for Loops and Ranges

A loop repeats code. Kotlin's for loop runs once for each item in something you can iterate over — a range, a list, or a string.

Ranges

fun main() {
    for (i in 1..3) print("$i ")
    println()
    for (i in 0..<3) print("$i ")
    println()
    for (i in 10 downTo 6 step 2) print("$i ")
    println()
    for (c in 'a'..'e') print(c)
    println()
}

1 2 3
0 1 2
10 8 6
abcde

• 1..3 includes both ends.
• 0..<3 excludes the end — handy for positions, which start at 0. (0 until 3 means the same thing and appears in older code.)
• downTo counts backwards; step skips values.
• A one-line loop body doesn't need braces, though braces are clearer for anything longer.

Looping over a collection

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

    for (fruit in fruits) {
        println("I like $fruit")
    }

    for ((index, fruit) in fruits.withIndex()) {
        println("${index + 1}. $fruit")
    }
}

I like apple
I like banana
I like cherry
1. apple
2. banana
3. cherry

withIndex() gives each element's position too; (index, fruit) unpacks both values.

repeat

When you just need to do something a number of times:

fun main() {
    repeat(3) {
        println("Hip hip hooray!")
    }
}

Hip hip hooray!
Hip hip hooray!
Hip hip hooray!

Common mistakes

• Expecting 1..3 to exclude 3. Use 1..<3 for that.
• Writing a reversed range like 5..1 — it's empty, so the loop never runs. Use 5 downTo 1.

Practice

Print the 5 times table from 5 x 1 = 5 to 5 x 10 = 50.

fun main() {
    for (n in 1..10) {
        println("5 x $n = ${5 * n}")
    }
}