Scala Map: Combine keys with the same value?

Question

Suppose I have a Map like

val x = Map(1 -> List("a", "b"), 2 -> List("a"), 3 -> List("a", "b"), 4 -> List("a"), 5 -> List("c"))

How would I create from this a new Map where the keys are Lists of keys from x having the same value, e.g., how can I implement

def someFunction(m: Map[Int, List[String]]): Map[List[Int], List[String]] = // stuff that would turn x into // Map(List(1, 3) -> List("a", "b"), List(2, 4) -> List("a"), List(5) -> List("c"))

?

akuiper · Accepted Answer · 2017-01-12 18:02:58Z

You can convert the Map to a List and then use groupBy to aggregate the first element of each tuple:

x.toList.groupBy(_._2).mapValues(_.map(_._1)).map{ case (x, y) => (y, x) } // res37: scala.collection.immutable.Map[List[Int],List[String]] = // Map(List(2, 4) -> List(a), List(1, 3) -> List(a, b), List(5) -> List(c))

Or as @Dylan commented, use _.swap to switch the tuples' elements:

x.toList.groupBy(_._2).mapValues(_.map(_._1)).map(_.swap)

You beat me by a few seconds with pretty much the exact same solution, so I'll give you the +1 and add that you can use _.swap in the final map call.
@Dylan Ah, didn't know the _.swap method before. Thank you!

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