Hey, my google fu lost me. Is there a built-in like `.sublist` that doesn't throw an `OutOfBoundsExc...

# getting-started

[Hildebrandt Tobias]

Hey, my google fu lost me.
Is there a built-in like .sublist that doesn't throw an OutOfBoundsException and just gives me all it can?
So if I say give me 4 elements from index 2 onwards, but the list has only 5 entries it should just give me back index 2, 3 and 4.

[Hildebrandt Tobias]

The way I see it the easiest way would be to do

myList.sublist(n, myList.size-1).take(m)

[CLOVIS]

With no intermediary lists:

fun <T> List<T>.safeSubList(from: Int, to: Int) =
    subList(from.coerceIn(indices), to.coerceIn(indices.start, indices.last + 1))

[CLOVIS]

it's not beautiful

[Szymon Jeziorski]

@CLOVIS's solution can be simplified to coercing min to 0 and max to size as follows:

subList(from.coerceAtLeast(0), to.coerceAtMost(size))

[CLOVIS]

No, it cannot: https://play.kotlinlang.org/#eyJ2ZXJzaW9uIjoiMS45LjI0IiwicGxhdGZvcm0iOiJqYXZhIiwiYXJncy[…]wgMiwgMywgNCwgNSkuY2FuZGlkYXRlU3ViTGlzdCg5LCAxMCkpXG59In0=

[CLOVIS]

actually that shows mine is broken as well, it shouldn't return any elements at all in this situation

[Szymon Jeziorski]

Oh, that's right, good catch!

I guess if you want the function to handle all possible error cases this should do the trick:
subList(from.coerceIn(0..size), to.coerceAtLeast(max(from, 0)).coerceAtMost(size))

https://pl.kotl.in/DNMKJkDTs

[CLOVIS]

I wonder if this could be a useful addition to the stdlib. At least, this is how I personally expected subList to behave

[Hildebrandt Tobias]

Yes I also kind of didn't expect sublist to throw

[Hildebrandt Tobias]

The way I have it now:

if(n < list.size) {
    list.subList(n, state.stationList.size - 1).take(m)
}

But I'll probably add an extension function with SJ's answer.

[Gurgen Nersesyan]

list.drop(2) ? Or maybe I didn't get the questio

[Hildebrandt Tobias]

I need to specify the start index from which to give an amount of elements.

[Gurgen Nersesyan]

So you skip (drop) first N

[Hildebrandt Tobias]

So list.drop(n).take(m)? Does that throw?

[Gurgen Nersesyan]

No

[Gurgen Nersesyan]

You get empty list at worse case

[Hildebrandt Tobias]

Huh that would also be a cool solution.

[CLOVIS]

This does create two temporary lists though.

[Gurgen Nersesyan]

Maybe list.slice(n..m) will do, not sure if that throws or not

[Gurgen Nersesyan]

https://pl.kotl.in/GM4IoFAGP

[Gurgen Nersesyan]

slice was both throwing and neither a list method 🙂

[Gurgen Nersesyan]

fun <T> List<T>.safeSubList(n: Int, m: Int): List<T> {
    val start  = min(max(n,0), min(m, this.size))
    val end = min(max(start, m), this.size)
    return this.subList(start, end)
}

[Gurgen Nersesyan]

this still looks messy, as good old messy javascript code won't throw but won't work as expected either

[Szymon Jeziorski]

List<T>.slice body looks like this:

if (indices.isEmpty()) return listOf()
return this.subList(indices.start, indices.endInclusive + 1).toList()

so it's not safe in case of index bounds, and also creates new list as a result.

One other thing to note is that resulting sublist from normal subList method reflects changes made to the original list, whereas drop, take etc create new lists with shallow copy of the data from original list.

So what you actually need depends on the context, I would put it like that:

Do you want sublist to reflect changes made to original list?
    Yes:
        Do you want implementation to be index safe?
            Yes: proposed `list.safeSubList(from, to)`
            No: `list.subList(from, to)`
    No:
        Do you want implementation to be index safe?
            Yes:
                Is the performance critical?
                    Yes: `list.safeSubList(from, to).toList()` // or even create yet another extension if you want to avoid SubList wrapping, though it should be cheap as it doesn't copy data
                    No: above or `take`/`takeLast` + `drop`/`dropLast` combo
            No: `list.slice(from, to)`

[Szymon Jeziorski]

One thing to note is that performance hits coming from intermediate `List`'s resulting from take / drop etc may not be affecting you much if you're working on relatively small collections. After all I would try to be pragmatic about it and think of it not only about in performance's aspect, but rather in wider context having readability and maintainability in mind (on the one hand dedicated extension can be super handy and make your life easier, but one the other, if you have a codebase with 200 of such micro extensions and use them all over the place then could just be an overkill making reading and writing code harder)