RPN

rpn.rb

class RPN
  def self.run(exp)
    res = tokenize(exp).inject([]) { |m, token| m << evaluate(token, m) }

    raise(ArgumentError, "expression is not completed") unless res.size==1
    res.first
  end

  def self.tokenize(s)
    s.split
  end

  def self.evaluate(token, m)
    basic_operators = /^[+\/-]|\*{1,2}$/
    number = /^[\d.]+$/
    case token
    when basic_operators
      m.pop(2).inject($&)
    when 'ln'
      Math.log(m.pop)
    when 'sum'
      m.inject(:+).tap{ m.clear }
    when number
      $&.to_f
    else
      raise ArgumentError, "operator `#{token}` not defined"
    end
  end
  private_class_method :tokenize, :evaluate
end

if __FILE__ == $0
  RPN.run("3 4 +") # => 7.0
  RPN.run("3 4 + 5.5 *") # => 38.5
  RPN.run("3 4 + 5 * 6 7 / -") # => 34.142857142857146
  RPN.run("3 4 **") # => 81.0
  RPN.run("2.7 ln") # => 0.9932517730102834
  RPN.run("1 2 3 4 5 sum 5 /") # => 3.0
end

rpn_spec.rb

require_relative "rpn"

describe RPN do
  describe "run" do
    it "raise operation error" do
      ->{ RPN.run("2 3 %") }.should raise_error(ArgumentError, 'operator `%` not defined')
    end

    it "raise expression error" do
      ->{ RPN.run("2 3 4 *") }.should raise_error(ArgumentError, 'expression is not completed')
    end

    it "calc plus" do
      RPN.run("2 3 +").should eql 5.0
    end

    it "calc plus-multi" do
      RPN.run("4 2 3 + *").should eql 20.0
    end

    it "calc plus-multi-minus" do
      RPN.run("10 4 3 + 2 * -").should eql -4.0
    end

    it "calc plus-multi-minus-plus" do
      RPN.run("34 12 33 55 66 + * - +").should eql -3947.0
    end

    it "calc minus" do
      RPN.run("90 3.8 -").should eql 86.2
    end
    
    it "calc divide" do
      RPN.run("90 3.8 /").should eql 90/3.8
    end

    it "calc exponential" do
      RPN.run("3 4 **").should eql 3**4.0
    end

    it "calc log" do
      RPN.run("2.7 ln").should eql Math.log(2.7)
    end

    it "calc sum" do
      RPN.run("10 10 10 10 sum 4 /").should eql 10.0
    end
  end
end