Kotlin List flatMap()
Syntax & Examples

Syntax of List.flatMap()

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

fun <T, R> Iterable<T>.flatMap( transform: (T) -> Iterable<R> ): List<R>

This flatMap() extension function of List returns a single list of all elements yielded from results of transform function being invoked on each element of original collection.


✐ Examples

1 Example

In this example,

  • We create a list named list containing integers.
  • We apply the flatMap function on list to double each element and create a new list containing both the original element and its double.
  • The resulting list is printed to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val list = listOf(1, 2, 3);
    val result = list.flatMap { listOf(it, it * 2) };
    println(result);
}

Output

[1, 2, 2, 4, 3, 6]

2 Example

In this example,

  • We create a list named list containing characters.
  • We apply the flatMap function on list to create a new list containing both the original character and its uppercase version.
  • The resulting list is printed to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val list = listOf('a', 'b', 'c');
    val result = list.flatMap { listOf(it, it.toUpperCase()) };
    println(result);
}

Output

[a, A, b, B, c, C]

3 Example

In this example,

  • We create a list named list containing strings.
  • We apply the flatMap function on list to split each string into individual characters and create a new list containing all these characters.
  • The resulting list is printed to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val list = listOf("apple", "banana", "cherry");
    val result = list.flatMap { it.toList() };
    println(result);
}

Output

[a, p, p, l, e, b, a, n, a, n, a, c, h, e, r, r, y]

Summary

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