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 t

Answer 1

An alternative to Miles Sabin's answer using Scala 2.13's new groupMapReduce method which is (as its name suggests) an equivalent (more efficient) of a groupBy followed by mapValues and a reduce step:

(l1 ::: l2).groupMapReduce(_._2)(_._1)(_ + _).toList.map(_.swap)
// List[(Int, String)] = List((3,b), (4,a), (4,c))

This:

• prepends l1 to l2

• groups elements based on their second tuple part (group part of groupMapReduce)

• maps grouped values to their first tuple part (map part of groupMapReduce)

• reduces values (_ + _) by summing them (reduce part of groupMapReduce)

• and finally swaps tuples' parts.

This is an equivalent version performed in one pass (for the group/map/reduce part) through the List of:

(l1 ::: l2).groupBy(_._2).mapValues(_.map(_._1).reduce(_ + _)).toList.map(_.swap)

Answer 2

With Scalaz, this is a snap.

import scalaz._
import Scalaz._

val l3 = (l1.map(_.swap).toMap |+| l2.map(_.swap).toMap) toList

The |+| method is exposed on all types T for which there exists an implementation of Semigroup[T]. And it just so happens that the semigroup for Map[String, Int] is exactly what you want.