while - Rust
Description: API documentation for the Rust `while` keyword in crate `std`.
Keywords: rust, rustlang, rust-lang, while

# [−]Keyword while

Loop while a condition is upheld.

A `while` expression is used for predicate loops. The `while` expression runs the conditional expression before running the loop body, then runs the loop body if the conditional expression evaluates to `true`, or exits the loop otherwise.

    let mut counter = 0;

    while counter < 10 {
        println!("{}", counter);
        counter += 1;
    }Run

Like the `for` expression, we can use `break` and `continue`. A `while` expression cannot break with a value and always evaluates to `()` unlike `loop`.

    let mut i = 1;

    while i < 100 {
        i *= 2;
        if i == 64 {
            break; // Exit when `i` is 64.
        }
    }Run

As `if` expressions have their pattern matching variant in `if let`, so too do `while` expressions with `while let`. The `while let` expression matches the pattern against the expression, then runs the loop body if pattern matching succeeds, or exits the loop otherwise. We can use `break` and `continue` in `while let` expressions just like in `while`.

    let mut counter = Some(0);

    while let Some(i) = counter {
        if i == 10 {
            counter = None;
        } else {
            println!("{}", i);
            counter = Some (i + 1);
        }
    }Run

For more information on `while` and loops in general, see the reference.