Looping

Often, you’ll want your computer to do the same thing over and over again. After all, that’s what they’re supposed to be good at doing.

When you tell your computer to keep repeating something, you also need to tell it when to stop. Computers never get bored, so if you don’t tell it when to stop, it won’t.

We make sure this doesn’t happen by telling the computer to repeat certain parts of a program while a certain condition is true. It works the way if works:

It’s not a fabulous program, though. For one thing, while tests your condition at the top of the loop. In this case we had to tweak our loop so it could test there. This made us puts a blank line before we did our first gets. In my mind, it just feels like the gets comes first and the echoing puts comes later. It’d be nice if we could say something like this:

That’s not valid Ruby code, but it’s close! To get it to loop forever, we just need to give while a condition that’s always true. And Ruby does have a way to break out of a loop:

Now, isn’t that better? OK, I’ll admit, the 'Spike' > 'Angel' thing is a little silly. When I get bored coming up with jokes for these examples, I’ll usually just use the actual true object:

And that’s a loop. It’s considerably trickier than a branch, so take a minute to look it over and let it sink in….

Loops are lovely things. However, like weapons-grade plutonium or bubble gum, they can cause big problems if handled improperly. Here’s a big one: what if your computer gets trapped in an infinite loop? If you think this may have happened, just go to your command line, hold down the Ctrl key, and press C. (You are running these from the command line, right?)

Before we start playing around with loops, though, let’s learn a few things to make our job easier.