Why does groupBy in Scala change the ordering of a list's items?

Question

This code is from a Scala Worksheet:

case class E(a: Int, b: String)

val l = List(
    E(1, \"One\"),
    E(1, \"Another One\"),
    E(2, \"Two\"),
    E(2,         


        

Answer 1

Unless you specifically use a subtype of SortedMap, a map (like a set) is always in an unspecified order. Since "groupBy" doesn't return a SortedMap but only a general immutable.Map and also doesn't use the CanBuildFrom mechanism, I think there's nothing that you can do here.

You can find more on this topic in answers to similar questions, e.g. here.

Edit:

If you want to convert the map afterwarts to a SortedMap (ordered by its keys), you can do SortedMap(l.groupBy(_.a).toSeq:_*) (with import scala.collection.immutable.SortedMap). Don't do ...toSeq.sortWith(...).toMap because that will not guarantee the ordering in the resulting map.