Clever Geek Handbook

Set map smoothing

I'm trying to flatten a map where the keys are shared, in the sense that:

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

should flatten:

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

Here's 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:

  • Is there an easier way to do this?

  • I don't really like Scala's type system, and I'm sure it can be improved. I have to specify types explicitly when 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)

PS: This function fuse

doesn't make much sense when the key intersections have a non-bubble intersection.

+3


source to share


4 answers


Like @Azzie I was thinking about zip, but maybe Azzie has an edge with these 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 is not clear how to generalize this well.

+2


source


This is basically what you do in an understanding, but simplified a little:

  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 can be done by type, I'm sure there are some types of lambda chenins you can do, I'm just not sure how to implement them ...



UPDATE Here's a bit better for the version, same as 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
  }
+4


source


It looks terrible and using 0 is cheating, but it gets the job done

 m.map( {case (s,c) => s.zipAll(Set(c),0,c)} ).flatten.toMap
+1


source


Since I seem to be in a "terribly generic implication", 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)
+1


source






More articles:


All Articles