How to clone an iterator?

Question

Suppose I have an iterator:

val it = List("a","b","c").iterator

I want a copy of it; my code is:

val it2 = it.toList.iterator

It's correct, but seems not good. Is there any other API to do it?

Answer 1

The method you are looking for is duplicate.

scala> val it = List("a","b","c").iterator
it: Iterator[java.lang.String] = non-empty iterator

scala> val (it1,it2) = it.duplicate
it1: Iterator[java.lang.String] = non-empty iterator
it2: Iterator[java.lang.String] = non-empty iterator

scala> it1.length
res11: Int = 3

scala> it2.mkString
res12: String = abc

Answer 2

Warning: as of Scala 2.9.0, at least, this leaves the original iterator empty. You can val ls = it.toList; val it1 = ls.iterator; val it2 = ls.iterator to get two copies. Or use duplicate (which works for non-lists also).

Rex's answer is by the book, but in fact your original solution is by far the most efficient for scala.collection.immutable.List's.

List iterators can be duplicated using that mechanism with essentially no overhead. This can be confirmed by a quick review of the implementation of iterator() in scala.collection.immutable.LinearSeq, esp. the definition of the toList method, which simply returns the _.toList of the backing Seq which, if it's a List (as it is in your case) is the identity.

I wasn't aware of this property of List iterators before investigating your question, and I'm very grateful for the information ... amongst other things it means that many "list pebbling" algorithms can be implemented efficiently over Scala immutable Lists using Iterators as the pebbles.