From: trans <transfire@...>
Date: 2009-06-12T19:30:23+09:00
Subject: Re: Idiomatic way to detect first/last iteration?



On Jun 11, 12:43 pm, Kendall Gifford <zettab...@gmail.com> 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?
>
> Specifically, I'm trying to avoid using each_with_index (as shown
> below):
>
> genres = %w{ classical punk rock jazz }
> genres.each_with_index do |genre, i|
>   desc = "one of my favorites"
>   desc = "my absolute favorite" if i.zero?
>   desc = "my least fav. of all my favorites" if i + 1 == genres.length
>   puts "#{genre} is #{desc}"
> end
>
> I just don't like having to compare an index value (especially as when
> detecting the final iteration) and hate even having it as a loop
> parameter unless absolutely necessary. I know, I'm picky. Just wanted
> to beat the bushes and see if any cool alternatives pop up.

I've experimented with something along these lines.

  # = Iteration
  #
  class It
    attr_reader :index, :value, :prior, :after
    def initialize(array)
      @array = array
      @index = 0
      @value = array[0]
      @prior = []
      @after = array[1..-1]
    end
    def first? ; @index == 0 ; end
    def last?
      if Enumerable === self
        nil
      else
        @index == @array.length
      end
    end
    private
    def next_iteration
      @index += 1
      @prior << @value
      @value = @after.shift
    end
  end

  class Array
    # Iterate over each element of array using an iteration object.
    #
    #   [1,2,3].each_iteration do |it|
    #     p it.index
    #     p it.value
    #     p it.first?
    #     p it.last?
    #     p it.prior
    #     p it.after
    #   end
    #
    # on each successive iteration produces:
    #
    #   0          1          2
    #   1          2          3
    #   true       false      false
    #   false      false      true
    #   []         [1]        [1,2]
    #   [2,3]      [3]        []
    #
    # CREDIT: Trans

    def each_iteration
      if block_given?
        it = It.new(self)
        each do |e|
          yield(it)
          it.send(:next_iteration)
        end
      else
        return Enumerable::Enumerator.new(self, :each_iteration)
      end
    end
  end

T.