How to get a subset of a map?

Question

How do I get a subset of a map?

Assume we have

val m: Map[Int, String] = ...
val k: List[Int]

Where all keys in k exist in m.

Now I would like to get a subsect of the Map m with only the pairs which key is in the list k.

Something like m.intersect(k), but intersect is not defined on a map.

One way is to use filterKeys: m.filterKeys(k.contains). But this might be a bit slow, because for each key in the original map a search in the list has to be done.

Another way I could think of is k.map(l => (l, m(l)).toMap. Here wie just iterate through the keys we are really interested in and do not make a search.

Is there a better (built-in) way ?

Answer 1

m filterKeys k.toSet

because a Set is a Function.

On performance: filterKeys itself is O(1), since it works by producing a new map with overridden foreach, iterator, contains and get methods. The overhead comes when elements are accessed. It means that the new map uses no extra memory, but also that memory for the old map cannot be freed.

If you need to free up the memory and have fastest possible access, a fast way would be to fold the elements of k into a new Map without producing an intermediate List[(Int,String)]:

k.foldLeft(Map[Int,String]()){ (acc, x) => acc + (x -> m(x)) }

Answer 2

val s = Map(k.map(x => (x, m(x))): _*)