Reverse a Map which has A Set as its value using HOF

May 25, 2022

I am trying to reverse a map that has a String as the key and a set of numbers as its value

My goal is to create a list that contains a tuple of a number and a list of strings that had the same number in the value set

I have this so far:

def flipMap(toFlip: Map[String, Set[Int]]): List[(Int, List[String])] = {
  toFlip.flatMap(_._2).map(x => (x, toFlip.keys.toList)).toList
}

but it is only assigning every String to every Int

val map = Map(
 "A" -> Set(1,2),
 "B" -> Set(2,3)
)

should produce:
List((1, List(A)), (2, List(A, B)), (3, List(B)))

but is producing:
List((1, List(A, B)), (2, List(A, B)), (3, List(A, B)))

>Solution :

This works to, but it’s not exactly what you might need and you may need some conversions to get the exact data type you need:

toFlip.foldLeft[Map[Int, Set[String]]](Map.empty) {
    case (acc, (key, numbersSet)) =>
      numbersSet.foldLeft[Map[Int, Set[String]]](acc) {
        (updatingMap, newNumber) =>
          updatingMap.updatedWith(newNumber) {
            case Some(existingSet) => Some(existingSet + key)
            case None => Some(Set(key))
          }
      }
  }

I used Set to avoid duplicate key insertions in the the inner List, and used Map for better look up instead of the outer List.