I have a list of ids
. I want to filter my list and only keep the values in that list that match the id.
fun filterHelper(ids: List<Int>, list: List<People>) {
list.filter { ids.contains(it.id) }
}
But this is very inefficient. It is essentially traversing the list O(n^2). Does Kotlin let me do better?
I asked a similar question about slicing maps recently. The answer is that there is no good built-in function, but you can work around by using a Set
instead of a List
for your ids, which gets you O(1) lookup time for the comparisons, so O(n) in total.
data class People(val id: Int)
fun main() {
val people = listOf(People(1), People(2), People(3), People(4))
val ids = setOf(2, 4)
val filtered = people.filter { it.id in ids }
println(filtered)
}
Output:
[People(id=2), People(id=4)]
It's worth mentioning that if you already have a list, you can easily convert to a set with:
list.toSet()
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