Hey, do we have a map function but where I can “sk...

# announcements

[galex]

Hey, do we have a map function but where I can “skip” returning elements depending on some condition? Kind of a reduce?

[gildor]

mapNotNull

[gildor]

don’t work with nullable of course, but usually fine for most cases

[galex]

Thanks, will check that!

[Stephan Schroeder]

of course, the general way would be to first run a filter and than a map. Regarding special cases: next to the useful mapNotNull, there is also the very useful filterIsInstance (if you want to only work with all the instances of the original list, that are of a specific subtype) https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/filter-is-instance.html

[raulraja]

IMO the general way is to always fold since all other methods can be derived from fold, including filter and map.
If you want to filter and map at the same time that is frequently known as mapFilter. It’s not in the std lib but the generalised version for all Iterable may look like:

fun <A, B: Any> Iterable<A>.mapFilter(f: (A) -> B?): List<B> =
  flatMap { a -> listOfNotNull(f(a)) }

fun main() {
  val evenBy100: List<String> = listOf(1,2,3,4,5).mapFilter {
    if (it % 2 == 0) "$it * 100"
    else null
  }
  println(evenBy100) //[200, 400]
}

[raulraja]

Implies you can filter and transform to new values of different types and null signals the ones you want to skip

[galex]

Thank you!

[ilya.gorbunov]

@raulraja That mapFilter is exactly what mapNotNull does