Programming Methodology (Stanford). Lecture 3

So a little bit of additional background on Karel before we dive into the real meat of things, okay? One thing you may have noticed is all the Karel code that you’ve either started writing, if you're already working on assignment number one or all the code that we write in class is all in file that ends with dot Java. If you haven't noticed that before, you'll – now you know, it ends with dot Java.

Because in fact, Karel is all implemented in Java. So one of the things you might be wondering is hey, I know a little Java; can I use some Java in conjunction with Karel? And the answer is no. For the purpose of the Karel assignments, you should just use the constructs that you’ve been shown in class and in the Karel reader, just keep it to those constructs. That still gives you plenty of stuff to do with Karel, it's a good time, but keep it to that, and actually starting on Monday we're gonna, like, sort of leave Karel behind and say bye-bye, Karel, and I'll be, like, bye-bye, I love you, call me. But we'll get into Java, so if you know Java now, just use the Karel stuff in the Karel assignments that we actually do.

All right, so with that said, any questions to start off with before we dive into something new? Um-hm?

Student:How do you stop the program? Like, I had a problem, it goes up into the corner and just kind of like it says an error message.

Instructor (Mehran Sahami):And it kept looping there forever?

Student:Yeah, and I tried making, like, an empty program called Stop, but that didn’t really work.

Instructor (Mehran Sahami):No, just go up to the little icon – or the little thing in the top of the window that allows you to close the window and just close the window. Karel will be okay, he knows how to deal with that.

As a matter of fact, that's – I am so glad you asked that question. It's just a wonderful thing, because it actually leads into the first topic that I want to talk about, which is common errors. And so there are some common errors that may come up, and these are good things to kind of know about. And one of them is the thing you just ran into.

But before we actually talk about that at sort of the level of Karel, I wanna ask you a question: [inaudible] another one of these strange questions. How many people have actually ever read the instructions on a bottle of shampoo? A few folks – oh, wow. Man, that's more than – I was, like, you’ve really gotta get out more often, right?

But I'm surprised that many. And what do those instructions say? Rinse, right, that's where you rinse your hair, and then you lather, and then you repeat. And if anyone actually followed these instructions, you would still be in the shower now. As a matter of fact, you'd be in the shower for the rest of your life. Why? Because you rinse, you lather, and you repeat, which means you rinse, you later, and you repeat.

And you just keep doing this, and it's like Karel just taking a shower, right? And he's like you keep telling me to rinse – to linse, rinse and lather together. Rinse, lather, and repeat, right? And this is what we refer to in program-speak as an infinite loop. Which as you can guess is a loop that keeps going forever. And this may come up in your Karel programs. Here is an example of an infinite loop. You might be standing somewhere and you might say while front is clear.