94 lines
1.8 KiB
Markdown
94 lines
1.8 KiB
Markdown
% while Loops
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Rust also has a `while` loop. It looks like this:
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```{rust}
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let mut x = 5; // mut x: i32
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let mut done = false; // mut done: bool
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while !done {
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x += x - 3;
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println!("{}", x);
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if x % 5 == 0 {
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done = true;
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}
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}
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```
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`while` loops are the correct choice when you’re not sure how many times
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you need to loop.
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If you need an infinite loop, you may be tempted to write this:
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```rust,ignore
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while true {
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```
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However, Rust has a dedicated keyword, `loop`, to handle this case:
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```rust,ignore
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loop {
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```
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Rust’s control-flow analysis treats this construct differently than a `while
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true`, since we know that it will always loop. In general, the more information
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we can give to the compiler, the better it can do with safety and code
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generation, so you should always prefer `loop` when you plan to loop
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infinitely.
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## Ending iteration early
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Let’s take a look at that `while` loop we had earlier:
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```rust
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let mut x = 5;
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let mut done = false;
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while !done {
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x += x - 3;
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println!("{}", x);
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if x % 5 == 0 {
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done = true;
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}
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}
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```
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We had to keep a dedicated `mut` boolean variable binding, `done`, to know
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when we should exit out of the loop. Rust has two keywords to help us with
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modifying iteration: `break` and `continue`.
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In this case, we can write the loop in a better way with `break`:
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```rust
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let mut x = 5;
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loop {
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x += x - 3;
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println!("{}", x);
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if x % 5 == 0 { break; }
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}
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```
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We now loop forever with `loop` and use `break` to break out early.
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`continue` is similar, but instead of ending the loop, goes to the next
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iteration. This will only print the odd numbers:
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```rust
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for x in 0..10 {
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if x % 2 == 0 { continue; }
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println!("{}", x);
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}
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```
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Both `continue` and `break` are valid in both `while` loops and [`for` loops][for].
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[for]: for-loops.html
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