Kotlin List flatMapTo()
Syntax & Examples

Syntax of List.flatMapTo()

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

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

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

✐ Examples

1 Example

In this example,

We create a list named list1 containing integers.
We create a mutable list named destination.
We use the flatMapTo function on list1 with a transform function that returns a list containing each element and its double.
The flatMapped list is stored in destination.
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 destination = mutableListOf<Int>()
    val result = list1.flatMapTo(destination) { listOf(it, it * 2) }
    println(result)
}

Output

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

2 Example

In this example,

We create a list named list2 containing strings.
We create a mutable list named destination.
We use the flatMapTo function on list2 with a transform function that returns a list containing each element and its reversed form.
The flatMapped list is stored in destination.
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 destination = mutableListOf<String>()
    val result = list2.flatMapTo(destination) { listOf(it, it.reversed()) }
    println(result)
}

Output

[apple, elppa, banana, ananab, cherry, yrrehc]

3 Example

In this example,

We create a list named list3 containing strings.
We create a mutable list named destination.
We use the flatMapTo function on list3 with a transform function that returns a list containing each element and its length.
The flatMapped list is stored in destination.
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 destination = mutableListOf<String>()
    val result = list3.flatMapTo(destination) { listOf(it, it.length.toString()) }
    println(result)
}

Output

[hello, 5, world, 5]

Summary

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