rpncalculator.rb

class RPNCalculator
  attr_accessor :calculator

  def initialize
    @stack = []
    @value = 0
  end

  def push(number)
    @stack << number
  end

  def plus
    raise "calculator is empty" if @stack[-2..-1] == nil
    @value = @stack[-2..-1].inject(:+)
    @stack[-2..-1] = @value
  end

  def minus
    raise "calculator is empty" if @stack[-2..-1] == nil
    @value = @stack[-2..-1].inject(:-)
    @stack[-2..-1] = @value
  end

  def divide
    raise "calculator is empty" if @stack[-2..-1] == nil
    items = @stack[-2..-1]
    @value = items[0].to_f / items[1].to_f
    @stack[-2..-1] = @value
  end

  def times
    raise "calculator is empty" if @stack[-2..-1] == nil
    @value = @stack[-2..-1].inject(:*)
    @stack[-2..-1] = @value
  end

  def value
    @value
  end

  def tokens(string)
    string_stack = string.split
    string_stack.map{|x| x.match(/[*+-\/]/) ? x.to_sym : x.to_i}
  end

  def evaluate_stack(symbol)
    operations = {"+"=>"plus", "-"=>"minus", "*"=>"times", "/"=>"divide"}
    operations.each do |operand, method|
      if operand.to_sym == symbol.to_sym
        self.send(method)
      end
    end
  end

  def evaluate(string)
    string_stack = tokens(string)
    updating_stack = string_stack

    while string_stack.length != 1
      string_stack.each_index do |i|
      symbol = string_stack[i]
      first_num = string_stack[i - 2]
      second_num = string_stack[i - 1]
      next unless symbol.is_a?(Symbol)
      @stack << first_num.to_i
      @stack << second_num.to_i
      evaluate_stack(symbol)
      symbol_index = updating_stack.index(symbol)
      updating_stack[symbol_index - 2..symbol_index] = @value
      break
    end
      string_stack = updating_stack
    end
    string_stack[0]
  end

  def input
    puts "Please enter input one operand or operator at a time."
    puts "Press enter on an empty line and calculator will run."
    puts "Please begin to enter items now."
    string = ""
    while true
      item = gets.chomp
      break if item.empty?
      string << item + " "
    end
    puts "Result: #{evaluate(string)}"
  end
end

if __FILE__ == $PROGRAM_NAME
  calc = RPNCalculator.new
  calc.input
end