How to get the element index when mapping an array in Scala?

Question

Let\'s consider a simple mapping example:

  val a = Array(\"One\", \"Two\", \"Three\")
  val b = a.map(s => myFn(s))

What I need is to use

Answer 1

A general tip: Use .iterator method liberally, to avoid creation of intermediate collections, and thus speed up your computation. (Only when performance requirements demand it. Or else don't.)

scala> def myFun(s: String, i: Int) = s + i
myFun: (s: String, i: Int)java.lang.String

scala> Array("nami", "zoro", "usopp")
res17: Array[java.lang.String] = Array(nami, zoro, usopp)

scala> res17.iterator.zipWithIndex
res19: java.lang.Object with Iterator[(java.lang.String, Int)]{def idx: Int; def idx_=(x$1: Int): Unit} = non-empty iterator

scala> res19 map { case (k, v) => myFun(k, v) }
res22: Iterator[java.lang.String] = non-empty iterator

scala> res22.toArray
res23: Array[java.lang.String] = Array(nami0, zoro1, usopp2)

Keep in mind that iterators are mutable, and hence once consumed cannot be used again.

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An aside: The map call above involves de-tupling and then function application. This forces use of some local variables. You can avoid that using some higher order sorcery - convert a regular function to the one accepting tuple, and then pass it to map.

scala> Array("nami", "zoro", "usopp").zipWithIndex.map(Function.tupled(myFun))
res24: Array[java.lang.String] = Array(nami0, zoro1, usopp2)