From: Paul Brannan <pbrannan@...>
Date: 2003-05-30T22:55:00+09:00
Subject: Re: Iteration - last detection

On Fri, May 30, 2003 at 06:12:38AM +0900, Dave wrote:
> this might work for you, it has simplicity going for it.
> 
> module Enumerable
>   def each_but_last(&block)
>     elements = entries
>     last = elements.pop
>     elements.each &block
>     return last
>   end
> end
> 
> last = %w{ one two three }.each_but_last { |i| puts "each_but_last #{i}" }
> puts "last #{last}"

I like that this solution lacks a flag to indicate whether I'm on the
last element; this makes it easy to use.

I don't like that all the elements are stored in a temporary array.

Combining this with Martin's idea, I get:

  module Enumerable
    def each_but_last
      prev = nil
      curr = nil
      each do |curr|
        yield prev if prev
        prev = curr
      end
      return prev
    end
  end

It uses less memory, but:

  1) It's surprisingly slower
  2) Both solutions have the problem of not being able to distinguish
     between [] and [nil].

Paul