The while loop

Learn loops that run while a condition holds with while.

The while loop

The while loop runs a block as long as a condition is true. The condition is checked at the top; if false, the block never runs.

var n = 3
while (n > 0) {
    println(n)
    n = n - 1
}

This prints 3, 2, 1. Each step n decreases by one; when n reaches 0, the condition becomes false and the loop ends.

We must update the loop variable ourselves; otherwise an infinite loop occurs. Make sure the condition eventually becomes false.

while with a counter

It is common to keep a counter in a while loop. The counter is updated each step; the condition depends on it.

var count = 0
while (count < 3) {
    println("Step $count")
    count = count + 1
}

This prints "Step 0", "Step 1", "Step 2". When count reaches 3, the condition is false. var is used because the counter changes.

Forgetting to update the condition causes an infinite loop; the program hangs.

Exercise Count down from 5 to 1

Write a fun main(). Assign 5 to var n. Use a while (n > 0) loop to print println(n) and decrease with n = n - 1 each step. The output should be 5, 4, 3, 2, 1.

Show example solution

fun main() {
    var n = 5
    while (n > 0) {
        println(n)
        n = n - 1
    }
}

Test yourself

Match the while behavior

condition true

Runs the block
condition false

Ends the loop
condition always true

Infinite loop
condition false at start

Never runs
while (x > 10) { ... } — x = 5: what happens?
- Runs the block
- Never runs
- Infinite loop
- Throws error
Show answer

Never runs — 5 > 10 is false, so the block never runs.

Complete the while condition

while (count ____ 3) { println(count) }

Show answer

while (count < 3) { println(count) }

Next lesson: The do-while loop

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