RPN

rpn.rb

# Create an RPNCalculator class which can evaluate expressions written in Reverse Polish notation.

# It should have an evaluate instance method which takes as its input a valid RPN expression and returns its evaluation. Your calculator only needs to handle addition, multiplication, and subtraction (not division).

# Operators and numbers should be separated by a single space.

# For example,

# calc = RPNCalculator.new

# calc.evaluate('1 2 +')   # => 3
# calc.evaluate('2 5 *')   # => 10
# calc.evaluate('50 20 -') # => 30

# # The general rule is that 'A B op' is the same as 'A op B'
# # i.e., 5 4 - is 5 - 4.
# calc.evaluate('70 10 4 + 5 * -') # => 0
# 70 - ((10 + 4) * 5 )
# 
# 
# Implement an RPN calculator that takes an expression like 
# 19 2.14 + 4.5 2 4.3 / - * 
# which is usually expressed as (19 + 2.14) * (4.5 - 2 / 4.3)

class RPNCalculator

  def initialize
    @array = Array.new
    @stack = Array.new

  end

  def evaluate(string)

    @array = string.split
    
    @array.each do |x|   

      if x.match(/(\d+)/)

        @stack << x.to_i

      else
        case x
          when '+'
            @stack << @stack.pop + @stack.pop
           
          when '-'
            @stack << - @stack.pop + @stack.pop
          when '*' 
            @stack << @stack.pop * @stack.pop
        end
      end

    end

    return @stack.join.to_i
      
  end

end



calculator = RPNCalculator.new 

#Test operation :Should print : 0
p calculator.evaluate("70 10 4 + 5 * -") 

p calculator.evaluate("12 2 3 + *")