two different versions of an RPN calculator

rpn.rb

OPERATORS = %w{ + - * / }

def tokenize(s)
  s.split.map do |x|
    if OPERATORS.member? x
      x.to_sym
    else
      x.to_i
    end
  end
end

def evaluate(s)
  stack = []
  tokenize(s).each do |x|
    y = x
    if x.is_a? Symbol
      y = stack[1].send x, stack[0]
      stack.shift 2
    end
    stack.unshift y
  end
  stack
end

puts "evaluate: #{evaluate('1 2 +')}"

def evaluate2(s)
  tokenize(s).inject([]) do |stack, x|
    if x.is_a? Symbol
      x = stack[1].send x, stack[0]
      stack.shift 2
    end
    stack.unshift x
  end
end