Loops

The for loop iterates over anything that provides an iterator. You have several options for looping with the for construct.

Looping over a range of values

for (index in 1..10) {
    // ... 
}

In this case, the loop runs for the values of index from 1 to 10 (inclusive).

Complete example:

fun main() {
    for (index in 1..10) {
        println(index)
    }
}

Descending loop

for (index in 10 downTo 1) {
    // ...
}

Here, the loop runs for the values of index from 10 to 1 (inclusive), in descending order.

Complete example:

fun main() {
    for (index in 10 downTo 1) {
        println(index)
    }
}

Loop with a custom step

for (index in 0 until 10 step 2) {
    // ...
}

In this example, the loop runs for the values of index from 0 to 10 (exclusive), with a step of 2. That is, the value of index will be 0, 2, 4, 6, 8.

Complete example:

fun main() {
    for (index in 0 until 10 step 2) {
        println(index)
    }
}

Looping over a collection

val values = arrayOf(1, 2, 3)

for (value in values) {
    // ...
}

The loop runs once for each element of the values collection.

Complete example:

fun main() {
    val values = arrayOf(1, 2, 3)
    
    for (value in values) {
        println(value)
    }
}

The while and do-while loops run repeatedly as long as the condition is true. The main difference between them is when the condition is checked.

While

Checks the condition before running the loop body. If the condition is met, the body runs and then the condition is checked again:

var value = 2

while (value > 0) {
    println(value)
    value--
}

Complete example:

fun main() {
    var value = 2
    
    while (value > 0) {
        println(value)
        value--
    }
}

Do-while

Runs the loop body at least once, regardless of the condition, and only then checks the condition:

var value = 2

do {
    println(value)
    value--
} while (value > 0)

Complete example:

fun main() {
    var value = 2
    
    do {
        println(value)
        value--
    } while (value > 0)
}