Kotlin List flatten()
Syntax & Examples

Syntax of List.flatten()

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

fun <T> Iterable<Iterable<T>>.flatten(): List<T>

This flatten() extension function of List returns a single list of all elements from all collections in the given collection.

✐ Examples

1 Example

In this example,

We create a list of lists named list1, where each inner list contains integers.
We use the flatten() function on list1 to combine all elements from inner lists into a single list.
The flattened 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(listOf(1, 2), listOf(3, 4), listOf(5, 6))
    val result = list1.flatten()
    println(result)
}

Output

[1, 2, 3, 4, 5, 6]

2 Example

In this example,

We create a list of lists named list2, where each inner list contains characters.
We use the flatten() function on list2 to combine all elements from inner lists into a single list.
The flattened 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(listOf('a', 'b'), listOf('c', 'd'), listOf('e', 'f'))
    val result = list2.flatten()
    println(result)
}

Output

[a, b, c, d, e, f]

3 Example

In this example,

We create a list of lists named list3, where each inner list contains strings.
We use the flatten() function on list3 to combine all elements from inner lists into a single list.
The flattened 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(listOf("hello", "world"), listOf("goodbye", "moon"))
    val result = list3.flatten()
    println(result)
}

Output

[hello, world, goodbye, moon]

Summary

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