Rust While

A while loop keeps running as long as a condition stays true. It’s the right tool when you know what you’re waiting for, even if you don’t know exactly how many times it’ll take.

Basic while loop

fn main() {
    let mut count = 1;

    while count <= 5 {
        println!("{count}");
        count += 1;
    }
}

Output:

1
2
3
4
5

▶ Try it Yourself

The condition (count <= 5) is checked before every run through the loop. As soon as it’s false, the loop ends — here, after count becomes 6.

Caution

Forgetting to update the variable the condition depends on (count += 1 here) is the classic way to write an infinite loop. If count never changes, count <= 5 is always true and the loop never stops.

while vs loop

You could write the same thing with loop and a manual if/break, but while says what you mean more directly: “keep going while this is true.” Save loop for cases where the stopping condition doesn’t fit naturally at the top of the loop.

// with while — clear and to the point
let mut n = 0;
while n < 3 {
    println!("{n}");
    n += 1;
}

// the same thing with loop — works, but more to read
let mut n = 0;
loop {
    if n >= 3 {
        break;
    }
    println!("{n}");
    n += 1;
}

A more realistic example

while is common for things like waiting for a value to reach a target:

fn main() {
    let mut balance = 100;

    while balance < 200 {
        balance += 25;
        println!("Balance is now {balance}");
    }

    println!("Reached the goal!");
}

▶ Try it Yourself

Recap

• while condition { ... } repeats as long as the condition is true, checked before each run.
• Make sure something inside the loop eventually makes the condition false, or it’ll run forever.
• Prefer while over loop whenever the stopping condition fits naturally as “while X holds.”

Quick check

Quick check

1. What's the classic mistake that causes an infinite `while` loop?

Forgetting to update the variable the condition depends onUsing `<=` instead of `<`Using `mut` on the loop variable

2. When should you prefer `while` over `loop`, per this page?

When you need to return a value with breakNever — loop is always betterWhen the stopping condition fits naturally as 'while X holds'

Score: 0 / 2

Read more

• Conditional Loops with while — The Rust Book