Sort List within Map in scala

Question

I have a map like:

val v = Map("01" -> List(34,12,14,23), "11" -> List(22,11,34))

I simply can't find a way to sort the list items within map.

I tried like:

v.flatMap(v => v._2.sortBy(list => list._))

but seems that it does not sort list items.

Does anybody know where I am wrong?

UPDATE: Actually my List are List of tuples like List[(Object1, Object2, Object3, Object4)], I put int for simplicity. (but I think _.sorted will not work for my objects case). So I basically want to sort by one of this objects column. (for instance Object1.int)

Tomasz Nurkiewicz · Accepted Answer · 2013-02-01 21:44:23Z

5

This seems to work:

v.mapValues(_.sorted)
// Map(01 -> List(12, 14, 23, 34), 11 -> List(11, 22, 34))

If your values are tuples, try this:

v.mapValues(_.sortBy(_._1))

This approach will also work for classes, just say e.g. _.sortBy(_.age).

Or if you want one, merged list:

v.values.flatten.toSeq.sorted
// List(11, 12, 14, 22, 23, 34, 34)

(for list of tuples):

v.values.flatten.toSeq.sortBy(_._1)

edited Feb 1, 2013 at 21:44

answered Feb 1, 2013 at 21:38

Tomasz Nurkiewicz

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3 Comments

Cristian Boariu Over a year ago

Is there any way to insert in this List of tuples another element on the fly? I mean without creating new map?

Tomasz Nurkiewicz Over a year ago

@CristianBoariu: you can use mapValues() as well to map from one tuple to another. Feel free to open another question to make it clearer.

alex Over a year ago

This doesn't sort in place. Is there any way to do this instead of setting it equal to a new map? Or would this be bad practice?

score 2 · Accepted Answer · 2013-02-01 21:47:24Z

Do you mean like this?

val v = Map("01" -> List(34,12,14,23), "11" -> List(22,11,34))
//v: scala.collection.immutable.Map[String,List[Int]] = Map(01 -> List(34, 12, 14, 23), 11 -> List(22, 11, 34))

v map { case (k, v) => (k -> v.sorted) }
//res0: scala.collection.immutable.Map[String,List[Int]] = Map(01 -> List(12, 14, 23, 34), 11 -> List(11, 22, 34))

Edit or:

v mapValues (_ sorted)
//res1: scala.collection.immutable.Map[String,List[Int]] = Map(01 -> List(12, 14, 23, 34), 11 -> List(11, 22, 34))

Another edit:

If there's an implicit Ordering in scope for all the types in your tuples, it should work exactly the same way:

val v = Map(
  "01" -> List((34, "thirty-four"), (12, "twelve"), (14, "fourteen"), (23, "twenty-three")), 
  "11" -> List((22, "twenty-two"), (11, "eleven"), (34, "thirty-four")))
//v: scala.collection.immutable.Map[String,List[(Int, String)]] =
//Map(
//  01 -> List((34,thirty-four), (12,twelve), (14,fourteen), (23,twenty-three)), 
//  11 -> List((22,twenty-two), (11,eleven), (34,thirty-four)))

v mapValues (_ sorted)
//res3: scala.collection.immutable.Map[String,List[(Int, String)]] =
//Map(
//  01 -> List((12,twelve), (14,fourteen), (23,twenty-three), (34,thirty-four)),
//  11 -> List((11,eleven), (22,twenty-two), (34,thirty-four)))

If you want to sort only by one member of the tuple (say ... in this case the second one):

v mapValues (_ sortBy (_ _2))
//res5: scala.collection.immutable.Map[String,List[(Int, String)]] =
//Map(
//  01 -> List((14,fourteen), (34,thirty-four), (12,twelve), (23,twenty-three)),
//  11 -> List((11,eleven), (34,thirty-four), (22,twenty-two)))

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