Getting values from Map given list of keys in Scala

Question

Basically I need to get a set of values from a map for a keys in a given list (in the same order keys are given).

val kv = Map("k3" -> "v3", "k1" -> "v1", "k2" -> "v2")
val ks = List("k1", "k2")

The best I could have improvised is foldRight over ks:

scala> (ks foldRight List[String]()) { (v, a) => kv(v) :: a }
res7: List[String] = List(v1, v2)

Are there any more convenient ways to do this in standard lib or just the shorter code, ideally something alike kv getVals ks? :)

Question is not so important of course, just trying to explore towards good style.

Wouldn't foldLeft be a better option here? It would be a better style I think, because it is implemented by iteration while foldRight is implemented by recursion in List.scala. — Core_Dumped
– Core_Dumped, Commented Jul 4, 2014 at 8:44

jrmn · Accepted Answer · 2013-01-31 19:29:07Z

30

Or:

ks collect kv

The result is composed of all the values of kv for which a key is in ks.

(Besides being a Function a Map is also a PartialFunction)

answered Jan 31, 2013 at 19:29

jrmn

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2 Comments

Great, collect happens to be safe variant when keys might be missing.

+1 I think this is as good as it gets. How blind was I to not exploit Map being a PartialFunction.

Faiz · Accepted Answer · 2013-01-31 10:27:08Z

21

Use flatMap:

 ks flatMap { k => kv get k }

Or better:

 ks flatMap (kv get)

And with for comprehensions:

for (k <- ks; v <- kv get k) yield v

But none are as succinct or as clear as your own one-liner: :-)

ks map kv
List("k42") map kv // will barf

However, note that this is exceptional - it may throw an exception if ks contains a key not defined in the map kv.

answered Jan 31, 2013 at 10:16

Faiz

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