Scala best way of turning a Collection into a Map-by-key?

scala map scala-collections
oxbow_lakes picture oxbow_lakes · Mar 23, 2009 · Viewed 147.5k times · Source

If I have a collection c of type T and there is a property p on T (of type P, say), what is the best way to do a map-by-extracting-key?

val c: Collection[T]
val m: Map[P, T]

One way is the following:

m = new HashMap[P, T]
c foreach { t => m add (t.getP, t) }

But now I need a mutable map. Is there a better way of doing this so that it's in 1 line and I end up with an immutable Map? (Obviously I could turn the above into a simple library utility, as I would in Java, but I suspect that in Scala there is no need)

Answer

Ben Lings picture Ben Lings · Jul 14, 2010

You can use

c map (t => t.getP -> t) toMap

but be aware that this needs 2 traversals.

Related questions

How to get the element index when mapping an array in Scala?

Let's consider a simple mapping example: val a = Array("One", "Two", "Three") val b = a.map(s => myFn(s)) What I need is to use not myFn(s: String): String here, but myFn(s: String, n: Int): String, where …

Read More
Scala: how to merge a collection of Maps

I have a List of Map[String, Double], and I'd like to merge their contents into a single Map[String, Double]. How should I do this in an idiomatic way? I imagine that I should be able to do this …

Read More
How to convert from from java.util.Map to a Scala Map

A Java API returns a java.util.Map<java.lang.String,java.lang.Boolean>;. I would like to put that into a Map[String,Boolean] So imagine we have: var scalaMap : Map[String,Boolean] = Map.empty val javaMap = …

Read More