Merge maps by key

Question

Say I have two maps:

val a = Map(1 -> \"one\", 2 -> \"two\", 3 -> \"three\")
val b = Map(1 -> \"un\", 2 -> \"deux\", 3 -> \"trois\")
         


        

Answer 1

Starting Scala 2.13, you can use groupMap which (as its name suggests) is an equivalent of a groupBy followed by map on values:

// val map1 = Map(1 -> "one", 2 -> "two",  3 -> "three")
// val map2 = Map(1 -> "un",  2 -> "deux", 3 -> "trois")
(map1.toSeq ++ map2).groupMap(_._1)(_._2)
// Map(1 -> List("one", "un"), 2 -> List("two", "deux"), 3 -> List("three", "trois"))

This:

• Concatenates the two maps as a sequence of tuples (List((1, "one"), (2, "two"), (3, "three"))). For conciseness, map2 is implicitly converted to Seq to align with map1.toSeq's type - but you could choose to make it explicit by using map2.toSeq.

• groups elements based on their first tuple part (_._1) (group part of groupMap)

• maps grouped values to their second tuple part (_._2) (map part of groupMap)