How can I use the stdlib efficiently and elegantly to subset a range between a start and a final ele...

# getting-started

[holgerbrandl]

How can I use the stdlib efficiently and elegantly to subset a range between a start and a final element in a list of unique items? My solution does not seem very pretty yet

val items = (1..10).shuffled()
val from =3
val to =7
 
val from3o7Incl = items.dropWhile { it != from }.takeWhile { it != to } + listOf(to)

[Wout Werkman]

It depends on the data structure. One thing you could do is add asSequence. Now it won't make intermediate lists, and it will not iterate any values after to.

val from3o7Incl = items.asSequence().dropWhile { it != from }.takeWhile { it != to } + sequenceOf(to)

If you are working on the JVM and you use a sorted set, you can efficiently lookup the range by calling sortedSet.subSet(from, to). Iterating this subset will have O(log(N) + M) time complexity, with N being the size of your initial set, and M being the size of the subset.
If it's a sorted list or array, you can use items.subList(items.binarySearch(from), items.binarySearch(to)) for the same O(log(N) + M) time complexity. You will need to handle the cases when from and to are not in items

[holgerbrandl]

Thanks. Yes, I am on JVM, and I'm sure that from and end are in items which is list of item.

Using subList or subSet felt a bit wrong to be, since I naively I would just need to traverse the list once to figure the subset. so it should be something like O(n) if the list is sorted and I'm sure that both from and end are included and showing up in order.

Essentially what I am hoping for is a flavor of takeWhile which is inclusive of the last element where the predicate fails. Something like ``takeWhileWithLast` ...

[Wout Werkman]

Using subList or subSet felt a bit wrong to be, since I naively I would just need to traverse the list once to figure the subset. so it should be something like O(n) if the list is sorted and I'm sure that both from and end are included and showing up in order.

Well, you are in luck, both subList aswell as `TreeSet`'s subSet functions only return a view. While might be dangerous in case of mutations to the original list collection*, it does not have the allocations intermediate lists* you fear.

Essentially what I am hoping for is a flavor of takeWhile which is inclusive of the last element where the predicate fails. Something like ``takeWhileWithLast` ...

This does not exist unfortunately, you'd have to add such extension method yourself.

[holgerbrandl]

Perfect, thanks for your kind support

[Joffrey]

There is an issue about having a new function to include that last element:
https://youtrack.jetbrains.com/issue/KT-50317/Add-the-ability-to-take-from-an-Iterable-while-some-predicate-is-satisfied-and-include-the-first-value-that-does-not-satisfy-the

[holgerbrandl]

Thanks for the pointer

[Wout Werkman]

Feel free to add your use case. The Kotlin team relies on strong use cases when designing their language :)