Array#each_with_position

array.rb

class Array
  class Position
    attr_accessor :idx, :max

    def initialize arr
      self.max = arr.size
    end

    def pos
      idx + 1
    end

    def inside? range
      range.include? pos
    end

    def nth? i
      pos == i
    end

    def first?
      nth? 1
    end

    def middle?
      not first? and not last?
    end

    def last?
      nth? max
    end

    def inside range
      yield if inside? range
    end

    def nth i
      yield if nth? i
    end

    def first
      yield if first?
    end

    def middle
      yield if middle?
    end

    def last
      yield if last?
    end
  end

  def each_with_position
    pos = Array::Position.new self
    each_with_index do |x,i|
      pos.idx = i
      yield x, pos
    end
  end
end

[1,2,3,4,5].each_with_position do |x,pos|
  puts "#{x} is first"  if pos.first?
  puts "#{x} is third"  if pos.nth? 3
  puts "#{x} is middle" if pos.middle?
  puts "#{x} is last"   if pos.last?
  puts
end

#1 is first
# 
#2 is middle
# 
#3 is third
#3 is middle
# 
#4 is middle
# 
#5 is last

%w{some short simple words}.each_with_position do |x, pos|
  pos.first do
    puts "#{x} is first"
  end

  pos.inside 2..3 do
    puts "#{x} is second or third"
  end

  pos.middle do
    puts "#{x} is middle"
  end

  pos.last do
    puts "#{x} is last"
  end
end


# some is first
# short is second or third
# short is middle
# simple is second or third
# simple is middle
# words is last