7: un poco mas tarde ...

[~/local/src/ruby/LPP/rspec_examples/rpcalculator(master)]$ cat lib/math/rpcalc.rb 
module Math
  class RPCalc
    attr_accessor :stack

    def initialize()
      @stack = []
    end

    def calc(expr)
      @stack = []
      expr.split(/\s+/).each do |x|
      case x
        when '+', '*', '-', '/' 
          op2 = @stack.pop or raise SyntaxError, "No first argument for '#{x}'"
          op1 = @stack.pop or raise SyntaxError, "No second argument for '#{x}'"
          @stack.push eval "(#{op1} #{x} #{op2})"
        when /^-?\s*\d+(\.\d+)?([eE][+-]?\d+)?\s*$/
          @stack.push x
        else
          raise SyntaxError, "Error. found '#{x}'. Expected number or operator"       
        end
      end
      @stack.pop
    end

  end  # class RPCalc
end    # Math

[~/local/src/ruby/LPP/rspec_examples/rpcalculator(master)]$ cat spec/math/rpcalc_spec.rb 
#require File.join(File.dirname(__FILE__), "/../spec_helper")
require "spec_helper"
module Math
  describe RPCalc do
    before :each do
      @c = Math::RPCalc.new
    end

    context "When an erroneous input is given" do
      context "not numeric input" do
        before :each do
          @e = proc { @c.calc('a') }
        end

        it "must raise an exception for not numeric input" do
          expect { @e[] }.to raise_error(SyntaxError)
          expect { @e[] }.to raise_error("Error. found 'a'. Expected number or operator")
        end
      end

      context "not enough operands" do
        it "must raise an exception when not enough operands" do
          expect { @c.calc('2 3 + *') }.to raise_error(SyntaxError)
          expect { @c.calc('2 3 + *') }.to raise_error("No second argument for '*'")
        end
      end
    end

    context "When a correct input is given" do
      it "must give the correct answer for integer expressions" do
        r = @c.calc('2 3 + 4 *')
        r.should eq 20
        @c.stack.should eq []
      end

      it "must give the correct answer for float expressions" do
        r = @c.calc('4 3 + 2.5 3.5 +')
        r.should be_within(0.01).of(6.0)
        @c.stack.should eq [ 7 ]
      end
    end

  end    # RPCalc
end      # Math

[~/local/src/ruby/LPP/rspec_examples/rpcalculator(master)]$ rake spec
rspec -Ilib -Ispec spec/math/rpcalc_spec.rb

Math::RPCalc
  When an erroneous input is given
    not numeric input
      must raise an exception for not numeric input
    not enough operands
      must raise an exception when not enough operands
  When a correct input is given
    must give the correct answer for integer expressions
    must give the correct answer for float expressions

Finished in 0.00526 seconds
4 examples, 0 failures