Question

I’m trying to find a (functional) way to add a collection of objects into a map that is keyed on a member of these objects.

Let’s say I have the following objects (they’re all instances of the same class O):

o1(a = 1, b = x)
o2(a = 1, b = y)
o3(a = 2, b = z)

I want to generate a Map keyed on member a that contains the following tuples:

(1, List(o1, o2))
(2, List(o3))

Now I could obviously do it iteratively, going through each object in my initial list and adding them as I go along. But I feel I am missing a functional way of doing that easily. I’ve been struggling with maps, flatMaps and filters to try to achieve that, no result so far.

Answer 1

groupBy is what you want:

scala> val os = List(O(1,2), O(1,3), O(2,4))
os: List[O] = List(O(1,2), O(1,3), O(2,4))

scala> os.groupBy(_.a)
res3: scala.collection.immutable.Map[Int,List[O]] = Map(1 -> List(O(1,2), O(1,3)), 2 -> List(O(2,4)))