CODEWITHSUNDEEP
scala
toto2
toto2

Reputation: 5326

Flattening a map of sets

I am trying to flatten a map where the keys are traversables, in the sense that:

Map( Set(1, 2, 3) -> 'A', Set(4, 5, 6) -> 'B')

should flatten to:

Map(5 -> B, 1 -> A, 6 -> B, 2 -> A, 3 -> A, 4 -> B)

Here is what I did:

def fuse[A, B, T <: Traversable[A]](mapOfTravs: Map[T, B]): Map[A, B] = {
  val pairs = for {
    trav <- mapOfTravs.keys
    key <- trav
  } yield (key, mapOfTravs(trav))
  pairs.toMap
}

It works. But:

  1. Is there a simpler way to do this?

  2. I'm not very comfortable with the Scala type system and I'm sure this can be improved. I have to specify the types explicitly whenever I use my function:

    val map2 = Map( Set(1, 2, 3) -> 'A', Set(4, 5, 6) -> 'B')
val fused2 = fuse[Int, Char, Set[Int]](map2)

val map1: Map[Traversable[Int], Char] = Map( Set(1, 2, 3) -> 'A', Set(4, 5, 6) -> 'B')
val fused1 = fuse[Int, Char, Traversable[Int]](map1)

P.S.: this fuse function does not make much sense when the key traversables have a non-null intersection.

Upvotes: 5

Views: 540

Answers (4)

Chris Martin
Chris Martin

Reputation: 30736

Since I'm apparently on a "hideously generic implicits" kick lately:

import scala.collection.MapLike
import scala.collection.TraversableLike
import scala.collection.generic.CanBuildFrom

implicit class Map_[
  A,
  B,
  T1 : ({type L[X] = X => TraversableLike[A, T2]})#L,
  T2,
  M1 : ({type L[X] = X => MapLike[T1, B, M2]})#L, 
  M2 <: MapLike[T1, B, M2] with Map[T1, B]
](map: M1) {

  def fuse[M3](implicit cbfM: CanBuildFrom[M2, (A, B), M3]) : M3 =
    map.flatMap({ case (k, v) => k.toTraversable.map((_, v)) })
}

Examples:

scala> Map(Set(1, 2, 3) -> 'A', Set(4, 5, 6) -> 'B').fuse
res: scala.collection.immutable.Map[Int,Char] =
     Map(5 -> B, 1 -> A, 6 -> B, 2 -> A, 3 -> A, 4 -> B)

scala> Map(Array(1, 2, 3) -> 'A', Array(4, 5, 6) -> 'B').fuse
res: scala.collection.immutable.Map[Int,Char] =
     Map(5 -> B, 1 -> A, 6 -> B, 2 -> A, 3 -> A, 4 -> B)

Upvotes: 1

som-snytt
som-snytt

Reputation: 39577

Like @Azzie, I was thinking zip, but maybe Azzie has the advantage with those zees.

scala> val m = Map( Set(1, 2, 3) -> 'A', Set(4, 5, 6) -> 'B')
m: scala.collection.immutable.Map[scala.collection.immutable.Set[Int],Char] = Map(Set(1, 2, 3) -> A, Set(4, 5, 6) -> B)

scala> (m map { case (k, v) => k zip (Stream continually v) }).flatten.toMap
res0: scala.collection.immutable.Map[Int,Char] = Map(5 -> B, 1 -> A, 6 -> B, 2 -> A, 3 -> A, 4 -> B)

scala> (m map { case (k, v) => k zipAll (Nil, null, v) }).flatten.toMap
res1: scala.collection.immutable.Map[Any,Char] = Map(5 -> B, 1 -> A, 6 -> B, 2 -> A, 3 -> A, 4 -> B)

scala> m flatMap { case (k, v) => k zip (Stream continually v) }
res2: scala.collection.immutable.Map[Int,Char] = Map(5 -> B, 1 -> A, 6 -> B, 2 -> A, 3 -> A, 4 -> B)

It's not obvious how to generalize it nicely.

Upvotes: 2

Azzie
Azzie

Reputation: 366

This looks horrible and using 0 is kind of cheating but it does the job

 m.map( {case (s,c) => s.zipAll(Set(c),0,c)} ).flatten.toMap

Upvotes: 1

Noah
Noah

Reputation: 13959

This is basically what you're doing in the for comprehension, but simplified a little bit:

  def fuse[A, B, T <: Traversable[A]](mapOfTravs: Map[T, B]): Map[A, B] = {
    mapOfTravs.flatMap({ case (s, c) => s.map(i => i -> c) })
  }

Not much you can do about the types, I'm sure there's some type lambda shenanigans that you can do, I'm just not sure how to do them...

UPDATE Here's a slightly better for version, same as the above flatMap:

  def fuse2[A, B, T <: Traversable[A]](mapOfTravs: Map[T, B]): Map[A, B] = {
    for {
      (keys, value) <- mapOfTravs
      key <- keys
    } yield key -> value
  }

Upvotes: 4

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