What is the proper way to remove elements from a scala mutable map using a predicate

Question

How to do that without creating any new collections? Is there something better than this?

val m = scala.collection.mutable.Map[String, Long]("1" -> 1, "2" -> 2, "3" -> 3, "4" -> 4)
m.foreach(t => if (t._2 % 2 == 0) m.remove(t._1))
println(m)

P.S. in Scala 2.8

Answer 1

retain does what you want. In 2.7:

val a = collection.mutable.Map(1->"one",2->"two",3->"three")
a: scala.collection.mutable.Map[Int,java.lang.String] = 
  Map(2 -> two, 1 -> one, 3 -> three)

scala> a.retain((k,v) => v.length < 4)   

scala> a
res0: scala.collection.mutable.Map[Int,java.lang.String] =
  Map(2 -> two, 1 -> one)

It also works, but I think is still in flux, in 2.8.

Answer 2

If you are using an immutable.Map, in 2.7 it might have to be something like:

def pred(k: Int, v: String) = k % 2 == 0

m --= (m.filter(pred(_, _)).keys

As there is no retain method available. Obviously in this case m must be declared as a var