The do-while loop

Learn loops that run at least once with do-while.

The do-while loop

The do-while loop runs the block at least once, then checks the condition. If the condition is true, the loop continues.

var n = 0
do {
    println(n)
    n = n + 1
} while (n < 3)

This prints 0, 1, 2. The block runs first, then n < 3 is checked. Unlike while, the condition is at the end, so the block always runs at least once.

It is useful when you need at least one pass, like reading input.

Exercise Count 0 to 2 with do-while

Write a fun main(). Assign 0 to var n. Use do { println(n); n = n + 1 } while (n < 3) to print 0, 1, 2.

Show example solution

fun main() {
    var n = 0
    do {
        println(n)
        n = n + 1
    } while (n < 3)
}

When to use do-while?

do-while is used when the block must run at least once, even if the condition is false the first time.

var n = 10
do {
    println(n)
    n = n - 1
} while (n > 10)

Here the condition n > 10 is false from the start; but the block still runs once and prints 10. A while loop would not run at all.

This matters especially for first-pass steps that must always execute.

Test yourself

Complete the while keyword

do {
    println(n)
    n = n + 1
} ____ (n < 3)

Show answer

do {
    println(n)
    n = n + 1
} while (n < 3)

Match the do-while behavior

do { ... } while

Runs first then checks
while (...)

Checks first then runs
condition true

Continues
condition false (first time)

do-while still runs
How does do-while differ from while?
- It runs faster
- It needs less code
- It runs at least once even if the condition is false
- It only works with numbers
Show answer

It runs at least once even if the condition is false — do-while checks the condition at the end; the block always runs at least once.

Next lesson: Range

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