All possible combinations of two lists
You could write these extension functions based on flatMap
stdlib function:
// Extensions
fun <T, S> Collection<T>.cartesianProduct(other: Iterable<S>): List<Pair<T, S>> {
return cartesianProduct(other, { first, second -> first to second })
}
fun <T, S, V> Collection<T>.cartesianProduct(other: Iterable<S>, transformer: (first: T, second: S) -> V): List<V> {
return this.flatMap { first -> other.map { second -> transformer.invoke(first, second) } }
}
// Example
fun main(args: Array<String>) {
val ints = listOf(0, 1, 2)
val strings = listOf("a", "b", "c")
// So you could use extension with creating custom transformer
strings.cartesianProduct(ints) { string, int ->
"$int $string"
}.forEach(::println)
// Or use more generic one
strings.cartesianProduct(ints)
.map { (string, int) ->
"$int $string"
}
.forEach(::println)
}
A possible alternative:
fun <S, T> List<S>.cartesianProduct(other: List<T>) = this.flatMap {
List(other.size){ i -> Pair(it, other[i]) }
}
Edit - nicer, using zip syntax:
fun <S, T> List<S>.cartesianProduct(other : List<T>) : List<Pair<S, T>> =
this.flatMap { s ->
List(other.size) { s }.zip(other)
}
Another (possibly more understandable) alternative to my previous answer. Both achieve the same result:
fun <S, T> List<S>.cartesianProduct(other: List<T>) = this.flatMap { thisIt ->
other.map { otherIt ->
thisIt to otherIt
}
}