Scala - 将地图列表展平到地图

Question

I need to map a key to a map inside a list in Scala. In other words, I need to go from here:

  Map(k1 -> List(Map(k2 -> v2)))

And get this:

  Map(k1 -> Map(k2 -> v2))

This stuff is working but I am looking for something more efficient.

  val m1 = Map("k1" -> List(Map("k2" -> "v2")))
  val m2 = m1("k1").flatten.toMap
  val m3 = Map("k1" -> m2)

Any thoughts?

Thanks in advance.

Answer 1

Like @Luis Miguel Mejía Suárez said it is mostly something like

val maps = Map(k1 -> List(Map(k2 -> v2)))
def flattenMaps[K, V]: List[Map[K, V]] => Map[K, V] = ???
maps.mapValues(flattenMaps).toMap

The devil is in the details of your flattening logic:

// assumes that list always of size 1, loses random elements if bigger, blows up if size=0
def flattenMaps[K, V]: List[Map[K, V]] => Map[K, V] = _.head

// last wins
def flattenMaps[K, V]: List[Map[K, V]] => Map[K, V] = _.foldLeft(Map.empty[K, V]) {
  (bigMap, map) => bigMap ++ map
}

// first wins
def flattenMaps[K, V]: List[Map[K, V]] => Map[K, V] = _.foldLeft(Map.empty[K, V]) {
  (bigMap, map) => map ++ bigMap
}

// if you want to optimize for some reason, you can use builder instead
def flattenMaps[K, V]: List[Map[K, V]] => Map[K, V] = _.foldLeft(Map.newBuilder[K, V]) {
  (builder, map) =>
     builder ++= map
     builder
}.result

// or basically any other logic that makes sense and is defined for any input that type-checks

You should define some correct behavior in case your list will have more than one element (or none), otherwise you might be surprised one day if you'll forget about your assumptions.