consume items from a scala Iterator

Question

I'm confused about behavior of method take in trait Iterator. It seems that it doesn't consume items. Here is an example:

scala> Iterator(1,2,3)
res0: Iterator[Int] = non-empty iterator

scala> res0 take 2 toArray
res1: Array[Int] = Array(1, 2)

scala> res0.next
res2: Int = 1

Apparently step 2 consumes two items, but in step 3 the Iterator is still at first item. Looking at the implementation, I can't see any kind of copying or buffering, just a new Iterator which delegates to the underlying one. How could it be possible? How can I manage to really consume n items?

Answer 1

The iterator in question is defined in IndexedSeqLike#Elements (source). A ticket was recently filed about the the inconsistent behaviour of take across different iterator implementations.

To really consume N items, call Iterator#next N times.

You might want to consider using Stream, which is a lazy (like Iterator), but is also immutable (unlike Iterator).

scala> val s = Stream(1, 2, 3)
s: scala.collection.immutable.Stream[Int] = Stream(1, ?)

scala> s.take(2).toList
res43: List[Int] = List(1, 2)

scala> s.take(2).toList
res44: List[Int] = List(1, 2)

scala> s.drop(2).toList
res45: List[Int] = List(3)

scala> {val (s1, s2) = s.splitAt(2); (s1.toList, s2.toList)}
res46: (List[Int], List[Int]) = (List(1, 2),List(3))

Answer 2

Thanks guys.

This is my solution to consume bunches of items from an Iterator:

  implicit def consumable(i: Iterator[_]) = new {
    def next(n: Int) = {
      (for (_ <- 1 to n) yield i.next()).iterator
    }
    def skip(n: Int) {
      (1 to n).foreach(_ => i.next())
    }
  }

any comments will be welcome.