GroupBy in scala

Question

I have

val a = List((1,2), (1,3), (3,4), (3,5), (4,5))

I am using A.groupBy(_._1) which is groupBy with the first element. Bu

Answer 1

You can also do it with a foldLeft to have only one iteration.

a.foldLeft(Map.empty[Int, List[Int]])((map, t) => 
  if(map.contains(t._1)) map + (t._1 -> (t._2 :: map(t._1))) 
  else map + (t._1 -> List(t._2)))

scala.collection.immutable.Map[Int,List[Int]] = Map(1 -> List(3, 2), 3 ->
       List(5, 4), 4 -> List(5))

If the order of the elements in the lists matters you need to include a reverse.

a.foldLeft(Map.empty[Int, List[Int]])((map, t) => 
  if(map.contains(t._1)) (map + (t._1 -> (t._2 :: map(t._1)).reverse)) 
  else map + (t._1 -> List(t._2)))

scala.collection.immutable.Map[Int,List[Int]] = Map(1 -> List(2, 3), 3 ->
       List(4, 5), 4 -> List(5))