Efficient iteration with index in Scala

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<         


        

Answer 1

The proposed solutions suffer from the fact that they either explicitly iterate over a collection or stuff the collection into a function. It is more natural to stick with the usual idioms of Scala and put the index inside the usual map- or foreach-methods. This can be done using memoizing. The resulting code might look like

myIterable map (doIndexed(someFunction))

Here is a way to achieve this purpose. Consider the following utility:

object TraversableUtil {
    class IndexMemoizingFunction[A, B](f: (Int, A) => B) extends Function1[A, B] {
        private var index = 0
        override def apply(a: A): B = {
            val ret = f(index, a)
            index += 1
            ret
        }
    }

    def doIndexed[A, B](f: (Int, A) => B): A => B = {
        new IndexMemoizingFunction(f)
    }
}

This is already all you need. You can apply this for instance as follows:

import TraversableUtil._
List('a','b','c').map(doIndexed((i, char) => char + i))

which results in the list

List(97, 99, 101)

This way, you can use the usual Traversable-functions at the expense of wrapping your effective function. Enjoy!