How do I get the index while iterating a list?

Use for ((index, value) in list.withIndex()) — this gives you both the zero-based index and the element value at each iteration.

Can I modify a list while iterating with for?

Avoid modifying a collection during iteration — it can cause ConcurrentModificationException or skipped elements. Build a new list or iterate a copy instead.

Beginner 7 min readKotlin 2.0

Kotlin For Loop — Iterating with Ranges and Collections

Kotlin's for loop iterates over any iterable — ranges, lists, maps, and strings. It is concise and removes the need for a counter variable.

What You Will Learn

Loop over a range with ..
Use downTo and step
Iterate over a list and map
Use withIndex for index + value
Loop over a String

Looping Over a Range

Use 1..5 to create an inclusive range and iterate over it.

Basic For Loop

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

Output

1 2 3 4 5 1 2 3 4

1..5 includes both endpoints: 1, 2, 3, 4, 5. 1 until 5 excludes the end: 1, 2, 3, 4.

Beginner Tip: Use until when you want to loop n times starting from 0: for (i in 0 until n)

downTo and step

Count down with downTo. Skip values with step.

downTo and step

kotlin
fun main() {
    for (i in 5 downTo 1) print("$i ")
    println()
    for (i in 0..10 step 2) print("$i ")
}

Output

5 4 3 2 1 0 2 4 6 8 10

downTo counts backward. step n skips every n values. Both can be combined: 10 downTo 0 step 2.

Iterating Collections

Iterate directly over lists, sets, maps, and strings.

Iterating a List and Map

kotlin
fun main() {
    val fruits = listOf("Apple", "Banana", "Cherry")
    for (fruit in fruits) {
        println(fruit)
    }

    val capitals = mapOf("India" to "Delhi", "France" to "Paris")
    for ((country, capital) in capitals) {
        println("$country: $capital")
    }
}

Output

Apple Banana Cherry India: Delhi France: Paris

for...in works on any Iterable. Map destructuring (country, capital) unpacks each entry automatically.

withIndex

Get both the index and value using withIndex().

withIndex

kotlin
fun main() {
    val items = listOf("Kotlin", "Java", "Python")
    for ((index, value) in items.withIndex()) {
        println("$index: $value")
    }
}

Output

0: Kotlin 1: Java 2: Python

withIndex() returns pairs of (index, value). Destructuring in the for loop unpacks each pair.

Practice Exercise

Exercisepredict output

What prints? for (i in 1..4) print("$i ")

Quick Quiz

What does 1 until 5 produce?

Kotlin For Loop — Iterating with Ranges and Collections

What You Will Learn

Looping Over a Range

Basic For Loop

downTo and step

downTo and step

Iterating Collections

Iterating a List and Map

withIndex

withIndex

Practice Exercise

Quick Quiz

Frequently Asked Questions

Related Tutorials