Kotlin List flatMapIndexedTo()
Syntax & Examples

Syntax of List.flatMapIndexedTo()

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

fun <T, R, C : MutableCollection<in R>> Iterable<T>.flatMapIndexedTo( destination: C, transform: (index: Int, T) -> Iterable<R> ): C

This flatMapIndexedTo() extension function of List appends all elements yielded from results of transform function being invoked on each element and its index in the original collection, to the given destination.

✐ Examples

1 Example

In this example,

We create a list named list containing strings.
We initialize an empty mutable list named result.
We apply the flatMapIndexedTo function on list to transform each element along with its index into a string in the format index: element and append the results to result.
The resulting list is printed to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val list = listOf("apple", "banana", "cherry");
    val result = mutableListOf<String>();
    list.flatMapIndexedTo(result) { index, element -> listOf("$index: $element") };
    println(result);
}

Output

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

2 Example

In this example,

We create a list named list containing integers.
We initialize an empty mutable list named result.
We apply the flatMapIndexedTo function on list to transform each element along with its index into pairs of index and double of the element and append the results to result.
The resulting list is printed to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val list = listOf(10, 20, 30);
    val result = mutableListOf<Int>();
    list.flatMapIndexedTo(result) { index, element -> listOf(index, element * 2) };
    println(result);
}

Output

[0, 20, 1, 40, 2, 60]

3 Example

In this example,

We create a list named list containing characters.
We initialize an empty mutable list named result.
We apply the flatMapIndexedTo function on list to transform each character along with its index into strings in the format index: character and index: uppercase character, and append the results to result.
The resulting list is printed to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val list = listOf('a', 'b', 'c');
    val result = mutableListOf<String>();
    list.flatMapIndexedTo(result) { index, element -> listOf("$index: $element", "$index: ${element.toUpperCase()}") };
    println(result);
}

Output

[0: a, 0: A, 1: b, 1: B, 2: c, 2: C]

Summary

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