From: Paul Brannan <pbrannan@...>
Date: 2002-11-15T08:01:58+09:00
Subject: Re: Array#next - Enumerable#next ?

On Fri, Nov 15, 2002 at 05:40:58AM +0900, ahoward wrote:
>   class Array
>     def next
>       # start iterating if we never have before
>       @itr = 0 unless @itr
>       @itr = @itr.succ
> 
>       # if there are no more elements return nil
>       # but also set itr back to zero so subsequent
>       # calls to next will succeed
>       if @itr > size
> 	@itr = 0
> 	return nil
>       end
> 
>       return self[@itr - 1]
>     end
>   end

One problem here is that I can't tell the difference between iterating
over a nil object in an array (e.g. if I have an array [1, 2, 3, nil,
4]) and hitting the end of the array.  This is pretty easy to fix;
change:
  return nil

to:
  return DoneIterating

A more serious problem here is that code that uses next() is not
reentrant; if you have a method that is iterating over an array, you'd
better not call another method that also iterates over that array.
E.g.:

  def print_array(arr)
    while (obj = arr.next) != DoneIterating do
      puts obj
    end
  end

  def print_all_ocurrances(arr, x)
    while (obj = arr.next) != DoneIterating do
      if x == obj then
        print "Found object x in array "
        print_array arr
        puts
      end
    end
  end

does not work as expected.  To work around this, you should instantiate
an iterator object whenever you want to iterate over your array.

rubycollections has an iterator library for doing this generically:

  http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/rubycollections/rubycollections/rbc/iterator/

You can use it like this:
  arr = [1, 2, 3, 4, 5]
  it = GenericIterator.new(arr)
  while not it.finished? do
    p it.elem
    it.advance
  end

though I regret that I haven't had time to work on this project as much
as I would like (one idea that was proposed was to merge my iterators
with Horste Duchene's streams, but that's a big project).  You might
also want to look at:

  http://www.rubygarden.org/article.php?sid=107
  http://www.ruby-lang.org/cgi-bin/cvsweb.cgi/rough/lib/generator.rb?rev=1.6&content-type=text/x-cvsweb-markup
  http://www.ruby-lang.org/en/raa-list.rhtml?name=stream.rb

for some alternate implementations.

Paul