scala foldLeft与地图地图

Question

I have a Map which is like

val v = Map("A" -> Map("C" -> "D", "F" -> "G"), "B" -> Map("C" -> "E", "M" -> "N"))

I have to create new map which will have list of value maps and their parents, so my resultant map will look like

Map("C" -> List(Map("A" -> Map("C" -> "D", "F" -> "G"), "B" -> Map("C" -> "E", "M" -> "N")), "F" -> List(Map("A" -> Map("C" -> "D", "F" -> "G")), "M" -> List("B" -> Map("C" -> "E", "M" -> "N")))

can this be possible with foldleft. I know this is confusing but any help will be appreciated.

Answer 1

Yes, it can be done with a foldLeft .

I don't know why you have Lists in there, because each List has exactly one Map as an element. Not more, not less.

Anyways.

I use the fold over the values (which are the maps) of v , so I can access the keys of those maps faster. I throw away all keys which are already in acc , so there won't be any duplicate computation. ( elem.keys.filter(!acc.isDefinedAt(_)) ).

Then for each key that is not yet in our result acc , traverse the original v and look where it occurs as a key in the value maps. ( v.filter{case (k, map) => map.isDefinedAt(key)}) )

Then put the filtered maps in a List and add them with their corresponding keys to acc .

Side note : maps of maps, keys of values, keys, values ... I'm not sure how I can explain this better without even confusing myself... I don't even know what I should highlight..

val v = Map("A" -> Map("C" -> "D", "F" -> "G"), "B" -> Map("C" -> "E", "M" -> "N"))

val result = v.values.foldLeft(Map.empty[String, List[Map[String, Map[String, String]]]])((acc, elem) => {
 val nextEntries = elem.keys.filter(!acc.isDefinedAt(_)).map(key => 
   key -> List(v.filter{case (k, map) => map.isDefinedAt(key)}))
 acc ++ nextEntries
})

println(result) //Map(C -> List(Map(A -> Map(C -> D, F -> G), B -> Map(C -> E, M -> N))), F -> List(Map(A -> Map(C -> D, F -> G))), M -> List(Map(B -> Map(C -> E, M -> N))))