How to transpose a map with list values in Scala

Question

How can this map of list,

Map (
   "a" -> List(1, 2)
)

be transposed to this list of maps primarily using methods from the Scala libraries?

List(
  Map("a" -> 1),
  Map("a" -> 2)
)

I can code a solution myself but I am more interested in using library functionality so the preferred solution should use the Scala library where possible while remaining compact and moderately legible.

This second example illustrates the required transformation with a map with more than one entry.

From this,

Map (
   10 -> List("10a", "10b", "10c"),
   29 -> List("29a", "29b", "29c")
)

to this,

List(
  Map(
    10 -> "10a",
    29 -> "29a"),
  Map(
    10 -> "10b",
    29 -> "29b"),
  Map(
    10 -> "10c",
    29 -> "29c")
)

It can be assumed that all values are lists of the same size.

Optionally the solution could handle the case where the values are empty lists but that is not required. If the solution supports empty list values then this input,

Map (
   "a" -> List()
)

should result in List().

It is more transpose, than flatten

om-nom-nom
– om-nom-nom

2013-07-25 08:30:59 +00:00
Commented Jul 25, 2013 at 8:30 — om-nom-nom
– om-nom-nom, Commented Jul 25, 2013 at 8:30
Updated to use transpose.

Janek Bogucki
– Janek Bogucki

2013-07-25 08:33:18 +00:00
Commented Jul 25, 2013 at 8:33 — Janek Bogucki
– Janek Bogucki, Commented Jul 25, 2013 at 8:33

Régis Jean-Gilles · Accepted Answer · 2013-07-25 08:51:18Z

9

val m = Map (
   10 -> List("10a", "10b", "10c"),
   29 -> List("29a", "29b", "29c")
)

m.map{ case (k, vs) =>
  vs.map(k -> _)
}.toList.transpose.map(_.toMap)

Note that this also handles your "empty list" case

answered Jul 25, 2013 at 8:51

Régis Jean-Gilles

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1 Comment

Janek Bogucki Over a year ago

This fits my requirements perfectly and passes all my unit tests.

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