From: Gary Wright <gwtmp01@...>
Date: 2009-06-12T03:32:53+09:00
Subject: Re: Idiomatic way to detect first/last iteration?


On Jun 11, 2009, at 12:43 PM, Kendall Gifford wrote:

> Hi there, is there any recognized ruby idiomatic way for detecting
> when you're on the first and/or last iteration over a collection? Or,
> if not, anyone know of any tricky ways?


The following solution uses each_with_index internally but doesn't
depend on #first, #last, or #length. On a single item collection
it will report 'last' but not 'first'.  You'll have to decide if
that works in your case. Obviously you could change it to report
'both' for a single item.

Gary Wright


module Enumerable
   def each_with_position
     previous = sigil = Object.new
     each_with_index { |next_item, index|
       case index
       when 0
         # do nothing
       when 1
         yield(previous, 'first')
       else
         yield(previous, 'middle')
       end
       previous = next_item
     }
     yield(previous, 'last') unless previous == sigil
   end
end

 >> [1,2,3,4].each_with_position { |*x| p x }
[1, "first"]
[2, "middle"]
[3, "middle"]
[4, "last"]
=> nil
 >> [].each_with_position { |*x| p x }
=> nil
 >> [1].each_with_position { |*x| p x }
[1, "last"]
=> nil