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. But, it gives me output as

Map(1 -> List((1,2) , (1,3)) , 3 -> List((3,4), (3,5)), 4 -> List((4,5)))

But, I want answer as

Map(1 -> List(2, 3), 3 -> List(4,5) , 4 -> List(5))

So, how can I do this?

Answer 1

You can do that by following up with mapValues (and a map over each value to extract the second element):

scala> a.groupBy(_._1).mapValues(_.map(_._2))
res2: scala.collection.immutable.Map[Int,List[Int]] = Map(4 -> List(5), 1 -> List(2, 3), 3 -> List(4, 5))