
other things are gonna happen after you execute all that, okay? 

635
00:31:51,148 --> 00:31:58,148
And so now the question comes - ah? [Inaudible.] 

636
00:32:01,660 --> 00:32:04,800
That's a good question. The only problem with doing that is Karel starts 

637
00:32:04,799 --> 00:32:07,428
off with an infinite number of beepers in his bag, right? 

638
00:32:07,429 --> 00:32:10,620
So if I pick up all five beepers at the corner that I'm at and then I take 

639
00:32:10,619 --> 00:32:14,359
a step back, or I pick up all five beepers and now I wanna put them all down 

640
00:32:14,359 --> 00:32:15,189
and double them, 

641
00:32:15,190 --> 00:32:17,990
I don't know that there were five anymore, because after I pick up the five, I look 

642
00:32:17,990 --> 00:32:19,650
in my beeper bag and I'm like 

643
00:32:19,650 --> 00:32:22,440
oh, man, I had infinitely many beepers. I've got 

644
00:32:22,440 --> 00:32:25,960
infinity plus five, and that looks like infinity, right? As a matter of fact, if you 

645
00:32:25,960 --> 00:32:28,600
take CS103, which I encourage you to do because I teach it in the 

646
00:32:28,599 --> 00:32:32,289
winter eventually, we talk about infinities, and we're like oh, infinities, and it all comes back 

647
00:32:32,289 --> 00:32:34,819
to Karel. No, it actually doesnft come back to Karel at all. 

648
00:32:34,819 --> 00:32:37,509
But infinity plus any number is 

649
00:32:37,509 --> 00:32:40,529
within reason - yeah, if you're a number theorist, you're gonna argue about that - is 

650
00:32:40,529 --> 00:32:41,549
still infinity. 

651
00:32:41,549 --> 00:32:43,460
So that's why Karel can't count. 

652
00:32:43,460 --> 00:32:47,519
If he had an empty beeper bag we could put all five in it and look inside and then we 

653
00:32:47,519 --> 00:32:50,408
could put down five. But now we don't have five more beepers to double the number 

654
00:32:50,409 --> 00:32:53,419
of beepers, which is why Karel had to start with infinitely many beepers, 

655
00:32:53,419 --> 00:32:56,230
so no matter how many were in the initial pile he would always have enough 

656
00:32:56,230 --> 00:32:57,190
beepers to double it. 

657
00:32:57,190 --> 00:32:58,179
But that's a good point. 

658
00:32:58,179 --> 00:33:01,419
So let's actually go ahead and run our program now. Let's save, and we'll go 

659
00:33:01,419 --> 00:33:03,910
ahead and run Our Double Beepers. 

660
00:33:03,910 --> 00:33:05,850
Doo, doo, doo, doo, doo. 

661
00:33:05,849 --> 00:33:08,648
Operate just to make sure it actually works. 

662
00:33:08,648 --> 00:33:11,819
And we'll sort of do it at slow speed. 

663
00:33:11,819 --> 00:33:14,558
So there's our world, let's start the program. There's Karel, 

664
00:33:14,558 --> 00:33:19,680
and you can see he's going [inaudible] beeper - whoa. He 

665
00:33:19,680 --> 00:33:25,160
just didnft want me to explain that program at all. All right, Karel, I'll remember this. 

666
00:33:25,160 --> 00:33:30,130
So for every beeper he picked up, he put two here, and now he's just transferring them all back. 

667
00:33:30,130 --> 00:33:31,370
Go, Karel. 

668
00:33:31,369 --> 00:33:35,919
And so now he's done with 10, and you'll notice he went back here after 10. Why? Because he 

669
00:33:35,920 --> 00:33:38,700
needed to check to see if there were any more here, and there werenft, so he moved 

670
00:33:38,700 --> 00:33:39,460
back 

671
00:33:39,460 --> 00:33:40,480
and then he was done. 

672
00:33:40,480 --> 00:33:43,710
So we just doubled the number of beepers. And this program, actually, notice the 

673
00:33:43,710 --> 00:33:45,179
initial state of the world 

674
00:33:45,179 --> 00:33:48,278
is now exactly the same as the ending state of the world, except we 

675
00:33:48,278 --> 00:33:50,788
doubled the number of beepers. So we could actually run this again if we wanted to - 

676
00:33:50,788 --> 00:33:52,599
let me speed it up a little bit - 

677
00:33:52,599 --> 00:33:54,178
and we'll go from 10 beepers to 20 beepers. And 

678
00:33:54,179 --> 00:33:57,580
there is Karel sort of doing his thing at faster speed, but you can 

679
00:33:57,579 --> 00:33:59,288
see there's 20 beepers over there, 

680
00:33:59,288 --> 00:34:02,429
now they all start coming back and saying, you'll be like, hey, [inaudible], how many times can I run it? 

