Is there a functional/idiomatic Kotlin way to crea...

# getting-started

[David Whittaker]

Is there a functional/idiomatic Kotlin way to create a list from a map where the value of each element is repeated the number of times as the key?
For instance, {3=7, 2=8, 4=1} turns into [7, 7, 7, 8, 8, 1, 1, 1, 1]? My naive way is to create a mutableList and iterate through the map using forEach and `repeat`edly add items to the list.

[Goetz Markgraf]

val map = mapOf(3 to 7, 2 to 8, 4 to 1)

map.flatMap { (key, value) -> List(key) { value} }

this returns

[7, 7, 7, 8, 8, 1, 1, 1, 1]

[David Whittaker]

thanks! flatMap was the key piece I was missing.

[David Whittaker]

Curious: since the final list could get quite large, and I only need the first n elements, is there a way to stop the generation of the flatMap once I've reached a list of size n?

[Vampire]

Writing from the top of my head, so could be wrong. But I think if you make a sequence first and then use take(n), it should probably work.

[CLOVIS]

Fully lazy version: https://play.kotlinlang.org/#eyJ2ZXJzaW9uIjoiMS44LjEwIiwicGxhdGZvcm0iOiJqYXZhIiwiYXJncy[…]AgLmZvckVhY2ggeyBwcmludGxuKFwiUmVzdWx0OiAkaXRcIikgfVxufSJ9

[David Whittaker]

Thank you @CLOVIS! - so if I do take(4).forEach{ println("Result: $it") } then the sequence will only produce 4 elements?

[Joffrey]

Yes, that's what sequences are for - they are lazy. Elements are produced and processed only when necessary for the end of the pipeline.

[CLOVIS]

When learning about this, don't hesitate to add .onEach { println("$it") } in the middle of the pipeline to see when elements are generated :)