input: Write a function to convert a list of string ratings to a list of float values. Return the list of float values. If the input list is empty, return an empty list.
  
  def str_to_float(ratings):
      return [float(x) for x in ratings]
  
  assert str_to_float(['4.5', '3.0', '2.5', '4.0'])==[4.5, 3.0, 2.5, 4.0]
  assert str_to_float([])==[]
output: def str_to_float(ratings):
      return [float(x) for x in ratings]