FlatmapValues on Map - scala

Question

Given a Seq of tuples like:
Seq(
("a",Set(1,2)),
("a",Set(2,3)),
("b",Set(4,6)),
("b",Set(5,6))
)
I would like to groupBy and then flatMap the values to obtain something like:
Map(
b -> Set(4, 6, 5),
a -> Set(1, 2, 3)
)
My first implementation is:
Seq(
("a" -> Set(1,2)),
("a" -> Set(2,3)),
("b" -> Set(4,6)),
("b" -> Set(5,6))
) groupBy (_._1) mapValues (_ map (_._2)) mapValues (_.flatten.toSet)
I was wondering if there was a more efficient and maybe simpler way to achieve that result.

Answer 1

You were on the right track, but you can simplify a bit by using a single mapValues and combining the map and flatten:
val r = Seq(
("a" -> Set(1,2)),
("a" -> Set(2,3)),
("b" -> Set(4,6)),
("b" -> Set(5,6))
).groupBy(_._1).mapValues(_.flatMap(_._2).toSet)
I actually find this a lot more readable than the foldLeft version (but note that mapValues returns a non-strict collection, which may or may not be what you want).

Answer 2

I would use foldLeft, I think it's more readable, you can avoid groupBy
val r = Seq(
("a",Set(1,2)),
("a",Set(2,3)),
("b",Set(4,6)),
("b",Set(5,6))
).foldLeft(Map[String, Set[Int]]()){
case (seed,(k,v)) => {
seed.updated(k,v ++ seed.getOrElse(k,Set[Int]()))
}
}

Answer 3

#grotrianster answer could be refined using the Semigroup binary operation |+| of Set and Map:
import scalaz.syntax.semigroup._
import scalaz.std.map._
import scalaz.std.set._
Seq(
("a",Set(1,2)),
("a",Set(2,3)),
("b",Set(4,6)),
("b",Set(5,6))
).foldLeft(Map[String, Set[Int]]()){case (seed, (k, v)) => seed |+| Map(k -> v)}
Using reduce instead of fold:
Seq(
("a", Set(1, 2)),
("a", Set(2, 3)),
("b", Set(4, 6)),
("b", Set(5, 6))
).map(Map(_)).reduce({_ |+| _})
Treating Set and Map as Monoids:
Seq(
("a", Set(1, 2)),
("a", Set(2, 3)),
("b", Set(4, 6)),
("b", Set(5, 6))
).map(Map(_)).toList.suml