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:

```{rust}
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:

```{rust,ignore}
while true {
```

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

```{rust,ignore}
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:

```{rust}
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`:

```{rust}
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:

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

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

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