Convert List of tuple to map (and deal with duplicate key ?)

Question

I was thinking about a nice way to convert a List of tuple with duplicate key [(\"a\",\"b\"),(\"c\",\"d\"),(\"a\",\"f\")] into map (\"a\" -> [\"b\", \"

Answer 1

Starting Scala 2.13, most collections are provided with the groupMap method which is (as its name suggests) an equivalent (more efficient) of a groupBy followed by mapValues:

List("a" -> "b", "c" -> "d", "a" -> "f").groupMap(_._1)(_._2)
// Map[String,List[String]] = Map(a -> List(b, f), c -> List(d))

This:

• groups elements based on the first part of tuples (group part of groupMap)

• maps grouped values by taking their second tuple part (map part of groupMap)

This is an equivalent of list.groupBy(_._1).mapValues(_.map(_._2)) but performed in one pass through the List.