How to get a set of all elements that occur multiple times in a list in Scala?

Question

E.g. for List(1, 1, 1, 2, 3, 3, 4) it would be Set(1, 3), because 1 and 3 are the only elements which occur multiple times.

Answer 1

val s = List(1, 1, 1, 2, 3, 3, 4) // a list with non-unique elements
(s diff s.distinct) toSet // Set(1, 3)

Answer 2

A bit more convoluted but you can avoid having to call toSet.toList, first group the integers:

scala> s.groupBy(identity)
res13: scala.collection.immutable.Map[Int,List[Int]] = 
  Map(2 -> List(2), 4 -> List(4), 1 -> List(1, 1, 1), 3 -> List(3, 3))

Then collect only the one were the list has length greater as 1:

scala> s.groupBy(identity).collect { case (v, l) if l.length > 1 => v }
res17: scala.collection.immutable.Iterable[Int] = List(1, 3)

If you want a Set just call toSet.