input: def convert_to_binary(num):
      """
      Convert a given number into binary format.
      >>> convert_to_binary(10)
      '1010'
      >>> convert_to_binary(0)
      0
      >>> convert_to_binary(7)
      '111'
      """
output:     if num == 0:
          return '0'
      binary_str = ''
      while num > 0:
          binary_str = str(num % 2) + binary_str
          num = num // 2
      return binary_str