While loops

while checks its condition before every iteration. If the condition is false at the start, the body does not run.

Count with a variable

var index: Int = 0;
while index < 4 {
    println(toString(index));
    index = index + 1;
}

Update the loop state inside the body. The condition is checked again after the body finishes.

Process until a condition changes

fun countdown(start: Int) {
    var remaining = start;
    while remaining > 0 {
        println(toString(remaining));
        remaining = remaining - 1;
    }
    println("done");
}

This form is useful when the next state comes from work performed during the iteration rather than from a fixed numeric range.

break

break leaves the nearest loop immediately:

var value: Int = 0;
while value < 100 {
    value = value + 1;
    if value * value > 50 {
        break;
    }
}

println(toString(value));

The statement after the loop runs after either the condition becomes false or break is reached.

continue

continue skips the rest of the current body and starts the next condition check:

var value: Int = 0;
while value < 10 {
    value = value + 1;
    if value % 2 != 0 {
        continue;
    }
    println(toString(value));
}

When using continue, update any counter before the statement. Otherwise the condition may never change and the loop will not terminate.

Nested loops

break and continue affect the nearest enclosing loop:

var row: Int = 0;
while row < 3 {
    var column: Int = 0;
    while column < 3 {
        if row == 1 && column == 1 {
            break;
        }
        column = column + 1;
    }
    row = row + 1;
}

There is no label on this break; it only exits the inner loop.

Avoid accidental infinite loops

Every terminating while needs a path that changes the condition:

var attempts: Int = 0;
while attempts < 3 {
    attempts = attempts + 1;
    println("trying");
}

Use for when the iteration already has a counter or a collection to traverse.