input: 
  def str_to_float(ratings):
      """ Given a list of strings representing ratings, convert each string to a float.
      >>> str_to_float(['4.5', '3.0', '2.5', '4.0'])
      [4.5, 3.0, 2.5, 4.0]
      >>> str_to_float(['1.0', '2.0', '3.5'])
      [1.0, 2.0, 3.5]
      """
  
output:     return [float(rating) for rating in ratings]