How to perform action for all combinations of elements in lists?

Question

I have multiple lists, for the sake of simplicity, let's say 2:

val names = listOf("John", "Tom")
val days = listOf(1, 2, 3)

And I want to iterate over all the combinations of these possible values with a function f: (String, Int): Unit:

f("John", 1)
f("John", 2)
f("John", 3)
f("Tom", 1)
f("Tom", 2)
f("Tom", 3)

I'd like to find out the idiomatic way to do this in kotlin. Off the top of my head, I imagine that this can be done with a nested map. Something along the lines of:

a.map { itA -> 
  b.map { itB ->
    f(itA, itB)
  }
}

EDIT: This doesn't seem to work for me though as it returns:

[() -> kotlin.collections.List<kotlin.Unit>, () -> kotlin.collections.List<kotlin.Unit>]

Answer 1

The name of the output you're looking for is a Cartesian Product.

If you don't need all of the combinations up front, and you're just searching for one or more, you could make it a lazily generated sequence:

private fun <A, B> lazyCartesianProduct(
    listA: Iterable<A>,
    listB: Iterable<B>
): Sequence<Pair<A, B>> =
    sequence {
        listA.forEach { a ->
            listB.forEach { b ->
                yield(a to b)
            }
        }
    }

Then you can map, filter etc only what you need:

val names = listOf("John", "Tom")
val days = listOf(1, 2, 3)

lazyCartesianProduct(names, days)
    .map { it.toSomethingElse() }
    .find { it.isWhatImLookingFor() }