input: Write a function to convert ratings in a list of strings to a float. The function should return a list of corresponding float values.
  
  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', '2.0', '3.0', '4.0'])==[1.0, 2.0, 3.0, 4.0]
  assert str_to_float(['0.5', '0.0', '5.0'])==[0.5, 0.0, 5.0]
  assert str_to_float([])==[]
output: def str_to_float(ratings):
      return [float(x) for x in ratings]