How to Use For Loop in Kotlin: Syntax and Examples

In Kotlin, use the for loop to repeat actions over ranges, arrays, or collections. The syntax is for (item in collection) { ... }, where item takes each value in the collection one by one.

Syntax

The for loop in Kotlin iterates over anything that provides an iterator, such as ranges, arrays, or collections.

• for: keyword to start the loop
• (item in collection): defines the loop variable item and the collection to iterate
• { ... }: block of code to run for each item

kotlin

for (item in collection) {
    // code to execute
}

Example

This example shows how to use a for loop to print numbers from 1 to 5 using a range.

kotlin

fun main() {
    for (number in 1..5) {
        println(number)
    }
}

Output

1
2
3
4
5

Common Pitfalls

One common mistake is trying to modify the loop variable inside the for loop, which is not allowed because it is read-only. Another is using incorrect range syntax or forgetting that ranges are inclusive.

Also, using for (i in 1 until 5) excludes 5, which can confuse beginners.

kotlin

fun main() {
    // Wrong: trying to change loop variable
    for (i in 1..3) {
        // i = i + 1 // Error: Val cannot be reassigned
        println(i)
    }

    // Correct: use a separate variable
    for (i in 1..3) {
        val j = i + 1
        println(j)
    }
}

Output

1
2
3
2
3
4

Quick Reference

Usage	Description
for (item in collection)	Iterate over each element in a collection or range
for (i in 1..5)	Iterate from 1 to 5 inclusive
for (i in 1 until 5)	Iterate from 1 to 4 (5 excluded)
for (c in "hello")	Iterate over each character in a string

Key Takeaways

Use for (item in collection) to loop over ranges, arrays, or collections.

Ranges like 1..5 include both ends; use until to exclude the end.

Loop variables are read-only; do not try to modify them inside the loop.

You can loop over strings, arrays, lists, and any iterable in Kotlin.

Use clear variable names inside loops for better readability.