Kotlin List flatMapIndexed()
Syntax & Examples

Syntax of List.flatMapIndexed()

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

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

This flatMapIndexed() extension function of List returns a single list of all elements yielded from results of transform function being invoked on each element and its index in the original collection.

✐ Examples

1 Example

In this example,

We create a list named list1 containing integers.
We use the flatMapIndexed function on list1 with a transform function that returns a list containing the index and value of each element.
The flatMapped list is stored in result.
Finally, we print the value of result to standard output using the println statement.

Kotlin Program

fun main(args: Array<String>) {
    val list1 = listOf(1, 2, 3)
    val result = list1.flatMapIndexed { index, value -> listOf(index, value) }
    println(result)
}

Output

[0, 1, 1, 2, 2, 3]

2 Example

In this example,

We create a list named list2 containing strings.
We use the flatMapIndexed function on list2 with a transform function that returns a list containing the index and value formatted as strings.
The flatMapped list is stored in result.
Finally, we print the value of result to standard output using the println statement.

Kotlin Program

fun main(args: Array<String>) {
    val list2 = listOf("apple", "banana", "cherry")
    val result = list2.flatMapIndexed { index, value -> listOf("$index: $value") }
    println(result)
}

Output

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

3 Example

In this example,

We create a list named list3 containing strings.
We use the flatMapIndexed function on list3 with a transform function that returns a list containing the reversed value of each string and its index.
The flatMapped list is stored in result.
Finally, we print the value of result to standard output using the println statement.

Kotlin Program

fun main(args: Array<String>) {
    val list3 = listOf("hello", "world")
    val result = list3.flatMapIndexed { index, value -> listOf(value.reversed(), index) }
    println(result)
}

Output

[olleh, 0, dlrow, 1]

Summary

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