681
00:34:02,429 --> 00:34:05,360
Can I just run it over and over and I'll get, like, you know, 40 beepers and 80 beepers 

682
00:34:05,359 --> 00:34:06,538
and 160 beepers? 

683
00:34:06,538 --> 00:34:09,489
You just can't. I think eventually the program dies when you reach, like, 100 

684
00:34:09,489 --> 00:34:12,158
and some-odd thousand beepers, because it's just like no more, man. 

685
00:34:12,159 --> 00:34:14,079
No more, all right? 

686
00:34:14,079 --> 00:34:18,109
But for all programs you write in this class, if you ever have more than 100,000 

687
00:34:18,108 --> 00:34:23,219
beepers on a corner, there's probably something wrong, so you don't need to worry about it, okay? Now, 

688
00:34:23,219 --> 00:34:26,608
that's - so congratulations, right? We just all wrote this together, and 

689
00:34:26,608 --> 00:34:29,460
you just made Karel do math. As a matter of fact, Karel, 

690
00:34:29,460 --> 00:34:34,050
in terms of raw computational power, can solve any arithmetic problem that 

691
00:34:34,050 --> 00:34:35,519
any other program can. 

692
00:34:35,518 --> 00:34:38,488
It just may take him a lot of beepers to do it. But theoretically, you can prove that. 

693
00:34:38,489 --> 00:34:39,739
We're not gonna do it in this class, 

694
00:34:39,739 --> 00:34:43,148
but if you're just kind of - you're like can Karel compute, like, square root and logarithm 

695
00:34:43,148 --> 00:34:45,838
and he can integrate? Yeah, he can. 

696
00:34:45,838 --> 00:34:50,318
It's just a whole lot of beepers, all right? 

697
00:34:50,318 --> 00:34:51,358
So 

698
00:34:51,358 --> 00:34:53,268
I could show you another program. 

699
00:34:53,268 --> 00:34:57,449
Here's another little program I happened to write in the office. 

700
00:34:57,449 --> 00:35:01,778
This is a program that happens to be called Do Your Thing, okay? 

701
00:35:01,778 --> 00:35:04,958
And I'll show you the full program, I'll scroll it down here so you can see the full body of the 

702
00:35:04,958 --> 00:35:05,509
program. 

703
00:35:05,509 --> 00:35:11,329
And all I will ask you, what does this program do? Doo, doo, doo, doo, doo, doo, doo, doo, 

704
00:35:11,329 --> 00:35:13,189
doo, doo, doo, doo, 

705
00:35:13,188 --> 00:35:15,608
doo. 

706
00:35:15,608 --> 00:35:20,009
This does example the same thing the code you just wrote did, okay? 

707
00:35:20,009 --> 00:35:22,409
This is really bad software engineering, 

708
00:35:22,409 --> 00:35:25,939
right? There is no decomposition, there's no comments, there's no indentation, 

709
00:35:25,938 --> 00:35:28,568
there's no notion of what's going on in this program. If you looked at this program and 

710
00:35:28,568 --> 00:35:29,188
I said, 

711
00:35:29,188 --> 00:35:32,188
hey, instead of doubling the number of beepers, I want you to say triple the 

712
00:35:32,188 --> 00:35:35,139
number of beepers, you'd have to go through this thing, and you might, because 

713
00:35:35,139 --> 00:35:38,708
you just wrote it, say hey, I know that there's two put beepers in a row, so I put 

714
00:35:38,708 --> 00:35:43,138
another put beeper in there and it'll probably work, right? Whereas 

715
00:35:43,139 --> 00:35:46,179
say you're a software engineer and you just got your first job, and you can look at either 

716
00:35:46,179 --> 00:35:49,679
that code, or you can come over here and look at the double beepers that's actually 

717
00:35:49,679 --> 00:35:50,969
in your assignment, 

718
00:35:50,969 --> 00:35:55,009
that has all the comments, like, hey, this doubles the number of beepers. And over here, 

719
00:35:55,009 --> 00:35:58,748
guess what? This is the version that's in your handout. This is all the same code we 

720
00:35:58,748 --> 00:36:02,658
just wrote except now with comments. Like for every beeper on the current corner, 

721
00:36:02,659 --> 00:36:05,499
Karel places two beepers on the corner immediately ahead of him, 

722
00:36:05,498 --> 00:36:08,728
and this says place two beepers on corner one avenue ahead. And you say hey, 

723
00:36:08,728 --> 00:36:10,759
if I wanna triple the number of beepers, I change the 

724
00:36:10,759 --> 00:36:13,858
comment from two to three and put another put beeper here, and I know 

725
00:36:13,858 --> 00:36:16,989
exactly where it goes and it's easy to understand. 

726
00:36:16,989 --> 00:36:19,028
That's why software engineering's important, 

727
00:36:19,028 --> 00:36:21,918
because that kind of modification to code happens 

728
00:36:21,918 --> 00:36:23,048
far more often 

729
00:36:23,048 --> 00:36:25,139
than writing code from scratch - 

730
00:36:25,139 --> 00:36:26,998
about 10 times more often. 

