flatMap and flatten
- Kotlin course
- Advanced
- Lesson 59/83
- 5 min
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.
Practice 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.
Starter code
// in main split two words into letters with flatMap
// print the result with printlnSample solution
fun main() {
val chars = listOf("apple", "pear").flatMap { it.toList() }
println(chars)
}Expected output
[a, p, p, l, e, p, e, a, r]
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the flatMap call
val chars = listOf("ab", "cd").____ { it.toList() }val chars = listOf("ab", "cd").flatMap { it.toList() }Explanation: flatMap turns each element into a list and concatenates.
2 Match the flatMap/flatten parts
| Term | Meaning |
|---|---|
flatMap { } |
Transform and merge |
flatten() |
One-level reduce |
it.toList() |
Convert to list |
[1, 2, 3, 4] |
Result |
3 What does listOf(listOf(1, 2), listOf(3, 4)).flatten() return?
- [[1, 2], [3, 4]]
- [1, 2, 3, 4] Correct answer
- [1, 2]
- [3, 4]
Explanation: flatten reduces a nested list by one level.