Kotlin for Loop

There is no traditional for loop in Kotlin unlike Java and other languages.

In Kotlin, for loop is used to iterate through ranges, arrays, maps and so on (anything that provides an iterator).

The syntax of for loop in Kotlin is:

for (item in collection) {
    // body of loop
}

Example: Iterate Through a Range

fun main(args: Array<String>) {

    for (i in 1..5) {
        println(i)
    }
}

Here, the loop iterates through the range and prints individual item.

Output

1
2
3
4
5

If the body of the loop contains only one statement (like above example), it's not necessary to use curly braces { }.

fun main(args: Array<String>) {
    for (i in 1..5) println(i)
}

It's possible to iterate through a range using for loop because ranges provides an iterator. To learn more, visit Kotlin iterators.

Example: Different Ways to Iterate Through a Range

fun main(args: Array<String>) {

    print("for (i in 1..5) print(i) = ")
    for (i in 1..5) print(i)

    println()

    print("for (i in 5..1) print(i) = ")
    for (i in 5..1) print(i)             // prints nothing

    println()

    print("for (i in 5 downTo 1) print(i) = ")
    for (i in 5 downTo 1) print(i)

    println()

    print("for (i in 1..5 step 2) print(i) = ")
    for (i in 1..5 step 2) print(i)

    println()

    print("for (i in 5 downTo 1 step 2) print(i) = ")
    for (i in 5 downTo 1 step 2) print(i)
}

Output

for (i in 1..5) print(i) = 12345
for (i in 5..1) print(i) = 
for (i in 5 downTo 1) print(i) = 54321
for (i in 1..5 step 2) print(i) = 135
for (i in 5 downTo 1 step 2) print(i) = 531

Iterating Through an Array

Here's an example to iterate through a String array.

fun main(args: Array<String>) {

    var language = arrayOf("Ruby", "Kotlin", "Python" "Java")

    for (item in language)
        println(item)
}

Output

Ruby
Kotlin
Python
Java

It's possible to iterate through an array with an index. For example,

fun main(args: Array<String>) {

    var language = arrayOf("Ruby", "Kotlin", "Python", "Java")

    for (item in language.indices) {

        // printing array elements having even index only
        if (item%2 == 0)
            println(language[item])
    }
}

Output

Ruby
Python

Note: Here, language.indices returns all all indices of each array elements.

If you want to learn more about arrays, visit Kotlin arrays.

Iterating Through a String

fun main(args: Array<String>) {

    var text= "Kotlin"

    for (letter in text) {
        println(letter)
    }
}

Output

K
o
t
l
i
n

Similar like arrays, you can iterate through a String with an index. For example,

fun main(args: Array<String>) {

    var text= "Kotlin"

    for (item in text.indices) {
        println(text[item])
    }
}

Output

K
o
t
l
i
n

You will learn to iterate over a map using for loop in Kotin map article.