731
00:36:26,998 --> 00:36:31,398
So if the code is bad to begin with, it gets 10 times more costly to go and 

732
00:36:31,398 --> 00:36:33,199
make fixes to it, and that's something that's 

733
00:36:33,199 --> 00:36:36,509
sort of well documented. Some people actually argue that the number is higher. But that's why 

734
00:36:36,510 --> 00:36:40,040
it's important that when you write your program the first time, or subsequent 

735
00:36:40,039 --> 00:36:43,309
revisions you make to it, you have good style because that's just way more important than 

736
00:36:43,309 --> 00:36:46,358
just getting the program working. And I've seen people actually 

737
00:36:46,358 --> 00:36:47,799
turn in programs 

738
00:36:47,800 --> 00:36:50,960
called Do Your Thing. That's where I actually got the name, was I actually knew someone once 

739
00:36:50,960 --> 00:36:54,480
who write a program called Do Your Thing. And I was, like, youfve gotta be kidding, 

740
00:36:54,480 --> 00:36:56,019
right? 

741
00:36:56,018 --> 00:36:58,828
And it was just like this, right? It was just a whole bunch of straight-line code, 

742
00:36:58,829 --> 00:37:02,380
no decomposition. They were like, well, it works. And I'm like yeah, 

743
00:37:02,380 --> 00:37:05,818
but that's not what software engineering is about. And we got into this - yeah, I won't call it an 

744
00:37:05,818 --> 00:37:07,219
argument, I'll call it a discussion. 

745
00:37:07,219 --> 00:37:10,829
And they got really adamant, because they were, like, but it works, so I must get an A. 

746
00:37:10,829 --> 00:37:12,250
And I was like, you know, 

747
00:37:12,250 --> 00:37:15,349
someday if you're teaching this class and you're willing to accept code like this, 

748
00:37:15,349 --> 00:37:17,239
you can give yourself an A. 

749
00:37:17,239 --> 00:37:18,489
I'm not, 

750
00:37:18,489 --> 00:37:19,378
because 

751
00:37:19,378 --> 00:37:23,239
this is just - anyone who sees this, not only in this class but outside of this 

752
00:37:23,239 --> 00:37:24,838
class, will be horrendous code, 

753
00:37:24,838 --> 00:37:28,248
and I don't want any people coming out of this class to go out into the industry and be like, 

754
00:37:28,248 --> 00:37:31,468
hey, yeah, this is what [inaudible] told me to do, that's why I'm writing it this way, right? 

755
00:37:31,469 --> 00:37:34,739
Because not only will they be, like, no, I'm sorry, you're fired, but find that 

756
00:37:34,739 --> 00:37:38,369
clown [inaudible] and fire him, too, right? So 

757
00:37:38,369 --> 00:37:40,219
don't write code like this. Is 

758
00:37:40,219 --> 00:37:44,309
there any questions about this? Um-hm? [Inaudible.] 

759
00:37:44,309 --> 00:37:47,439
And let me bring up the good code again, just so you don't get, you know, think 

760
00:37:47,438 --> 00:37:50,949
like that other code is the good thing to do. When you're naming 

761
00:37:50,949 --> 00:37:53,259
the private [inaudible] methods, do 

762
00:37:53,259 --> 00:37:56,039
you - should the first letter be capitalized, or not? 

763
00:37:56,039 --> 00:37:57,239
Ah, good question. 

764
00:37:57,239 --> 00:38:01,929
So in terms of - someone asked before, all the words or methods that we 

765
00:38:01,929 --> 00:38:03,588
define are all case sensitive. 

766
00:38:03,588 --> 00:38:05,538
You'll notice that 

767
00:38:05,539 --> 00:38:09,028
sometimes we use a naming convention. I'm actually bad because I mixed up the naming 

768
00:38:09,028 --> 00:38:10,659
convention a little bit where 

769
00:38:10,659 --> 00:38:14,568
the first name of the methods, we can either have a lower-case character or an 

770
00:38:14,568 --> 00:38:17,570
upper-case character, it's up to you how you want to do it. 

771
00:38:17,570 --> 00:38:21,349
Just be consistent. Sometimes, this notation is referred to as camel case, 

772
00:38:21,349 --> 00:38:25,619
and the reason why it's referred to as camel case, the first letter of every word that 

773
00:38:25,619 --> 00:38:29,230
actually appears sort of all stuck together is upper cased, and it's kind of like 

774
00:38:29,230 --> 00:38:32,849
a camel with humps, because every once in a while you get this capital letter with a hump. 

