Group, map & reduce with two different reducer-operators

October 12, 2022

I have these tuples:

("T1",2,"x1"),
("T1",2,"x2"),
// … etc

And i want to reduce it to ("T1", 4, List("x1", "x2")). How can i do this ?

I did something like .group(_._1).map{case (key,list) => key-> list.map(_._2).reduce(_+_)}
But this is not working, and just sums the numbers without appending the list.

>Solution :

With groupMapReduce:

val xs = List(
  ("T1",40,"x1"),
  ("T1",2,"x2"),
  ("T2",58,"x3")
)

println(xs.groupMapReduce(_._1)
  (e => (e._2, List(e._3)))
  ({ case ((x, y), (z, w)) => (x + z, y ++ w)})
)

with groupBy:

val xs = List(
  ("T1",40,"x1"),
  ("T1",2,"x2"),
  ("T2",58,"x3")
)
println(xs.groupBy(_._1)
  .view
  .mapValues(ys => (ys.view.map(_._2).sum, ys.map(_._3)))
  .toMap
)

If you want to do it in one pass per list, and not use ++ you could try sth. like this:

xs.groupBy(_._1)
  .view
  .mapValues(ys =>
     ys.foldRight((0, List.empty[String])){
       case ((_, n, x), (sum, acc)) => (n + sum, x :: acc)
     }
  )
  .toMap

All three variants give

Map(T2 -> (58,List(x3)), T1 -> (42,List(x1, x2)))

Note that combining many lists with ++ might become very inefficient if the number of lists becomes large. It depends on your use-case whether this is acceptable or not.