Scala GroupBy preserving insertion order?

Question

The groupBy method in Lists, Maps, etc., generate a Map after the function.

Is there a way to use the groupBy to generate a Map that preserves insertion order (Link

Answer 1

groupBy as defined on TraversableLike produces an immutable.Map, so you can't make this method produce something else.

The order of the elements in each entry is already preserved, but not the order of the keys. The keys are the result of the function supplied, so they don't really have an order.

If you wanted to make an order based on the first occurrence of a particular key, here's a sketch of how you might do it. Say we want to group integers by their value / 2:

val m = List(4, 0, 5, 1, 2, 6, 3).zipWithIndex groupBy (_._1 / 2)
val lhm = LinkedHashMap(m.toSeq sortBy (_._2.head._2): _*)
lhm mapValues (_ map (_._1))
// Map(2 -> List(4, 5), 0 -> List(0, 1), 1 -> List(2, 3), 3 -> List(6))
// Note order of keys is same as first occurrence in original list