Efficient iteration with index in Scala

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.

Solution 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)

Solution 2:

It has been mentioned that Scala does have syntax for for loops:

for (i <- 0 until xs.length) ...

or simply

for (i <- xs.indices) ...

However, you also asked for efficiency. It turns out that the Scala for syntax is actually syntactic sugar for higher order methods such as map, foreach, etc. As such, in some cases these loops can be inefficient, e.g. How to optimize for-comprehensions and loops in Scala?

(The good news is that the Scala team is working on improving this. Here's the issue in the bug tracker: https://issues.scala-lang.org/browse/SI-4633)

For utmost efficiency, one can use a while loop or, if you insist on removing uses of var, tail recursion:

import scala.annotation.tailrec

@tailrec def printArray(i: Int, xs: Array[String]) {
  if (i < xs.length) {
    println("String #" + i + " is " + xs(i))
    printArray(i+1, xs)
  }
}
printArray(0, Array("first", "second", "third"))

Note that the optional @tailrec annotation is useful for ensuring that the method is actually tail recursive. The Scala compiler translates tail-recursive calls into the byte code equivalent of while loops.