CODEWITHSUNDEEP
scala
Paul Nikonowicz
Paul Nikonowicz

Reputation: 3903

Composing Iterators

Say I have the following function definition:

def print(input:Iterator[String]) : Iterator[String] = {
  Iterator.continually {
     val item = input.next
     item + item
  }
}

given the input

List("1", "2", "3").iterator

I want the output to be equal to 

List("11", "22", "33").iterator

The problem is that when I use Iterator.continually, I am forced to iterate through the list using a takeWhile. I would like to be able to use any Iterator function that exists (fold, filter, etc...) without getting a "next on empty iterator" exception.

You can do this in F# using seq and yield. I was hoping that Iterator.continually would be just as robust.

F# example:

seq {
   for i in input |> yield i + i
}

P.S. I realize I could just compose a bunch of functions and then pass them to an Iterator function but I'm trying to compose Iterators in a way that I could in F#.

Upvotes: 1

Views: 537

Answers (1)

DaoWen
DaoWen

Reputation: 33029

I think List.map will do what you want. However, you might want to use Scala's for comprehension syntax, which the compiler just translates into a map call. Here's a snippet from a test in my REPL. Seems to work.

scala> def print(input:Iterator[String]) : Iterator[String] =
     |   for (i <- input) yield i + i
print: (input: Iterator[String])Iterator[String]

scala> print( List("a", "b", "c").iterator )
res0: Iterator[String] = non-empty iterator

scala> res0.toList
res1: List[String] = List(aa, bb, cc)

Upvotes: 3

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