while and do-while Loops

A while loop repeats as long as its condition stays true — useful when you don't know in advance how many times to loop.

Real-world analogy: "While the kettle isn't boiling, keep waiting."

while

fun main() {
    var balance = 100
    var months = 0

    while (balance < 200) {
        balance += balance / 10
        months++
    }

    println("Doubled after $months months: $balance")
}

Doubled after 8 months: 212

The condition is checked before each pass. If it's false from the start, the body never runs.

do-while

do { } while (condition) checks after each pass, so the body always runs at least once:

fun main() {
    var attempts = 0

    do {
        attempts++
        println("Attempt $attempts")
    } while (attempts < 3)
}

Attempt 1
Attempt 2
Attempt 3

It's the natural fit for "keep asking until the input is valid".

Common mistakes

• Forgetting to change the variable in the condition, so the loop never ends. Stop a runaway program with the IDE's red stop button or Ctrl + C.
• Using while with a counter to walk through a list. A for loop is clearer and can't go out of bounds.

Practice

Starting from 1, keep doubling a number while it's below 1,000, then print the first value that reached 1,000 or more.

fun main() {
    var n = 1
    while (n < 1000) {
        n *= 2
    }
    println(n)
}

1024