Transform Sequence of Maps into Map of Keys to Sequences of Values in Scala

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!

marstran · Accepted Answer · 2018-08-21 08:57:03Z

4

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.

edited Aug 21, 2018 at 8:57

answered Aug 21, 2018 at 8:27

marstran

28.4k5 gold badges67 silver badges71 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Tim Over a year ago

You can just use flatten rather than flatMap(_.toSeq)

marstran Over a year ago

@Tim Oh, that's true. Fixed.

Collectives™ on Stack Overflow

Transform Sequence of Maps into Map of Keys to Sequences of Values in Scala

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related