From: Brian Candler <B.Candler@...>
Date: 2005-07-12T18:04:03+09:00
Subject: Re: accessing index inside map

On Mon, Jul 11, 2005 at 08:07:37PM +0900, David A. Black wrote:
> I'm probably the person who has asked for map_with_index the most, and
> I promise not to ask for *_with_index :-)
> 
> I can understand that a general solution makes sense, though I'm still
> very unclear on the whole notion of "index" as it pertains to
> Enumerable.

I'm joining this converstation late, but ISTM that the fundamental issue
here is that one object may have multiple enumeration methods (e.g. #each
and #each_with_index), but the methods in Enumerable are only able to call
#each. So perhaps a solution is to be able to choose the name of the
enumerator method to call.

e.g.

  module Enumerable
    def altmap(meth=:each)
      res = []
      send(meth) { |*args| res << yield(*args) }
      res
    end
  end

  a = [10,20,30,40]
  p a.altmap { |x| x*2 }			#=> [20, 40, 60, 80]
  p a.altmap(:each_with_index) { |x,i| x+i } 	#=> [10, 21, 32, 43]

  h = {1=>"one", 2=>"two", 3=>"three"}
  p h.altmap(:each_key) { |x| x + x }		#=> [2, 4, 6]
  p h.altmap(:each_value) { |x| x + x }		#=> ["oneone", "twotwo", "threethree"]

Then, Enumerable doesn't have to care about the concept of what's an "index"
- conceptually we move #each_with_index into the object being enumerated,
and it can provide whatever index makes sense to it. (Enumerable could still
provide a default #each_with_index as it does now, though)

Aside: I always found each_with_index to be odd, because we have

   myhash.each { |key,value| ... }
   myarray.each_with_index { |value,key| ... }

I'd like to be able to think of an Array as a special case of Hash, where
the keys just happen to be Integers starting from 0.

This could be regularised if Hash#each_with_index did { |k,v| yield v,k }

Regards,

Brian.