Elegant way to invert a map in Scala

Assuming values are unique, this works:

(Map() ++ origMap.map(_.swap))

On Scala 2.8, however, it's easier:

origMap.map(_.swap)

Being able to do that is part of the reason why Scala 2.8 has a new collection library.

You can avoid the ._1 stuff while iterating in few ways.

Here's one way. This uses a partial function that covers the one and only case that matters for the map:

Map() ++ (origMap map {case (k,v) => (v,k)})

Here's another way:

import Function.tupled        
Map() ++ (origMap map tupled {(k,v) => (v,k)})

The map iteration calls a function with a two element tuple, and the anonymous function wants two parameters. Function.tupled makes the translation.

Mathematically, the mapping might not be invertible (injective), e.g., from Map[A,B], you can't get Map[B,A], but rather you get Map[B,Set[A]], because there might be different keys associated with same values. So, if you are interested in knowing all the keys, here's the code:

val m = Map(1 -> "a", 2 -> "b", 4 -> "b")
m.groupBy(_._2).mapValues(_.keys)

res0: Map[String,Iterable[Int]] = Map(b -> Set(2, 4), a -> Set(1))

Starting scala 2.13, this becomes even simpler and more efficient:

m.groupMap(_._2)(_._1)