for Loops
for (i in 1..5) { } // inclusive range
for (i in 0 until n) { } // excludes n
for (item in collection) { }Looping Over a Range
for (i in 1..5) iterates over the inclusive range 1 through 5, running the loop body once for each number in that sequence.
Example: Looping Over a Range
// Entry point: execution of the program starts here
fun main() {
// Loop over `1..5`, binding each item to `i`
for (i in 1..5) {
// Print "Number: $i" to the console, with a trailing newline
println("Number: $i")
}
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Looping with until
for (i in 0 until n) loops from 0 up to, but not including, n -- the idiomatic way to repeat something exactly n times using zero-based indices.
Example: Looping with until
// Entry point: execution of the program starts here
fun main() {
// Declare a read-only variable `n`, set to `4`
val n = 4
// Loop over `0 until n`, binding each item to `i`
for (i in 0 until n) {
// Print "Iteration $i" to the console, with a trailing newline
println("Iteration $i")
}
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Looping Over a Collection
A for loop can iterate directly over a List, Set, or array, giving you each element in turn without needing to track an index manually.
Example: Looping Over a Collection
// Entry point: execution of the program starts here
fun main() {
// Declare a read-only variable `fruits`, set to `listOf("apple", "banana", "cherry")`
val fruits = listOf("apple", "banana", "cherry")
// Loop over `fruits`, binding each item to `fruit`
for (fruit in fruits) {
// Print "Fruit: $fruit" to the console, with a trailing newline
println("Fruit: $fruit")
}
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Looping Over a Map
Iterating a Map lets each entry be destructured directly into a key and a value inside the loop header, avoiding a separate .key/.value access.
Example: Looping Over a Map
// Entry point: execution of the program starts here
fun main() {
// Declare a read-only variable `prices`, set to `mapOf("apple" to 3, "banana" to 1)`
val prices = mapOf("apple" to 3, "banana" to 1)
for ((fruit, price) in prices) {
// Print "$fruit costs $$price" to the console, with a trailing newline
println("$fruit costs $$price")
}
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- Trying to use a classic C-style
for (i = 0; i < 10; i++)loop; Kotlin only has thefor (x in range)style. - Iterating a
Mapand forgetting the destructuring shortcut, writing more verbose entry-based access than necessary. - Modifying a mutable list while iterating over it directly with a
forloop, which can cause aConcurrentModificationException.
- Kotlin's
forloop iterates over anything that provides an iterator: ranges, collections, arrays, and strings. for (i in 0 until n)is the idiomatic way to loop a fixed number of times without an off-by-one error.- Iterating a
Maplets you destructure each entry directly into(key, value)in the loop header. - Kotlin does not support a C-style three-part
forloop; ranges replace that use case.
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