Turning Map(“a” -> 2, “b” -> 1) into seq(“a”,“a”,“b”) using map

Question

I am trying to turn a Map("a" -> 2, "b" -> 1) into seq("a","a","b") through the map function, Currently I am trying to ru

Answer 1

I am not sure exactly what the shape of your data is. That part is a little bit unclear from the question.

Map('a' -> 3, 'b' -> 1)

Is indeed a Map. Whereas

('a' -> 3, 'b' -> 1)

desugars in a Tuple

(('a', 3), ('b', 1))

If it is the former case you can fold like this

val m : Map[String, Int] =  Map("a" -> 2, "b" -> 1) 
val res = m.foldLeft(List[String]())((a, b) => a ++ List.fill(b._2)(b._1))

here res will be List('a', 'a', 'b')

What is going on here is that we start with an empty accumulator, iterate through the key-value pairs of the Map, create a list that repeats the given key, value times and concat it to the accumulator

The latter is unfortunately a little harder without using something like Shapeless since in Scala 2 the type information will get lost when converting from a Tuple. You need to do some type juggling with the annotations

val ml = ("a" -> 2, "b" -> 1).productIterator
ml.foldLeft(List[String]())((a, b) => b match{ 
   case (k: String, v : Int) => a ++ List.fill(v)(k)
})