Elegant way to reverse a map in Scala

Question

Greetings,

Learning Scala currently (2.7.7) and needed to reverse a Map to do some reverse value->key lookups. I was looking for a simple way to do this, but came up with only:

(Map() ++ origMap.map(kvp=>(kvp._2->kvp._1)))

Anybody have a more elegant approach?

Answer 1

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.

Answer 2

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.