From: Robert Klemme <shortcutter@...>
Date: 2009-06-12T20:18:29+09:00
Subject: Re: Idiomatic way to detect first/last iteration?

2009/6/11 Gary Wright <gwtmp01@mac.com>:
>
> 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

Turns out we think along similar lines, although my solution is a bit
more wasteful:

module Enumerable
  # will yield elements along with
  # a symbol :first, :last, :middle or :only
  def each_special
    last = nil

    each do |x|
      if last
        yield *last
        last = [x, :middle]
      else
        # first iter
        last = [x, :first]
      end
    end

    yield last.first, last.last == :middle ? :last : :only if last
    self
  end
end

[
  [],
  [1],
  [1,2],
  [1,2,3],
  [1,2,3,4,5],
].each do |en|
  p en

  en.each_special do |x,t|
    printf "%-10p %p\n", t, x
  end

  puts "end"
end

Kind regards

robert
-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/