I have a list of numbers: `val numbers = listOf(1,2,4,8,11,14)` I would like to create a new list w...

# getting-started

[Daniele B]

I have a list of numbers:
val numbers = listOf(1,2,4,8,11,14)

I would like to create a new list with the differences between a number and the one that is 2 indexes before it. In this case the results would be:
val differencesTwoIndexesApart = listOf(3,6,7,6)

is there an easy way to do that with Kotlin high-order functions?
or is an imperative for loop the best way?

[akatkov]

val differencesTwoIndexesApart = numbers.windowed(3).map { it.last() - it.first() }

[ephemient]

also

numbers.zip(numbers.drop(2)) { a, b -> b - a }

[Vampire]

@ephemient that would throw away the first two numbers and then calculate the differences between subsequent numbers, that is not what OP wanted

[ephemient]

@Vampire why do you think that? it clearly works according to @Daniele B's specification

[ephemient]

https://pl.kotl.in/giebfvRu3

[Vampire]

Ah, sorry, confused with zipWithNext, you are totally right of course

[Daniele B]

Many thanks to @akatkov and @ephemient!
Both solutions fulfill the specification.
Any comment about the efficiency of the two?
Are they comparable?

[ephemient]

it depends. .windowed() creates a new list for every window, while .drop() creates a copy of the trailing list

[ephemient]

on List<T> specifically, that could be optimized to .slice(2 until size) which returns a view of the trailing list instead of a copy

[ephemient]

if you're on an Array or ArrayList, you might as well just do List(numbers.size - 2) { numbers[it + 2] - numbers[it] }

[Daniele B]

@ephemient so does windowed() create a new list for every window or is it also a view?

[ephemient]

new list

[ephemient]

https://github.com/JetBrains/kotlin/blob/95edcea9a97464c6ae2b7f5a911e4ad17d28802e/libraries/stdlib/common/src/generated/_Collections.kt#L3147

[ephemient]

actually I lied; the version with `transform`does create a view, https://github.com/JetBrains/kotlin/blob/95edcea9a97464c6ae2b7f5a911e4ad17d28802e/libraries/stdlib/common/src/generated/_Collections.kt#L3183

[Daniele B]

cool! so that does it mean it reuses the same data?

[ephemient]

https://github.com/JetBrains/kotlin/blob/d66f07a84ecc163a4225fc5639a8a754f47da546/libraries/stdlib/src/kotlin/collections/SlidingWindow.kt#L64 it's a view into the original list. this is an implementation detail though