In Scala, is there a way to take convert two lists into a Map?

Question

I have a two lists, a List[A] and a List[B]. What I want is a Map[A,B] but I want the semantics of zip. So started out like so:

var tuplesOfAB = listOfA zip listOfB

Now I'm not sure how to construct a Map from my tuplesOfAB.

As a follow-up question, I also want to invert my map so that from a Map[A,B] I can create a Map[B,A]. Can anyone hit me with a clue-stick?

Solution 1

In 2.8 this is really simple using the CanBuildFrom functionality (as described by Daniel) and using breakOut with a type instruction to the compiler as to what the result type should be:

import scala.collection.breakOut
val m = (listA zip listB)(breakOut): Map[A,B]

The following would also work:

val n: Map[A,B] = (listA zip listB)(breakOut)

And (as EastSun, below, has pointed out) this has been added to the library as toMap

val o = (listA zip listB).toMap

As for reversing the map, you can do:

val r = m.map(_.swap)(breakOut): Map[B, A]

Solution 2

Now that you've got a list of tuples it is easy to make it into a map by writing Map(tuplesOfAB: _*). The : _* notation means to call the varargs overload with the arguments taken from the sequence. This seems like a funny bit of syntax, but it helps to think that varargs are declared like Map[A,B](pairs: (A,B)*) and the : _* is a type annotation to convert to varargs because of the common * part.

To reverse a map m use Map(m.map(_.swap): _*). In scala a map is also a collection of pairs. This transforms those pairs by swapping the elements and passing them to the Map constructor.

Solution 3

There's yet another way to do it, beyond those already shown. Here:

Map() ++ tuplesOfAB

Solution 4

scala> List( "a", "f", "d") zip List(7, 5, 4, 8) toMap
res0: scala.collection.immutable.Map[java.lang.String,Int] = Map(a -> 7, f -> 5, d -> 4)

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