input: Write a function to count down from a given positive integer to 1 using a while loop and print each number. If the input is not a positive integer, return an appropriate message.
  def count_down(num):
      if num <= 0:
          return "Input must be a positive integer."
      while num > 0:
          print(num)
          num -= 1
      print("Done!")
  
  
  assert count_down(6)
  # Output:
  # 6
  # 5
  # 4
  # 3
  # 2
  # 1
  # Done!
  assert count_down(-3) == "Input must be a positive integer."
  assert count_down(0) == "Input must be a positive integer."
  assert count_down(3)
  # Output:
  # 3
  # 2
  # 1
  # Done!
output: def count_down(num):
      if num <= 0:
          return "Input must be a positive integer."
      while num > 0:
          print(num)
          num -= 1
      print("Done!")