loop + break

Goal of This Episode

Create an infinite loop with loop, then use break to jump out at the right moment.

Main Text

Up to now, our programs run once and end. But what if something needs to be done repeatedly? Like a countdown: 5, 4, 3, 2, 1, liftoff!

That calls for a loop.

loop — the Infinite Loop

loop just keeps running and running, forever:

fn main() {
    loop {
        println!("I can't stooooop");
    }
}

If you actually run this program, it will print and print and print… You’ll have to force-stop it with Ctrl + C.

So we need an “exit.”

break — Jumping Out of the Loop

fn main() {
    let mut count = 5;
    loop {
        if count == 0 {
            println!("Liftoff!");
            break;
        }
        println!("{}", count);
        count -= 1;
    }
}

How Does It Work?

1. count starts at 5.
2. Enter the loop. First check: is count == 0? No, so print 5, then count -= 1 makes count 4.
3. Back to the top of the loop. Is count == 0? No, print 4, count becomes 3.
4. …and so on…
5. When count reaches 0, if count == 0 holds: print “Liftoff!”, then break out of the loop.
6. The program ends.

Recap

• loop repeats the code inside the curly braces forever.
• break jumps out of the loop so the program can continue on.
• A loop without a break is an infinite loop — remember to have an exit.