Kotlin List mapIndexed()
Syntax & Examples

Syntax of List.mapIndexed()

The syntax of List.mapIndexed() extension function is:

fun <T, R> Iterable<T>.mapIndexed( transform: (index: Int, T) -> R ): List<R>

This mapIndexed() extension function of List returns a list containing the results of applying the given transform function to each element and its index in the original collection.

✐ Examples

1 Example

In this example,

We create a list named list1 containing the integers 10, 20, 30.
We then apply the mapIndexed function to list1, which takes a lambda with two parameters: index and value.
Within the lambda, we concatenate the index and value into a string format.
The mapIndexed function returns a new list containing the transformed elements.
The resulting list, containing strings representing each element along with its index, is stored in result.
Finally, we print the value of result to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val list1 = listOf(10, 20, 30);
    val result = list1.mapIndexed { index, value -> "\$index: \$value" }
    print(result);
}

Output

[0: 10, 1: 20, 2: 30]

2 Example

In this example,

We create a list named list1 containing the characters 'a', 'p', 'p', 'l', 'e'.
We then apply the mapIndexed function to list1, which takes a lambda with two parameters: index and value.
Within the lambda, we concatenate the index and value into a string format.
The mapIndexed function returns a new list containing the transformed elements.
The resulting list, containing strings representing each character along with its index, is stored in result.
Finally, we print the value of result to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val list1 = listOf(&quot;a&quot;, &quot;p&quot;, &quot;p&quot;, &quot;l&quot;, &quot;e&quot;);
    val result = list1.mapIndexed { index, value -> "\$index: \$value" }
    print(result);
}

Output

[0: a, 1: p, 2: p, 3: l, 4: e]

3 Example

In this example,

We create a list named list1 containing the strings 'apple', 'banana', 'cherry'.
We then apply the mapIndexed function to list1, which takes a lambda with two parameters: index and value.
Within the lambda, we concatenate the index and value into a string format.
The mapIndexed function returns a new list containing the transformed elements.
The resulting list, containing strings representing each element along with its index, is stored in result.
Finally, we print the value of result to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val list1 = listOf("apple", "banana", "cherry");
    val result = list1.mapIndexed { index, value -> "\$index: \$value" }
    print(result);
}

Output

[0: apple, 1: banana, 2: cherry]

Summary

In this Kotlin tutorial, we learned about mapIndexed() extension function of List: the syntax and few working examples with output and detailed explanation for each example.