input: Write a function to convert ratings in a list of strings to their corresponding 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(['1.5', '2.0', '3.5'])==[1.5, 2.0, 3.5]
  assert str_to_float([])==[]
  assert str_to_float(['5.0'])==[5.0]
  assert str_to_float(['0.5', '4.75'])==[0.5, 4.75]
output: def str_to_float(ratings):
      return [float(x) for x in ratings]