What is the use case for flatMap vs map in kotlin

Question

in https://try.kotlinlang.org/#/Kotlin%20Koans/Collections/FlatMap/Task.kt

it has sample of using flatMap and map

seems both are doing

Answer 1

There are three functions in play here. map(), flatten(), and flatMap() which is a combination of the first two.

Consider the following example

data class Hero (val name:String)
data class Universe (val heroes: List)

val batman = Hero("Bruce Wayne")
val wonderWoman = Hero (name = "Diana Prince")

val mailMan = Hero("Stan Lee")
val deadPool = Hero("Wade Winston Wilson")

val marvel = Universe(listOf(mailMan, deadPool))
val dc = Universe(listOf(batman, wonderWoman))

val allHeroes: List = listOf(marvel, dc)

Map

allHeroes.map { it.heroes }

// output: [[Hero(name=Stan Lee), Hero(name=Wade Winston Wilson)], [Hero(name=Bruce Wayne), Hero(name=Diana Prince)]]

Map allows you to access each universe in {allHeroes} and (in this case) return its list of heroes. So the output will be a list containing two lists of heroes, one for each universe. The result is a List>

Flatmap

allHeroes.flatMap { it.heroes } 

// output: [Hero(name=Stan Lee), Hero(name=Wade Winston Wilson), Hero(name=Bruce Wayne), Hero(name=Diana Prince)]

FlatMap allows you to do the same as map, access the two lists of heroes from both universes. But it goes further and flattens the returned list of lists into a single list. The result is a List

Flatten

allHeroes.map { it.heroes }.flatten() 

// output: [Hero(name=Stan Lee), Hero(name=Wade Winston Wilson), Hero(name=Bruce Wayne), Hero(name=Diana Prince)]

This produces the same result as flatMap. So flatMap is a combination of the two functions, map{} and then flatten()