Refactoring feedback for Reverse Polish Notation (RPN) or Postfix Notation

Question

One of the pre-work exercises for Dev Bootcamp is an RPN calculator. I made it work but would like refactoring feedback. Any and all help to make this code cleaner is greatly appreciated.

class RPNCalculator
  def evaluate(rpn)
    a = rpn.split(' ')
    array = a.inject([]) do |array, i|    
      if i =~ /\d+/ 
        array << i.to_i
      else
        b = array.pop(2)
        case 
          when i == "+" then array << b[0] + b[1]  
          when i == '-' then array << b[0] - b[1]  
          when i == '*' then array << b[0] * b[1]  
          when i == '/' then array << b[0] / b[1]  
        end
      end
    end
    p array.pop
  end
end

calc = RPNCalculator.new
calc.evaluate('1 2 +')   # => 3
calc.evaluate('2 5 *')   # => 10
calc.evaluate('50 20 -') # => 30
calc.evaluate('70 10 4 + 5 * -') # => 0  

Answer 1

class RPNCalculator
  def evaluate rpn
    array = rpn.split(" ").inject([]) do |array, i|    
      if i =~ /\d+/ 
        array << i.to_i
      else
        b = array.pop(2)
        array << b[0].send(i, b[1])
      end
    end
    p array.pop
  end
end