Check if a List is Sorted in Descending Order

Python offers several elegant ways to determine if a list is sorted in descending order. This is a common task in data processing and algorithm design. Let’s explore efficient methods to achieve this, along with clear code examples.

Method 1: Using a Loop

The most straightforward approach involves iterating through the list and comparing adjacent elements. If at any point, a preceding element is smaller than its successor, the list is not sorted in descending order.

def is_sorted_descending_loop(data):
  """Checks if a list is sorted in descending order using a loop.

  Args:
    data: The input list.

  Returns:
    True if the list is sorted in descending order, False otherwise.
  """
  for i in range(len(data) - 1):
    if data[i] < data[i+1]:
      return False
  return True

my_list = [9, 7, 5, 3, 1]
print(f"Is {my_list} sorted descending (loop)? {is_sorted_descending_loop(my_list)}")  # Output: True

my_list = [9, 7, 10, 3, 1]
print(f"Is {my_list} sorted descending (loop)? {is_sorted_descending_loop(my_list)}")  # Output: False

my_list = []
print(f"Is {my_list} sorted descending (loop)? {is_sorted_descending_loop(my_list)}")  # Output: True (empty list considered sorted)

my_list = [5]
print(f"Is {my_list} sorted descending (loop)? {is_sorted_descending_loop(my_list)}")  # Output: True (single-element list considered sorted)

Method 2: Using all() and zip()

This method leverages Python’s built-in functions for a more concise solution. zip() pairs consecutive elements, and all() checks if the condition (preceding element >= succeeding element) holds true for all pairs.

def is_sorted_descending_all(data):
  """Checks if a list is sorted in descending order using all() and zip().

  Args:
    data: The input list.

  Returns:
    True if the list is sorted in descending order, False otherwise.
  """
  return all(data[i] >= data[i+1] for i in range(len(data)-1))


my_list = [9, 7, 5, 3, 1]
print(f"Is {my_list} sorted descending (all/zip)? {is_sorted_descending_all(my_list)}")  # Output: True

my_list = [9, 7, 10, 3, 1]
print(f"Is {my_list} sorted descending (all/zip)? {is_sorted_descending_all(my_list)}")  # Output: False

my_list = []
print(f"Is {my_list} sorted descending (all/zip)? {is_sorted_descending_all(my_list)}")  # Output: True

my_list = [5]
print(f"Is {my_list} sorted descending (all/zip)? {is_sorted_descending_all(my_list)}")  # Output: True

Method 3: Using sorted() (for comparison)

While not the most efficient for just checking if a list is sorted, using sorted() provides a simple way to compare the original list with its descending sorted version. Note that this method is less efficient than the previous methods for large lists.

def is_sorted_descending_sorted(data):
    """Checks if a list is sorted in descending order by comparing to a sorted version.

    Args:
      data: The input list.

    Returns:
      True if the list is sorted in descending order, False otherwise.
    """
    return data == sorted(data, reverse=True)

my_list = [9, 7, 5, 3, 1]
print(f"Is {my_list} sorted descending (sorted)? {is_sorted_descending_sorted(my_list)}")  # Output: True

my_list = [9, 7, 10, 3, 1]
print(f"Is {my_list} sorted descending (sorted)? {is_sorted_descending_sorted(my_list)}")  # Output: False

my_list = []
print(f"Is {my_list} sorted descending (sorted)? {is_sorted_descending_sorted(my_list)}")  # Output: True

my_list = [5]
print(f"Is {my_list} sorted descending (sorted)? {is_sorted_descending_sorted(my_list)}")  # Output: True

Each method offers a different approach to solving the problem. The loop method is explicit, while the all() and zip() method is more concise. The sorted() method offers a simpler, albeit less efficient, alternative. Choose the method that best suits your coding style and performance requirements.