input: Write a function to convert ratings in a list of strings to a float. The function should take a list of strings as input and return a list of floats.
  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(['1.0', '2.1', '3.2', '4.3'])==[1.0, 2.1, 3.2, 4.3]
  assert str_to_float([])==[]
  assert str_to_float(['5.0'])==[5.0]
output: def str_to_float(ratings):
      return [float(x) for x in ratings]