RPN Calculator - Steven Vo

rpn_calculator.rb

# RPN Calculator
#
# SPECIFICATIONS:
# REQ 1: The calculator should use standard input and standard output, unless the language makes that impossible.
# REQ 2: It should implement the four standard arithmetic operators
# REQ 3: It should support negative and decimal numbers, and should not have arbitrary limits on the number of operations.
# REQ 4: The calculator should not allow invalid or undefined behavior.
# REQ 5: The calculator should exit when it receives a q command or an end of input indicator (EOF).

class RpnStack

  def initialize
    @stack = Array.new
  end

  def pop
    @stack.pop
  end

  def push(element)
    is_number = true if Float(element) rescue false
    if is_number
      @stack.push(element)
      return element
    else
      return self.calculate(element)
    end
  end

  def calculate(operator)
    is_operator = true if ['+', '-', '*', '/'].include?(operator) else false
    # REQ #2: supports 4 standard arithmetic operators

    if !is_operator
      return 'Invalid Input'
    end

    rescue_operands = Array.new # to save popped item for future disaster recovery
    begin

      # Retrieve and convert operand 1
      val = @stack.pop
      rescue_operands.push(val) if !val.nil? # save popped item for future disaster recovery
      operand_1 = Float(val)

      # Retrieve and convert operand 2
      val = @stack.pop
      rescue_operands.push(val) if !val.nil? # save popped item for future disaster recovery
      operand_2 = Float(val)

      result = 0
      case operator
        when '+'
          result = operand_2 + operand_1
        when '-'
          result = operand_2 - operand_1
        when '*'
          result = operand_2 * operand_1
        when '/'
          result = operand_2 / operand_1
          if result == Float::INFINITY || result.nan? then raise ZeroDivisionError end
          # Use ZeroDivisionError to handle both NaN, Infinity, or ZeroDivisionError as below
          # ruby-1.9.2-p180 :018 > 0.0 / 0
          # => NaN
          #
          # ruby-1.9.2-p180 :020 > 3.0 / 0
          # => Infinity
          #
          # ruby-1.9.2-p180 :021 > 3 / 0
          # ZeroDivisionError: divided by 0

        else
          puts 'Not a valid operator!' # REQ #4: disallow invalid or undefined behavior
      end
      @stack.push(result)
      return result
    rescue TypeError => e
      # Put the "already popped" items back to the stack
      while !(item = rescue_operands.pop).nil?
        @stack.push(item)
      end
      return 'Invalid Input' # REQ #4: disallow invalid or undefined behavior
    rescue ZeroDivisionError
      # Put the "already popped" items back to the stack
      while !(item = rescue_operands.pop).nil?
        @stack.push(item)
      end
      return 'Division by ZERO is detected, please try with different operator!' # REQ #4: disallow invalid or undefined behavior
    end

    return 'Invalid Input!' # REQ #4: disallow invalid or undefined behavior
  end

  def size
    @stack.size
  end

  def to_s
    @stack.to_s
  end
end


rpn_stack = RpnStack.new

while true
  print "#{rpn_stack.to_s} > " # print the stack content before the prompt for better visibility
  input = gets.chomp.to_s.strip()
  break if 'q'.eql?(input) or input.empty? # REQ #4: exit when enter 'q' or EOF
  puts rpn_stack.push(input)
end