Is there a method to convert a `List` to a `Map` b...

# getting-started

[CLOVIS]

Is there a method to convert a List to a Map but where each initial element that lead to multiple keys?

class A
class B

data class C(
    val a: List<A>,
    val b: B,
)

val input: List<C> = …
val output: Map<A, List<B>> = input.???

There is .groupBy which does the opposite (each item can create multiple values), but I'm searching for the case where each item can be put into the list in different places.

I guess I can do

val output: Map<A, List<B>> = input.fold(HashMap<A, ArrayList<B>>()) { acc, it ->
    for (a in it.a) {
        acc.merge(a, arrayListOf(it)) { a, b -> a.addAll(b); a }
    }
    acc
}

but that's not very elegant

[Sam]

I'd start by flattening List<C> into a list of individual key-value pairs:

val pairs = input.flatMap { (aa, b) -> aa.map { a -> a to b } }
val output = pairs.groupBy({ it.first }, { it.second })

I'm not sure it's clearer than what you already had, though...

[Youssef Shoaib [MOD]]

If you make your fold into a buildMap call, I think it'll be clear enough

[phldavies]

val output: Map<A, List<B>> = buildMap<A, MutableList<B>> {
        for((aa, b) in input) {
            for(a in aa) {
                getOrPut(a, { mutableListOf<B>() }).add(b)
            }
        }
    }

fairly clear of intent