Count occurrences of each element in a List[List[T]] in Scala

Try this:

scala> docs.flatten.groupBy(identity).mapValues(_.size)
res0: Map[String,Int] = Map(one -> 1, two -> 2, three -> 1)

If you are going to be accessing the counts many times, then you should avoid mapValues since it is "lazy" and, thus, would recompute the size on every access. This version gives you the same result but won't require the recomputations:

docs.flatten.groupBy(identity).map(x => (x._1, x._2.size))

The identity function just means x => x.

docs.flatten.foldLeft(new Map.WithDefault(Map[String,Int](),Function.const(0))){
  (m,x) => m + (x -> (1 + m(x)))}

What a train wreck!

[Edit]

Ah, that's better!

docs.flatten.foldLeft(Map[String,Int]() withDefaultValue 0){
  (m,x) => m + (x -> (1 + m(x)))}