rust/src/doc/trpl/while-loops.md

% while loops

The other kind of looping construct in Rust is the while loop. It looks like this:

let mut x = 5; // mut x: u32
let mut done = false; // mut done: bool

while !done {
    x += x - 3;
    println!("{}", x);
    if x % 5 == 0 { done = true; }
}

while loops are the correct choice when you're not sure how many times you need to loop.

If you need an infinite loop, you may be tempted to write this:

while true {

However, Rust has a dedicated keyword, loop, to handle this case:

loop {

Rust's control-flow analysis treats this construct differently than a while true, since we know that it will always loop. The details of what that means aren't super important to understand at this stage, but in general, the more information we can give to the compiler, the better it can do with safety and code generation, so you should always prefer loop when you plan to loop infinitely.

Ending iteration early

Let's take a look at that while loop we had earlier:

let mut x = 5;
let mut done = false;

while !done {
    x += x - 3;
    println!("{}", x);
    if x % 5 == 0 { done = true; }
}

We had to keep a dedicated mut boolean variable binding, done, to know when we should exit out of the loop. Rust has two keywords to help us with modifying iteration: break and continue.

In this case, we can write the loop in a better way with break:

let mut x = 5;

loop {
    x += x - 3;
    println!("{}", x);
    if x % 5 == 0 { break; }
}

We now loop forever with loop and use break to break out early.

continue is similar, but instead of ending the loop, goes to the next iteration. This will only print the odd numbers:

for x in 0..10 {
    if x % 2 == 0 { continue; }

    println!("{}", x);
}

Both continue and break are valid in both kinds of loops.