Is there a way to split a list in two at a given index? I tried `partition` but I can't give it a t...

# getting-started

[Colton Idle]

Is there a way to split a list in two at a given index?

I tried partition but I can't give it a true/false predicate. I just want to give it a location.
I tried slice but it only gives me a list after the index.

I just wanna be able to slice a list into two parts at a given index and get both parts in one shot (that should be more performant than calling slice twice... right?)

[Youssef Shoaib [MOD]]

Just use sublist twice. It returns a view on the list, and so it just results in an object allocation. Do keep in mind that sublist does depend on the list, and so if the list updates, the sublists would update, but that can be simply remedied by copying first before `sublist`ing

[Colton Idle]

Thanks!

[Francesc]

you can also use take and drop,

val firstN = list.take(n)
val remainder = list.drop(n)

[Pihentagy]

What was the problem with slice?

Untitled.kt

[Pihentagy]

https://pl.kotl.in/4Mqoh0I27

[Pihentagy]

Btw is there a way to create an unbounded range to avoid specifying list.size?

[CLOVIS]

n until Int.MAX_VALUE ?

[Klitos Kyriacou]

That gives IndexOutOfBoundsException. It occurs to me that String has a substring function that takes one Int argument and returns the substring up to the end. Why is List different?

[CLOVIS]

Ah, no, slice expects the end limit to be a valid index. Using take /`drop` is probably the best, then.

[CLOVIS]

String has a substring function that takes one Int argument and returns the substring up to the end.

That's List.drop

[CLOVIS]

But yeah, String and List have a lot of differences. length /`size` , substring /`subList` …

[Fabrizio Scarponi]

@Pihentagy Follow upvoted solution or feel free to use a listIterator, and use the content while your are iterating, is will be probably the best effort solution

list.listIterator()
    .withIndex()
    .forEachRemaining { (index, it) ->
        when {
            index < list.size / 2 -> println("first half")
            else -> println("second half")
        }
    }

[Michael de Kaste]

This is why, locally, we have introduced the concept of

val leftBound = n until null
val rightBound = null until n
val open = openRange<Int>() // null until null, typesafe

[Michael de Kaste]

but I would love for these mathematical properties to exist in kotlin itself as well, we can't live without them

[Colton Idle]

I know I can call slice twice or sublist twice. I was hoping there was a method that returned a pair with the first chunk of the list, and the second chunk of the list.

[CLOVIS]

fun <T> List<T>.partition(n: Int) =
    take(n) to drop(n)

[CLOVIS]

Maybe it's worth making a feature request to the standard lib?

[Colton Idle]

have never filed a feature request for kotlin. i might look that up.

[CLOVIS]

Just explain what you want here: https://kotl.in/issue
Do mention that you know about take/drop, but you'd like a function that does both at once.

[CLOVIS]

If you do create it, please send the link here afterwards, I'd like to subscribe to it

[Klitos Kyriacou]

I think this does what you want:

list.withIndex().partition { it.index <= n }.toList().map { it.map { it.value } }

But it's so unreadable that any of the other solutions are better.

[CLOVIS]

↑ it's also much slower: it goes through the list 2 times, whereas the take/drop solution doesn't even go through the list once