post_fix_calculator.rb

class PostFixCalculator

  attr_accessor :list, :original_list

  def initialize(list)
    @original_list = list
    @list = list.split(' ')
  end

  def evaluation
    list.each_with_object(stack=[]) do |x,o|
      case x
      when /\d/
        o.push(x)
      when "-", "/", "*", "+", "**", '%'
        result = process_operands(o.pop(2), x)
        o.push(result)
      end
    end

    raise 'ArgumentError' if stack.count > 1
    stack.first
  end

  def print_result
    puts "#{original_list} = #{evaluation}"
  end

  def self.tests_passing?
    [
      self.new('1 2 +').evaluation == 3,
      self.new('10 5 -').evaluation == 5,
      self.new('3 2 *').evaluation == 6,
      self.new('5 2 **').evaluation == 25,
      self.new('1 2.0 %').evaluation == 1,
      self.new('1 2 /').evaluation == 0,
      self.new('1 2.0 /').evaluation == 0.5,

      self.new('1 2 3 + -').evaluation == -4,
    ].all?
  end

  private

  def process_operands(operands=[], operator)
    operands.map{ |i| Integer(i) rescue Float(i) }.inject(operator)
  end

end

raise 'FaultyCalculator' unless PostFixCalculator.tests_passing?

puts PostFixCalculator.new(ARGV[0]).print_result