Question

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.

Answer 1

Much worse than traversing twice, it creates an intermediary array of pairs.
You can use view. When you do collection.view, you can think of subsequent calls as acting lazily, during the iteration. If you want to get back a proper fully realized collection, you call force at the end. Here that would be useless and costly. So change your code to

for((x,i) <- xs.view.zipWithIndex) println("String #" + i + " is " + x)