Mutating a mutable collection using map?

If you have a mutable data structure like an Array, is it possible to use map operations or something similar to change its values?

Say I have val a = Array(5, 1, 3), what the best way of say, subtracting 1 from each value? The best I've come up with is

for(i <- 0 until a.size) { a(i) = a(i) - 1 }

I suppose way would be make the array a var rather than a val so I can say

a = a map (_-1)

edit: fairly easy to roll my own if there's nothing built-in, although I don't know how to generalise to other mutable collections

scala> implicit def ArrayToMutator[T](a: Array[T]) = new {
     |   def mutate(f: T => T) = a.indices.foreach {i => a(i) = f(a(i))}
     | }
ArrayToMutator: [T](a: Array[T])java.lang.Object{def mutate(f: (T) => T): Unit}

scala> val a = Array(5, 1, 3)
a: Array[Int] = Array(5, 1, 3)

scala> a mutate (_-1)

scala> a
r开发者_如何学Ces16: Array[Int] = Array(4, 0, 2)

If you don't care about indices, you want the transform method.

scala> val a = Array(1,2,3)
a: Array[Int] = Array(1, 2, 3)

scala> a.transform(_+1)
res1: scala.collection.mutable.WrappedArray[Int] = WrappedArray(2, 3, 4)

scala> a
res2: Array[Int] = Array(2, 3, 4)

(It returns a copy of the wrapped array so that chaining transforms is more efficient, but the original array is modified as you can see.)

How about using foreach since you don't really want to return a modified collection, you want to perform a task for each element. For example:

a.indices.foreach{i => a(i) = a(i) - 1}

or

a.zipWithIndex.foreach{case (x,i) => a(i) = x - 1}