flatMap and flatten

Learn flattening with flatMap and flatten.

flatMap and flatten

flatMap transforms each element into a list and concatenates them. flatten flattens a nested list by one level.

Example: listOf("ab", "cd").flatMap { it.toList() } returns [a, b, c, d]. listOf(listOf(1, 2), listOf(3, 4)).flatten() gives [1, 2, 3, 4]. flatMap = map + flatten. This pattern is used to flatten nested structures.

Flatten a nested list

flatten reduces a nested list by one level. When no transform is needed, flatten is more readable than flatMap.

Example: listOf(listOf(1, 2), listOf(3, 4), listOf(5)).flatten(). This returns [1, 2, 3, 4, 5]. flatten only reduces one level; deeper structures require multiple calls. This pattern is used to expand grouped data into a single list.

Exercise Split words into letters with flatMap

In main assign the result of listOf("apple", "pear").flatMap { it.toList() } to a variable and print it with println.

Show example solution

fun main() {
    val chars = listOf("apple", "pear").flatMap { it.toList() }
    println(chars)
}

Test yourself

Complete the flatMap call

val chars = listOf("ab", "cd").____ { it.toList() }

Show answer

val chars = listOf("ab", "cd").flatMap { it.toList() }

Match the flatMap/flatten parts

flatMap { }

Transform and merge
flatten()

One-level reduce
it.toList()

Convert to list
[1, 2, 3, 4]

Result
What does listOf(listOf(1, 2), listOf(3, 4)).flatten() return?
- [[1, 2], [3, 4]]
- [1, 2, 3, 4]
- [1, 2]
- [3, 4]
Show answer

[1, 2, 3, 4] — flatten reduces a nested list by one level.

Next lesson: Summary - Collection stdlib

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