while and do-while Loops

while and do-while loops keep repeating an action as long as a condition stays true, like knocking on a door until someone answers.

In this page:

The while Loop
The do-while Loop
Infinite Loops with break
Choosing Between while and do-while

The while Loop

while repeatedly runs its body as long as the condition remains true, checking the condition before every iteration -- including the very first one.

Example: The while Loop

markup

fun main() {
    var count = 0
    while (count < 5) {
        println("Count is $count")
        count++
    }
}

The do-while Loop

do-while runs its body first and checks the condition afterward, guaranteeing the body executes at least once even if the condition is false from the start.

Example: The do-while Loop

markup

fun main() {
    var attempts = 0
    do {
        println("Attempt number ${attempts + 1}")
        attempts++
    } while (attempts < 3)
}

Infinite Loops with break

while (true) combined with a break statement inside the body creates a loop whose exit condition is easier to express in the middle of the logic rather than up front.

Note: Always make sure a while (true) loop has a reachable break, or it will run forever.

Example: Infinite Loops with break

markup

fun main() {
    var n = 1
    while (true) {
        println("n is $n")
        n *= 2
        if (n > 20) break
    }
}

Choosing Between while and do-while

Use while when the body might legitimately need to run zero times, and do-while when the body must always run at least once before checking whether to continue.

Example: Choosing Between while and do-while

markup

fun main() {
    val condition = false
    while (condition) {
        println("This never prints")
    }
    do {
        println("This always prints once")
    } while (condition)
}

Common Mistakes

Forgetting to update the loop's condition variable inside the body, causing an infinite loop that never terminates.
Using while when a do-while is actually needed, resulting in a block that should run at least once being skipped entirely when the condition starts false.
Confusing the check order: while checks before running the body, do-while checks after, changing how many times the body might run.

Chapter Summary

while checks its condition before each iteration, so the body may run zero times.
do-while checks its condition after each iteration, guaranteeing the body runs at least once.
Both loops require the condition to eventually become false, usually by updating a variable inside the body.
while (true) { ... break ... } is a common pattern for loops whose exit condition is easier to express inside the body.

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while and do-while Loops

The while Loop

The do-while Loop

Infinite Loops with break

Choosing Between while and do-while

Chapter Quiz — Complete all 7 topics to unlock

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