775
00:38:32,849 --> 00:38:36,329
So that when you - if you hear someone refer to oh, we use capital case or camel 

776
00:38:36,329 --> 00:38:39,670
case notation, that's what they mean. It's all strung together as one word 

777
00:38:39,670 --> 00:38:41,240
with the first 

778
00:38:41,239 --> 00:38:43,739
letter of each word capitalized. 

779
00:38:43,739 --> 00:38:46,669
Sometimes what people will do is they'll actually put in underscores. You can put in 

780
00:38:46,670 --> 00:38:50,150
underscores between words and that's perfectly fine, too. When we get into 

781
00:38:50,150 --> 00:38:51,670
Java, I'll actually spent 

782
00:38:51,670 --> 00:38:54,389
probably like half of a lecture talking about the different conventions 

783
00:38:54,389 --> 00:38:57,859
for all kinds of things, because when we get into the Java world there's actually a bunch of things 

784
00:38:57,860 --> 00:39:00,009
besides just method names you need to worry about, 

785
00:39:00,009 --> 00:39:02,568
and we use all different sorts of conventions for them. For right now, I'd 

786
00:39:02,568 --> 00:39:06,458
just say pick one convention and stick with it, and the convention you can use is, 

787
00:39:06,458 --> 00:39:09,208
you know, the two most common ones are either camel case all the way 

788
00:39:09,208 --> 00:39:13,778
through or camel case except for the first character, which is lower case. 

789
00:39:13,778 --> 00:39:20,778
Any other questions? Um-hm? [Inaudible.] Instructor: 

790
00:39:25,949 --> 00:39:29,909
Yeah, so it starts at run and every time it encounters some method that it doesnft 

791
00:39:29,909 --> 00:39:32,959
know is a primitive method, it just says hey, did you define it somewhere? 

792
00:39:32,958 --> 00:39:35,379
And it goes to that definition. And guess what? If it encounters one that you didnft 

793
00:39:35,380 --> 00:39:37,108
define, that's an error. 

794
00:39:37,108 --> 00:39:39,909
And usually it'll tell you that when you try to run the program. It'll tell you 

795
00:39:39,909 --> 00:39:43,900
that this thing doesnft actually exist. All righty? 

796
00:39:43,900 --> 00:39:50,318
Any other questions about - um-hm? [Inaudible] as to what order you should put your [inaudible]? 

797
00:39:50,318 --> 00:39:53,179
No, you can put them in whatever order you want. Usually I just put them in the 

798
00:39:53,179 --> 00:39:56,469
order as I need to define them, right? So just like we did in class, I just keep going 

799
00:39:56,469 --> 00:39:59,450
down, which means run starts at the top, and everything comes underneath it. 

800
00:39:59,449 --> 00:40:02,649
A couple things related to that. One question that always comes up is 

801
00:40:02,650 --> 00:40:06,329
how do I know how much to decompose, right? Like aren't there different ways I 

802
00:40:06,329 --> 00:40:09,639
could decompose? And in fact, yeah, there are multiple different ways you can decompose. 

803
00:40:09,639 --> 00:40:12,619
So let me just give you some sort of quickie guidelines about how you might 

804
00:40:12,619 --> 00:40:15,038
think about decomposing, okay? 

805
00:40:15,039 --> 00:40:17,940
In terms of decomposition, the thing that you really wanna think about 

806
00:40:17,940 --> 00:40:19,999
is every one of your methods 

807
00:40:19,998 --> 00:40:23,248
should basically solve one problem, okay? The 

808
00:40:23,248 --> 00:40:27,018
only thing is that that problem can be at different levels. So when you think 

809
00:40:27,018 --> 00:40:29,959
about decomposition, 

810
00:40:29,960 --> 00:40:31,760
what are some good guidelines? 

811
00:40:31,760 --> 00:40:36,749
One guideline is think about each method solving one problem, 

812
00:40:36,748 --> 00:40:40,108
okay? That problem can be high-level, like double the number of beepers . 

813
00:40:40,108 --> 00:40:41,889
That's a high-level single problem. 

814
00:40:41,889 --> 00:40:43,449
When you get into doubling 

815
00:40:43,449 --> 00:40:46,309
the number of beepers, you don't just wanna say oh, double the number of beepers 

816
00:40:46,309 --> 00:40:47,259
again, because you're 

817
00:40:47,259 --> 00:40:50,708
already at that level of decomposition. You need to go one step down and say, 

818
00:40:50,708 --> 00:40:53,908
what are the things that make sense in terms of solving one problem at a time 

819
00:40:53,909 --> 00:40:54,979
to break it down? 

820
00:40:54,978 --> 00:40:56,519
Now a lot of times, people wonder 

821
00:40:56,519 --> 00:41:00,230
but [inaudible], what does that mean in terms of lines of code? Because sometimes it's 

822
00:41:00,230 --> 00:41:03,369
different for me, at least conceptually, to think about one problem. And here's just a 

823
00:41:03,369 --> 00:41:04,159
rough 

824
00:41:04,159 --> 00:41:04,618
guideline. 

