Ophelia Ophelia - 1 month ago 11
Scala Question

Scala sort one list according to the values in another one

I have two IndexedSeq

works[Work]
order[Int]


Each object Work has an id field with integer value: Work.id
In order lists there are ids, which are in the order we need to sort out the work. Like, in position 0 there is a first id, so we need to find the work in the works array with id that corresponds this one, and put it in the 0 place and so on.
Is there any way to do this with scala without going through two loops?Like, some elegant way?
Some pseudo data, for example:

order = 33, 22, 11, 55

works = (33, "some text"), (55, "eeeee"), (22, "fdsfs"), (11, "fdsffds")


After the sorting:

order = 33, 22, 11, 55

works = (33, "some text"),(22, "fdsfs"), (11, "fdsffds"), (55, "eeeee"),

Answer

You may use dictionary for that, the complexity is O(N) here (which is better in comparision with N*N for two nested cycles):

scala> val order = List(33, 22, 11, 55)
order: List[Int] = List(33, 22, 11, 55)

scala> val works = List((33, "some text"), (55, "eeeee"), (22, "fdsfs"), (11, "fdsffds"))
works: List[(Int, String)] = List((33,some text), (55,eeeee), (22,fdsfs), (11,fdsffds))

scala> val worksMap = works.toMap
worksMap: scala.collection.immutable.Map[Int,String] = Map(33 -> some text, 55 -> eeeee, 22 -> fdsfs, 11 -> fdsffds)

scala> val newWorks = order zip order.map(worksMap)
newWorks: List[(Int, String)] = List((33,some text), (22,fdsfs), (11,fdsffds), (55,eeeee))

if your entities is something more than tuples:

scala> val worksMap = (works map (_._1) zip works).toMap //any other key extraction, like `_.myKey` may be applied instead of `_._1`
worksMap: scala.collection.immutable.Map[Int,(Int, String)] = Map(33 -> (33,some text), 55 -> (55,eeeee), 22 -> (22,fdsfs), 11 -> (11,fdsffds))

scala> order.map(worksMap)
res13: List[(Int, String)] = List((33,some text), (22,fdsfs), (11,fdsffds), (55,eeeee))

if you don't want to spent a memory for Map - just use find instead of Map.apply (but it would be O(N*N), so it's slower):

scala> val newWorks = order.map(x => works.find(_._1 == x).get)
newWorks: List[(Int, String)] = List((33,some text), (22,fdsfs), (11,fdsffds), (55,eeeee))

If you don't want to have an exception in case if order doesn't contain your key, you may use flatMap:

scala> val newWorks = order.flatMap(x => works.find(_._1 == x))
newWorks: List[(Int, String)] = List((33,some text), (22,fdsfs), (11,fdsffds), (55,eeeee))

scala> order.flatMap(worksMap.get)
res15: List[(Int, String)] = List((33,some text), (22,fdsfs), (11,fdsffds), (55,eeeee))