在Scala中将映射序列转换为键映射到值序列

Question

I'm trying to turn a sequence of maps, like for instance

val input = Seq(Map("a" -> 1, "b" -> 2),
                Map("a" -> 10,  "c" -> 30),
                Map("b" -> 200, "c" -> 300, "d" -> 400))

into a map from the keys in those maps to the sequence of values they map to across each of the maps in the original sequence.

So the above should transform into

val output = Map("a" -> Seq(1, 10),
                 "b" -> Seq(2, 200),
                 "c" -> Seq(30, 300),
                 "d" -> Seq(400))

I can think of a few ways to go about this but I'm looking for a solution that is idiomatic or the best scala style for this sort of transformation? Ideally without being really wasteful but performance isn't a great concern.

Thanks!

Answer 1

You can do this:

val output = input.flatten.groupBy(_._1).mapValues(_.map(_._2))

It will first flatten your map into a Seq of all map entries. Then it groups the map entries on their keys. Finally, you map the values (which is now a list of map entries) to a list of map-values.