CODEWITHSUNDEEP
scaladictionary
John Threepwood
John Threepwood

Reputation: 16143

How to get a subset of a map?

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 ?

Upvotes: 4

Views: 4607

Answers (3)

Luigi Plinge
Luigi Plinge

Reputation: 51109

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)) }

Upvotes: 16

Pablo Lalloni
Pablo Lalloni

Reputation: 2755

I think this is most readable and good performer:

k zip (k map m) toMap

Or, method invocation style would be:

k.zip(k.map(m)).toMap

Upvotes: 1

Sergey Weiss
Sergey Weiss

Reputation: 5974

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

Upvotes: 2

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