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;
}

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.
    }
}

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);
    }
}

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

See also, for, loop.