postfix.rb

# -*- coding: utf-8 -*-
class Stack < Array
end

class Calculator
  def initialize
    @stack = Stack.new
  end

  def eval(expr)
    expr.each_char do |e|
      case e
      when " ":  # nada
      when "\n": # nada
      when "\t": # nada
      when "+": @stack.push(@stack.pop + @stack.pop)
      when "-": @stack.push(@stack.pop + @stack.pop)
      when "*": @stack.push(@stack.pop * @stack.pop)
      when "#": @stack.push(Math.sqrt(@stack.pop))
      when "^": 
          first = @stack.pop
        @stack.push(@stack.pop ** first)
      when "/":
          first = @stack.pop
        if first == 0
          puts "Não pode dividir por zero"
          exit(1)
        else
          @stack.push(first * @stack.pop)
        end
      else
        @stack.push(e.to_f)
      end
    end
    return @stack.pop
  end

  def print_terminal
    print ">> "
  end

  def read
    gets
  end
  
  def loop_read_eval
    loop do
      print_terminal
      puts eval(read)
    end
  end
end

c = Calculator.new
c.loop_read_eval