Loops

for iterates ranges and collections directly rather than indices, with downTo and step for counting backward — and a range is a real, storable value in Kotlin.

for over a range

Kotlin's for loop iterates over anything iterable — most often a range, written with ..:

fun main() {
    for (i in 1..5) {
        print("$i ")
    }
    println()
}

1 2 3 4 5

1..5 is inclusive on both ends. Use until instead of .. for an exclusive upper bound (1 until 5 stops at 4), and downTo to count backward, optionally with step to skip values:

fun main() {
    for (i in 10 downTo 0 step 2) {
        print("$i ")
    }
    println()
}

10 8 6 4 2 0

while and iterating collections directly

while works as in most languages, and for can iterate a collection's elements directly without indices:

fun main() {
    val fruits = listOf("apple", "banana", "cherry")

    var i = 0
    while (i < fruits.size) {
        println(fruits[i])
        i++
    }

    for (fruit in fruits) {
        println(fruit.uppercase())
    }
}

apple
banana
cherry
APPLE
BANANA
CHERRY

The for (fruit in fruits) form is what you'll actually reach for almost every time — index-based loops over collections are rare in idiomatic Kotlin.