Combining two Map[String,Int] with key-wise addition.

combine-maps.scala

object CombineMaps {
  type Counts = Map[String,Int]	
  def combine(x: Counts, y: Counts): Counts = {
    val x0 = x.withDefaultValue(0)
    val y0 = y.withDefaultValue(0)
    val keys = x.keys.toSet.union(y.keys.toSet)
    keys.map{ k => (k -> (x0(k) + y0(k))) }.toMap
  }
}

val counts1 = Map("boats" -> 1, "against" -> 2, 
 		  "the" -> 10, "current" -> 1)  
val counts2 = Map("burned" -> 1, "the" -> 5, "boats" -> 1)

println(CombineMaps.combine(counts1,counts2))