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scalascala-collections

Find a number of occurrences in internal collections in a functional way

What is the best way to find a number of occurrences of each element in a functional/Scala way?

Seq(Set("a", "b", "c"), Set("b"), Set("b", "c"))

As a result, I need something like 

Set(("a", 1), ("b", 3), ("c", 2))

Thank you!

Solution

  • Use flatten and groupBy:

    scala> val s = Seq(Set("a", "b", "c"), Set("b"), Set("b", "c"))
s: Seq[scala.collection.immutable.Set[String]] =
  List(Set(a, b, c), Set(b), Set(b, c))

scala> s.flatten
res0: Seq[String] =
  List(a, b, c, b, b, c)

scala> s.flatten.groupBy(identity)
res3: scala.collection.immutable.Map[String,Seq[String]] =
  Map(b -> List(b, b, b), a -> List(a), c -> List(c, c))

scala> s.flatten.groupBy(identity).map { case (k, v) => (k, v.size) }.toSet
res7: scala.collection.immutable.Set[(String, Int)] =
  Set((b,3), (a,1), (c,2))