Efficient iteration with index in Scala

user_750995 0
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Since Scala does not have old Java style for loops with index, 

// does not work
val xs = Array("first", "second", "third")
for (i=0; i<xs.length; i++) {
  println("String #" + i + " is " + xs(i))
}

How can we iterate efficiently, and without using var's?

You could do this

val xs = Array("first", "second", "third")
val indexed = xs zipWithIndex
for (x <- indexed) println("String #" + x._2 + " is " + x._1)

but the list is traversed twice - not very efficient.

iteration scala
12 Answers

Indeed, calling zipWithIndex on a collection will traverse it and also create a new collection for the pairs. To avoid this, you can just call zipWithIndex on the iterator for the collection. This will just return a new iterator that keeps track of the index while iterating, so without creating an extra collection or additional traversing.

This is how scala.collection.Iterator.zipWithIndex is currently implemented in 2.10.3:

  def zipWithIndex: Iterator[(A, Int)] = new AbstractIterator[(A, Int)] {
    var idx = 0
    def hasNext = self.hasNext
    def next = {
      val ret = (self.next, idx)
      idx += 1
      ret
    }
  }

This should even be a bit more efficient than creating a view on the collection.

user_7751380
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