input: Write a function to convert ratings in a list of strings to a floating-point number 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(['5.0', '4.8', '3.2'])==[5.0, 4.8, 3.2]
  assert str_to_float(['2.1', '1.9', '4.7'])==[2.1, 1.9, 4.7]
  assert str_to_float([])==[]
output: def str_to_float(ratings):
      return [float(x) for x in ratings]