825
00:41:04,619 --> 00:41:09,160
It's not a guideline that always holds, but a rough guideline is most of your methods 

826
00:41:09,159 --> 00:41:15,848
will probably be somewhere between one and 15 lines long, okay? 

827
00:41:15,849 --> 00:41:17,419
And you're like, one line? 

828
00:41:17,418 --> 00:41:20,759
Yeah, that's perfectly fine. It's perfectly fine for you to have a method that's 

829
00:41:20,759 --> 00:41:22,929
just one line long that calls some other method, 

830
00:41:22,929 --> 00:41:24,589
and you might say what's the purpose of that? 

831
00:41:24,590 --> 00:41:29,509
The purpose of that is giving that thing that you call a different name, right? So 

832
00:41:29,509 --> 00:41:32,670
remember when we did ascend hurdle, which basically brought you all the way down until 

833
00:41:32,670 --> 00:41:35,619
you were facing a wall, and then it did one last turn at the end to get you facing the right 

834
00:41:35,619 --> 00:41:36,599
direction? 

835
00:41:36,599 --> 00:41:39,989
But basically all it was doing was move to wall. We could have actually defined ascend - or 

836
00:41:39,989 --> 00:41:41,429
sorry, descend hurdle 

837
00:41:41,429 --> 00:41:44,858
to be move to wall and then do the turn afterwards. 

838
00:41:44,858 --> 00:41:48,598
That would have been perfectly fine, too, and then descend hurdle would have just been a 

839
00:41:48,599 --> 00:41:49,410
one-line method. 

840
00:41:49,409 --> 00:41:52,949
But it would have changed the concept, the way the programmer thinks 

841
00:41:52,949 --> 00:41:53,599
about it, 

842
00:41:53,599 --> 00:41:55,438
from being hey, I'm moving to a wall 

843
00:41:55,438 --> 00:41:57,848
to be hey, I'm descending a hurdle. 

844
00:41:57,849 --> 00:42:01,039
So that's another thing that kind of comes up in terms of the methods is you wanna 

845
00:42:01,039 --> 00:42:06,079
think about them as having good names. The names should describe what 

846
00:42:06,079 --> 00:42:09,140
the method is actually doing, what problem it's solving, 

847
00:42:09,139 --> 00:42:12,779
because you want your programs to read like good English, right? When 

848
00:42:12,780 --> 00:42:16,019
someone - again, programs are written for people. 

849
00:42:16,018 --> 00:42:18,628
Computers execute them, but they're really written for people to be able to 

850
00:42:18,628 --> 00:42:20,079
go and look at and modify, 

851
00:42:20,079 --> 00:42:23,889
so good names explains what the program actually does, and every one of the 

852
00:42:23,889 --> 00:42:27,279
methods that you have in terms of decomposition should also have some 

853
00:42:27,280 --> 00:42:28,540
comments associated with it 

854
00:42:28,539 --> 00:42:32,820
that explains either at some more detailed level what the method is doing, or 

855
00:42:32,820 --> 00:42:34,548
if the method is 

856
00:42:34,548 --> 00:42:37,210
very short and it's fairly self-explanatory, 

857
00:42:37,210 --> 00:42:39,969
pre-conditions and post-conditions are sometimes a good idea as well. 

858
00:42:39,969 --> 00:42:43,009
So think about these things for your assignment. When you're actually writing 

859
00:42:43,009 --> 00:42:46,889
the code for your assignment, you wanna think about the sort of decomposition using 

860
00:42:46,889 --> 00:42:50,159
the step-wise refinement process and make sure to 

861
00:42:50,159 --> 00:42:53,588
comment - both a comment for your whole program and a comment for each method. 

862
00:42:53,588 --> 00:42:54,719
Um-hm, question? 

863
00:42:54,719 --> 00:42:59,149
The book mentioned procedural programming and decomposition under 

864
00:42:59,150 --> 00:43:03,349
that. Is that what we're doing right now, or are we doing the - Well, there's a difference between procedural 

865
00:43:03,349 --> 00:43:07,140
and object-oriented programming. Decomposition actually applies to both of them, 

866
00:43:07,139 --> 00:43:10,879
but what we are doing is decomposition whether it's procedural or object-oriented. 

867
00:43:10,880 --> 00:43:11,919


868
00:43:11,918 --> 00:43:13,588
They're just two different - 

869
00:43:13,588 --> 00:43:15,739
come up to me after class and I'll set you up. 

870
00:43:15,739 --> 00:43:18,149
So I did wanna show you one more thing, 

871
00:43:18,150 --> 00:43:21,709
and the one thing I wanna show you is yet another interesting problem, 

872
00:43:21,708 --> 00:43:23,669
which is called Clean-Up Karel. 

