From: "Ara.T.Howard" <ahoward@...>
Date: 2003-10-09T02:49:42+09:00
Subject: Re: map/collect iterating over multiple arrays/arguments

On Wed, 8 Oct 2003, quent wrote:

> zoranlazarevic@yahoo.com (Zoran Lazarevic) wrote in message news:<32c0bb6a.0310071950.55c75dd0@posting.google.com>...
>
> > Apparently it is not possible to iterate using Collection.each over
> > multiple collections. It is possible by using one thread per
> > collection, but I do not want to go there.
>
> a continuations multi_each has been placed on the wiki at rubygarden.org
> if you are interested

heres another one. it also defines an iterator class:


----CUT----
module Enumerable
  class Iterator
  public

    attr_reader :enumerable, :has_next

    def initialize enumerable, end_value = nil, &end_block
      @enumerable = enumerable
      @end_value = end_value
      @end_block = end_block
      initialize_fetch_block
    end

    def next
      @has_next ? fetch_next_element : fetch_end_value
    end

    def rewind
      initialize_fetch_block
      self
    end

  protected

    def initialize_fetch_block
      callcc do |@after_fetch|
	@has_next = true
	@enumerable.each do |@next_element|
	  callcc do |@next_fetch| @after_fetch.call end
	end
	@has_next = false
	@next_fetch = nil
	@after_fetch.call
      end
      @after_fetch = nil
    end

    def fetch_next_element
      result = @next_element
      callcc do |@after_fetch| @next_fetch.call end
      @after_fetch = nil
      result
    end

    def fetch_end_value
      @end_block ? @end_block.call : @end_value
    end
  end

  # enumerable instance methods
  def iterator; @iterator = Iterator.new self; end

  # enumerables class methods
  class << self
    def each(*enumerables, &block)
      iterators = enumerables.collect{|e| e.iterator}
      while true
        args = iterators.collect{|i| i.next}
        if args.detect{|arg| arg}
          block.call *args
        else
          return enumerables
        end
      end
    end
    def collect(*enumerables, &block)
      ret = []
      each(*enumerables){|*args| ret << (block.call *args)}
      ret
    end
    alias map collect
  end
end

if $0 == __FILE__
  a = ['fee','fie','foe','fum']
  h = {'k' => 'v', 'K' => 'V'}

  ai = a.iterator
  hi = h.iterator

  while ((n = ai.next)) do
    puts n.inspect
  end

  while ((n = hi.next)) do
    puts n.inspect
  end

  Enumerable::each a, h do |elem, kv|
    puts elem.inspect
    puts kv.inspect
  end

  p Enumerable::collect(a,h){|e,kv| [e,kv]}
end


__END__

OUTPUT:

"fee"
"fie"
"foe"
"fum"
["K", "V"]
["k", "v"]
"fee"
["K", "V"]
"fie"
["k", "v"]
"foe"
nil
"fum"
nil
[["fee", ["K", "V"]], ["fie", ["k", "v"]], ["foe", nil], ["fum", nil]]
----CUT----


-a
  ====================================
  | Ara Howard
  | NOAA Forecast Systems Laboratory
  | Information and Technology Services
  | Data Systems Group
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  | Email: ara.t.howard@noaa.gov
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  | The difference between art and science is that science is what we understand
  | well enough to explain to a computer.  Art is everything else.
  |   -- Donald Knuth, "Discover"
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  ====================================