While Loop

A while loop is a conditional control flow construct that repeatedly executes a block of code as long as a specified boolean expression evaluates to true. The condition is evaluated before each iteration, meaning the loop body may execute zero or more times depending on the initial state of the condition.

while boolean_expression {
    // statements executed as long as boolean_expression is true
}

Unlike languages such as C, C++, or Java, Rust does not require parentheses around the while loop condition. The Rust compiler actively discourages unnecessary parentheses via the unused_parens lint, enforcing a cleaner, idiomatic syntax.

Technical Characteristics

• Strict Boolean Typing: The condition must explicitly evaluate to the bool type. Rust does not support implicit type coercion; integers, pointers, or standard objects cannot be evaluated as “truthy” or “falsy”.
• Expression Type: A while loop is an expression that always evaluates to the unit type ().
• No Value Return on Break: Unlike Rust’s unconditional loop construct, you cannot return a value from a while loop using the break keyword.

Control Flow Modifiers

Execution within the loop body can be altered using standard control flow keywords:

• continue: Immediately halts the current iteration and jumps back to the condition evaluation at the top of the loop.
• break: Immediately terminates the loop entirely, transferring execution to the first statement following the loop block.

let mut counter = 0;

while counter < 10 {
    counter += 1;
    
    if counter % 2 == 0 {
        continue; // Skips to the next condition evaluation
    }
    
    if counter >= 7 {
        break; // Exits the while loop completely
    }
}

Loop Labels

When dealing with nested while loops, Rust allows you to annotate loops with labels (prefixed by a single quote '). This enables break and continue statements to target specific outer loops rather than the innermost loop they reside in.

let mut x = 0;

'outer: while x < 5 {
    x += 1;
    let mut y = 0;
    
    while y < 5 {
        y += 1;
        
        if x * y == 6 {
            break 'outer; // Terminates the loop labeled 'outer
        }
        if y == 3 {
            continue 'outer; // Skips to the next iteration of 'outer
        }
    }
}

The while let Variant

Rust provides a specialized syntactic sugar called while let that combines a while loop with pattern matching. It repeatedly executes the loop body as long as a value successfully matches a specified pattern, automatically destructuring the value in the process.

let mut vector = vec![1, 2, 3];

// The loop continues as long as `pop()` returns `Some(value)`
// It terminates automatically when `pop()` returns `None`
while let Some(element) = vector.pop() {
    // `element` is bound and available in this scope
    let _squared = element * element;
}

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