873
00:43:23,670 --> 00:43:27,329
And what Clean-Up Karel was basically gonna do is Karel starts off, it's kind of 

874
00:43:27,329 --> 00:43:30,598
like after - we should call it After-Fraternity Party Karel. It could 

875
00:43:30,599 --> 00:43:32,900
be After-Sorority Party Karle, too. 

876
00:43:32,900 --> 00:43:36,000
I don't wanna make any kind of claims. But basically what ends up 

877
00:43:36,000 --> 00:43:36,940
happening 

878
00:43:36,940 --> 00:43:39,329
is that Karel kind of comes into his world 

879
00:43:39,329 --> 00:43:40,719
and it's a mess. 

880
00:43:40,719 --> 00:43:43,798
He's just like what happened? When last I was in this world I just doubled the number 

881
00:43:43,798 --> 00:43:47,038
of beepers and shown you I can do math. And now you bring me into this world 

882
00:43:47,039 --> 00:43:51,528
and there's just crap everywhere, right? And we could call him Angry Karel, too, right? 

883
00:43:51,528 --> 00:43:53,190
He's the one who's gotta live here. And there's, 

884
00:43:53,190 --> 00:43:56,400
like, cups strewn all around and they're just on random corners, and the things that 

885
00:43:56,400 --> 00:44:00,719
you know about Karel in this world is he starts at location 1-1 

886
00:44:00,719 --> 00:44:04,398
and every corner of the world can have at most one beeper on it. It means it can 

887
00:44:04,398 --> 00:44:05,868
either be empty, 

888
00:44:05,869 --> 00:44:08,818
like some of these corners, or have one beeper on them. And what Karel needs to 

889
00:44:08,818 --> 00:44:11,058
do is he needs to go through his whole 

890
00:44:11,059 --> 00:44:14,189
world and clean it up, which means he needs to gather up all the beepers that are 

891
00:44:14,188 --> 00:44:14,808
out there. 

892
00:44:14,809 --> 00:44:18,309
And he can end up wherever he wants to end up, okay? 

893
00:44:18,309 --> 00:44:20,979
Now when we kind of think about a problem like this, one thing we might be 

894
00:44:20,978 --> 00:44:22,179
inclined to do 

895
00:44:22,179 --> 00:44:25,768
is you say hey, if there's a whole world that I wanna clean up, 

896
00:44:25,768 --> 00:44:29,488
I might wanna break this problem down into a series of steps. 

897
00:44:29,489 --> 00:44:32,798
One step - and something that is, like, oh, I can go and write it right now because I know how to 

898
00:44:32,798 --> 00:44:35,838
write it is maybe to clean up one row at a time, right? 

899
00:44:35,838 --> 00:44:38,429
Because I probably know how to clean up one row, I probably just wanna keep 

900
00:44:38,429 --> 00:44:42,460
moving until I get to a wall, and if every time I move there happens to be a beeper 

901
00:44:42,460 --> 00:44:45,778
there, just pick it up. And so you get all excited and you go and you write the 

902
00:44:45,778 --> 00:44:47,708
code to do that, 

903
00:44:47,708 --> 00:44:49,009
and so you start off 

904
00:44:49,009 --> 00:44:51,858
by having something like this - let me 

905
00:44:51,858 --> 00:44:54,199
just focus on the top code up here - 

906
00:44:54,199 --> 00:44:54,998
right, which is 

907
00:44:54,998 --> 00:44:57,688
hey, I wanna clean a row. This thing just 

908
00:44:57,688 --> 00:44:59,908
does 

909
00:44:59,909 --> 00:45:02,999
not wanna stay on me. It's like no, you will not talk into the microphone. 

910
00:45:02,998 --> 00:45:06,438
So cleaning up a row I could say hey, you know, [inaudible] told me about the OBOB 

911
00:45:06,438 --> 00:45:10,808
too, he told me about the off-by-one bug. So before I even do my first move, 

912
00:45:10,809 --> 00:45:13,839
I'm gonna see if there's a beeper where I'm standing now, and if there is, I'm gonna 

913
00:45:13,838 --> 00:45:14,750
pick it up. 

914
00:45:14,750 --> 00:45:17,130
And then I'll check if my front is clear, 

915
00:45:17,130 --> 00:45:18,690
and if it is I know I can move 

916
00:45:18,690 --> 00:45:21,400
and I should check to see if there's a beeper there, and I'll just keep doing 

917
00:45:21,400 --> 00:45:24,139
this over and over until I've picked up all the beepers. 

918
00:45:24,139 --> 00:45:26,538
And that's a perfectly fine thing to do, 

919
00:45:26,539 --> 00:45:29,759
right? So now youfve written some method called clean row that can clean up a 

920
00:45:29,759 --> 00:45:33,318
whole row, and you know youfve sort of dealt with the off-by-one bug because 

921
00:45:33,318 --> 00:45:37,509
you sort of do one extra thing outside of your loop, either before or after. In this 

922
00:45:37,509 --> 00:45:40,630
case, you happened to do it before just so you can see a different kind of 

923
00:45:40,630 --> 00:45:42,088
formulation of it than doing it after, 

924
00:45:42,088 --> 00:45:43,139
and that's great. 

