Array#each_with_position (cached) Useful if you call the pos methods often.

array.rb

class Array
  class Position
    def initialize ary
      @pos = 1
      @max = ary.size
      @cache = Hash.new
    end

    def advance
      @pos += 1
      @cache.clear
    end

    def inside? range
      return @cache[range] unless @cache[range].nil?
      @cache[range] = range.include? @pos
    end

    def nth? i
      inside? i..i
    end

    def middle?
      inside? 2..@max-1
    end

    def first?
      nth? 1
    end

    def last?
      nth? @max
    end

    %w{inside nth first middle last}.each do |pos_name|
      define_method pos_name do |*args, &block|
        block.call if block and send :"#{pos_name}?", *args, &block
      end
    end
  end

  def each_with_position
    pos = Position.new self
    each do |x|
      yield x, pos
      pos.advance
    end
  end
end