Is there a neat way of joining two different lists...

# arrow

[Kev]

Is there a neat way of joining two different lists of different types where they share the same attribute that they need to be matched on (like i id field) and then return a list of matching pairs?

[Youssef Shoaib [MOD]]

associateBy on one of them, then a map on the other, pairing it with the matching value from the first one

[simon.vergauwen]

+1 to what @Youssef Shoaib [MOD] said, a code example:

inline fun <A, B, C, Key> Iterable<A>.zipBy(
  other: Iterable<B>,
  keySelectorA: (A) -> Key,
  keySelectorB: (B) -> Key,
  combine: (A, B) -> C
): List<C> {
    val firstByKey = associateBy(keySelector)
    val secondByKey = other.associateBy(keySelector)
    return (firstByKey.keys intersect secondByKey.keys)
        .map { key -> firstByKey.getValue(it) to secondByKey.getValue(it) }
}

• intersect creates a Set of all common Key
• getValue returns the value for Key, (and throws NoSuchKeyElementException but that is impossible in this case)
• keySelector maps A and B to Key, in your case id field
• combine allows to transform common value to be transformed in your case ::Pair.

[simon.vergauwen]

A more optimised implementation might be possible, but not sure if it's worth implementing.

[Youssef Shoaib [MOD]]

Yeah I was thinking of only doing associateBy on one and iterate over the other with mapNotNull, but I think it's likely an unnecessary optimizatuon

[simon.vergauwen]

Nope, that's definitely a lot better @Youssef Shoaib [MOD] 😄 Code is not more complex IMO.

public inline fun <A, B, C, Key> Iterable<A>.zipBy(
  other: Iterable<B>,
  keySelectorA: (A) -> Key,
  keySelectorB: (B) -> Key,
  combine: (A, B) -> C
): List<C> {
  val aByKey = associateBy(keySelectorA)
  return other.mapNotNull { b ->
    val key = keySelectorB(b)
    val aOrNull = aByKey[key]
    aOrNull?.let { a -> combine(a, b) }
  }
}