Combining two lists in Scala

Question

From 2 lists of the form List[(Int, String):

l1 = List((1,"a"),(3,"b"))
l2 = List((3,"a"),(4,"c"))

how can I combine the Integers where the Strings are the same to get this third list:

l3 = List((4,"a"),(3,"b"),(4,"c"))

Right now I'm traversing both of the lists and adding if the strings are the same, but I think there should be a simple solution with pattern matching.

Answer 1

val l = l1 ::: l2
val m = Map[String, Int]()
(m /: l) {
  case (map, (i, s)) => { map.updated(s, i + (map.get(s) getOrElse 0))}
}.toList // Note: Tuples are reversed.

But I suppose there is a more elegant way to do the updated part.

Answer 2

How about,

(l1 ++ l2).groupBy(_._2).mapValues(_.unzip._1.sum).toList.map(_.swap)

Unpacking this a little on the REPL helps to show what's going on,

scala> l1 ++ l2
res0: List[(Int, java.lang.String)] = List((1,a), (3,b), (3,a), (4,c))

scala> res0.groupBy(_._2)
res1: ... = Map(c -> List((4,c)), a -> List((1,a), (3,a)), b -> List((3,b)))

scala> res1.mapValues(_.unzip)
res2: ... = Map(c -> (List(4),List(c)), a -> (List(1, 3),List(a, a)), b -> (List(3),List(b)))                         

scala> res1.mapValues(_.unzip._1)                                                                                                                                                                      
res3: ... = Map(c -> List(4), a -> List(1, 3), b -> List(3))                                                                                    

scala> res1.mapValues(_.unzip._1.sum)
res4: ... = Map(c -> 4, a -> 4, b -> 3)                                                                                                               

scala> res4.toList                                                                                                                                                                                     
res5: List[(java.lang.String, Int)] = List((c,4), (a,4), (b,3))                                                                                                                                        

scala> res5.map(_.swap)
res6: List[(Int, java.lang.String)] = List((4,c), (4,a), (3,b))