scala ArrayBuffer remove all elements with a predicat

Question

Scala is very elegant in filtering immutable sequences:

var l = List(1,2,3,4,5,6)
l = l.filter(_%2==1)

But how do I do this with a mutable collections like ArrayBuffer? All I found is the removal of single elements or slices, or remove elements from another sequence, but nothing that removes elements given by a predicate.

Edit: I was hoping to find something similar tho this:

trait Removable[A] extends Buffer[A]{ 
  def removeIf(p: A => Boolean){
    var it1 = 0
    var it2 = 0

    while(it2 < length){
      if( p( this(it2) ) ){
        it2 += 1;
      } 
      else {
        this(it1) = this(it2)
        it1 += 1;
        it2 += 1;
      }
    }

    trimEnd(it2-it1)
  }
}

this filters in linear time and can be mixed into any Buffer, but only ArrayBuffer makes sense, on ListBuffers it would be slow, because indexing does take linear time.

Answer 1

My guess is that it's more efficient to filter by building a new buffer, so you would normally just use filter and use its result. Otherwise you can write your own in-place filter method:

def filterInPlace[A](b: collection.mutable.Buffer[A])(fun: A => Boolean): Unit = {
  var sz = b.size
  var i = 0; while(i < sz) {
    if (fun(b(i))) {
      i += 1
    } else {
      sz -= 1
      b.remove(i)
    }
  }
}

val b = collection.mutable.ArrayBuffer((1 to 6): _ *)
filterInPlace(b)(_ % 2 == 1)
println(b)

Answer 2

There has been discussion about having a set of methods that work by performing mutation, but coming up with a good, generic set is surprisingly hard, and, on the other hand, there just hasn't been enough demand for it.