Scala filter tuples (x, y) == (y, x)

Question

I have a list of tuples, for example:

  (0,2)
  (0,5)
  (2,0)
  (2,5)
  (3,4)
  (4,3)
  (5,0)
  (5,2)

There are some tuples where (x, y)

Answer 1

You can groupBy the tuples represented as a Set, which will make Set(0, 5) == Set(5, 0), etc. Then map the resulting Map to the groups of tuples and convert it back to a List, and finally grab the head of each list for one representative of the group. Calling head is ok here, because the groups will never be empty (they would otherwise just not be there at all).

val list = List((0,2), (0,5), (2,0), (2,5), (3,4), (4,3), (5,0), (5,2))

list.groupBy { case (x, y) => Set(x, y) } // Map[Set, List[(Set, (Int, Int))]]
    .map(_._2)                            // Iterable[List[(Int, Int)]] 
    .toList                               // List[List[(Int, Int)]]
    .map(_.head)                          // List[(Int, Int)]


res10: List[(Int, Int)] = List((0,5), (3,4), (2,5), (0,2))