925
00:45:43,139 --> 00:45:46,269
The only thing is, now you need to figure out where clean row is gonna go in 

926
00:45:46,268 --> 00:45:47,389
your program, 

927
00:45:47,389 --> 00:45:52,318
okay? This is an example of sort of doing more, the notion of bottom-up as opposed 

928
00:45:52,318 --> 00:45:55,599
to top-down design. It's easy to get excited, to say oh, there's something I 

929
00:45:55,599 --> 00:45:59,798
know how to do, and just go write it. And if you happen to write the right thing 

930
00:45:59,798 --> 00:46:04,188
that fits cleanly into the rest of your program, that's great. But sometimes it 

931
00:46:04,188 --> 00:46:07,558
doesnft, and you might need to go back and modify it, so keep that in mind. But if you 

932
00:46:07,559 --> 00:46:10,600
do this, that's perfectly okay. It turns out that in this case, 

933
00:46:10,599 --> 00:46:14,150
that's a fine thing to do, to clean one row at a time. And so if we wanna think 

934
00:46:14,150 --> 00:46:17,389
about how that fits into the larger program, 

935
00:46:17,389 --> 00:46:18,979
here's kind of a funky thing. 

936
00:46:18,978 --> 00:46:22,398
And let me just explain the algorithm to you, because the algorithm here is somewhat 

937
00:46:22,398 --> 00:46:25,719
related to an algorithm, for example, on one of your programming assignments. 

938
00:46:25,719 --> 00:46:28,979
I'm gonna clean up the first row, because I know that first row I always need to 

939
00:46:28,978 --> 00:46:31,288
clean. So I'll just go clean up the first row, 

940
00:46:31,289 --> 00:46:33,819
and now I wanna check to see am I done. 

941
00:46:33,818 --> 00:46:37,628
Well, if I've just cleaned up a row, the only way I know I'm done is if the 

942
00:46:37,628 --> 00:46:41,418
ceiling's above me, right? I've reached the end of my world. And so I'm facing east, so I 

943
00:46:41,418 --> 00:46:44,308
check my left to see if there's a ceiling up there. 

944
00:46:44,309 --> 00:46:48,339
And if there's a ceiling up there, then I'm done. But if there isnft a ceiling up 

945
00:46:48,338 --> 00:46:50,619
there, then I need to do more work. So 

946
00:46:50,619 --> 00:46:54,419
while my left is clear, right, there's no ceiling up there, I'm gonna 

947
00:46:54,418 --> 00:46:57,969
reposition for it to clean up a row to the west, because I just cleaned up a row sort 

948
00:46:57,969 --> 00:46:59,420
of moving to the east - am 

949
00:46:59,420 --> 00:47:00,999
I facing your east? Yeah. 

950
00:47:00,998 --> 00:47:04,088
I just cleaned up a row to the east, and I wanna say hey, no ceiling here, that 

951
00:47:04,088 --> 00:47:08,148
means there's another row to clean up here. So what I wanna do is move up one 

952
00:47:08,148 --> 00:47:11,509
row to be able to now head west and clean up the next row, because I'm gonna 

953
00:47:11,509 --> 00:47:15,148
go back and forth, that's my algorithm. So I'm just gonna write some 

954
00:47:15,148 --> 00:47:18,438
function that says reposition for a row to the west, and go ahead and clean that 

955
00:47:18,438 --> 00:47:22,489
row. Now what do I need to do 

956
00:47:22,489 --> 00:47:23,079
again? 

957
00:47:23,079 --> 00:47:24,890
I need to check my roof. 

958
00:47:24,889 --> 00:47:28,929
But now my roof, because I'm not facing that way, isnft on my left 

959
00:47:28,929 --> 00:47:32,659
anymore. I need to check my right to actually check my roof, so I say if my right is 

960
00:47:32,659 --> 00:47:36,578
clear, then reposition for a row to the east. You're gonna go back the other way 

961
00:47:36,579 --> 00:47:38,818
and clean that row, okay? 

962
00:47:38,818 --> 00:47:43,759
Now there's this else here, and we'll get to the else in just a second, but just to 

963
00:47:43,759 --> 00:47:47,248
figure out what reposition to west and east are, they're actually very simple. I'll 

964
00:47:47,248 --> 00:47:49,828
just show you those down here. 

965
00:47:49,829 --> 00:47:53,410
Reposition for row to west means I just finished a row to the east, how do I get 

966
00:47:53,409 --> 00:47:56,149
up to harvest the next row or to clean up the next row? 

