Recursive RPN Calculator without iteration (inject, each, map, etc. not allowed)

gistfile1.rb

def evaluate(string, answer=[])
  return answer[0] if string.empty?

  expressions = string.split(' ')
  obj = expressions.shift 
    
  if obj.to_s[/\d+/]
    answer << obj.to_f
  else ["*","+","-","/"].include?(obj) 
    operands = answer.pop(2)
    answer << (operands[0].send(obj,operands[1]))
  end
  
  evaluate(expressions.join(' '), answer)
end

puts evaluate('1 2 +') == 3   # => 3
puts evaluate('2 5 *') == 10  # => 10
puts evaluate('3 50 20 - +') == 33 # => 30
puts evaluate('70 10 4 + 5 * -') == 0 # => 0

Nice! I re-did the RPN calculator a few days ago and my code is very similar. I didn't know you could use
operands = answer.pop(2)

I did
operand_one, operand_two = answer.pop, answer.pop