How to use mutable collections in Scala

For both immutable and mutable collections, :+ and +: create new collections. If you want mutable collections that automatically grow, use the += and +=: methods defined by collection.mutable.Buffer.

Similarly, map returns a new collection — look for transform to change the collection in place.

map operation applies the given function to all the elements of collection, and produces a new collection.

The operation you are looking for is called transform. You can think of it as an in-place map except that the transformation function has to be of type a -> a instead of a -> b.

scala> import collection.mutable.Buffer
import collection.mutable.Buffer

scala> Buffer(6, 3, 90)
res1: scala.collection.mutable.Buffer[Int] = ArrayBuffer(6, 3, 90)

scala> res1 transform { 2 * }
res2: res1.type = ArrayBuffer(12, 6, 180)

scala> res1
res3: scala.collection.mutable.Buffer[Int] = ArrayBuffer(12, 6, 180)