967
00:47:56,150 --> 00:47:59,108
I do one turn left that gets me to face north, 

968
00:47:59,108 --> 00:48:02,699
I move up one step so now imagine I've floated up one row, 

969
00:48:02,699 --> 00:48:05,739
and then I do another turn left and I'm facing like this, and now I'm ready 

970
00:48:05,739 --> 00:48:07,598
to plow over the other direction. 

971
00:48:07,599 --> 00:48:11,109
And I do sort of the opposite for repositioning for a row to the 

972
00:48:11,108 --> 00:48:12,119
east. So I'm standing here, 

973
00:48:12,119 --> 00:48:16,108
I do a turn right, I look up, I move up one row, I do another turn right, and now 

974
00:48:16,108 --> 00:48:17,719
I'm ready to go back that way. 

975
00:48:17,719 --> 00:48:21,099
So these are very simple, right? They're in terms of primitives. 

976
00:48:21,099 --> 00:48:24,139
So this is the whole program. Youfve just seen the whole program now. 

977
00:48:24,139 --> 00:48:26,689
The only question that comes up 

978
00:48:26,688 --> 00:48:28,899
is what happens 

979
00:48:28,900 --> 00:48:32,528
with this little turn around here. What's going on there? So you said hey, [inaudible] you 

980
00:48:32,528 --> 00:48:33,599
just finished 

981
00:48:33,599 --> 00:48:36,869
cleaning a row and you just checked to see with your 

982
00:48:36,869 --> 00:48:40,189
right hand, actually, if my right is clear, if there was a ceiling above you. 

983
00:48:40,188 --> 00:48:42,998
And if there was a ceiling above you, right? 

984
00:48:42,998 --> 00:48:45,958
Or if there wasn't a ceiling above you, you went ahead and got ready to clean 

985
00:48:45,958 --> 00:48:49,969
another row. But if there was a ceiling above you, what do you wanna do? And you say hey, 

986
00:48:49,969 --> 00:48:53,168
if there was a ceiling above me, I wanna stop - just stop. Is there some way to get 

987
00:48:53,168 --> 00:48:56,719
Karel to stop? For the love of god, Karel, stop. No, Karel's 

988
00:48:56,719 --> 00:49:01,900
just like no, man, I'm going. I'm going. You're like, there's no way to just say stop, Karel? 

989
00:49:01,900 --> 00:49:05,889
How do you get Karel to stop? You need him to finish executing what he's 

990
00:49:05,889 --> 00:49:06,528
executing. 

991
00:49:06,528 --> 00:49:10,318
What is he executing, right? If we just got here and we cleaned up a row and I checked my 

992
00:49:10,318 --> 00:49:11,909
right and there's a ceiling there, 

993
00:49:11,909 --> 00:49:16,298
if I don't do anything, what's Karel gonna do? He's gonna go back around and 

994
00:49:16,298 --> 00:49:20,018
say hey, is your left clear? Yeah, because my left is below me, it's clear. And it's 

995
00:49:20,018 --> 00:49:23,738
gonna go and try to clean more, even though I just finished the last row. 

996
00:49:23,739 --> 00:49:27,298
So what I actually do is when my right is blocked over here I say 

997
00:49:27,298 --> 00:49:31,429
hey, you know what, Karel? To get you to think that you're done, I'm gonna have you 

998
00:49:31,429 --> 00:49:35,179
turn around. So now when you check your left, the ceiling is gonna be there and 

999
00:49:35,179 --> 00:49:36,969
you're gonna break out of that loop. 

1000
00:49:36,969 --> 00:49:39,729
So you wanna think about the conditions on your loop to actually get 

1001
00:49:39,728 --> 00:49:43,528
this to happen. So in our final 10 seconds together, I'll just run this to show you that it 

1002
00:49:43,528 --> 00:49:47,289
actually works. Doo, doo, doo. 

1003
00:49:47,289 --> 00:49:50,819
And you're like actually, [inaudible] you're in, like, our, you know, final minus four minutes 

1004
00:49:50,818 --> 00:49:54,579
together. I know, I'm sorry. Education's one of those odd things that the more - you 

1005
00:49:54,579 --> 00:49:55,709
get more of it 

1006
00:49:55,708 --> 00:49:59,659
for paying the same amount, and somehow you feel like you were cheated. 

1007
00:49:59,659 --> 00:50:00,478


1008
00:50:00,478 --> 00:50:03,608
So let's start Karel. Look, if I 

1009
00:50:03,608 --> 00:50:07,348
let you out 10 minutes early, everyone would be like oh, yeah, rock on, but if I gave 

1010
00:50:07,349 --> 00:50:10,769
you a car that only had three doors on it, you'd be like what's up with that? 

1011
00:50:10,768 --> 00:50:11,848
Strange, how that works. 

1012
00:50:11,849 --> 00:50:14,939
And there's Karel, just cleaned up his world. All righty, so I will see you on 

1013
00:50:14,938 --> 00:50:15,998
Monday. Have a good weekend.