Sorting in Reverse Order in Python
Sorting in reverse order means arranging elements from highest to lowest or in the opposite of the usual sorting order. Python provides several ways to achieve this, both with and without custom sorting criteria.
Basic Reverse Order Sorting with sort()
The sort() method allows you to sort a list in place and can also sort in reverse order using the reverse parameter.
Example of Basic Reverse Order Sorting
# List of numbers
numbers = [5, 2, 9, 1, 5, 6]
# Sort in reverse order
numbers.sort(reverse=True)
print("Reverse order sort:", numbers)
"""
Output:
Reverse order sort: [9, 6, 5, 5, 2, 1]
"""
In this example, numbers.sort(reverse=True) sorts the list from largest to smallest.
Reverse Order Sorting with sorted()
The sorted() function returns a new list that is sorted in reverse order, leaving the original list unchanged.
Example with sorted()
# List of numbers
numbers = [5, 2, 9, 1, 5, 6]
# Sort in reverse order with sorted()
sorted_numbers = sorted(numbers, reverse=True)
print("Reverse order sort with sorted():", sorted_numbers)
print("Original list:", numbers)
"""
Output:
Reverse order sort with sorted(): [9, 6, 5, 5, 2, 1]
Original list: [5, 2, 9, 1, 5, 6]
"""
Here, sorted(numbers, reverse=True) creates a new list sorted in reverse order without modifying the original list.
Reverse Order Sorting with Custom Key Function
You can combine reverse order sorting with a custom key function to control the sorting criteria while still sorting in reverse.
Example with Custom Key and reverse=True
# List of strings
words = ["apple", "Banana", "cherry", "kiwi"]
# Sort by length of strings in reverse order
words.sort(key=len, reverse=True)
print("Reverse order sort by length:", words)
"""
Output:
Reverse order sort by length: ['Banana', 'cherry', 'apple', 'kiwi']
"""
In this example, words.sort(key=len, reverse=True) sorts the list by string length in descending order.
Reverse Order Sorting with Case Insensitivity
Combine reverse order sorting with case-insensitive sorting for customized results.
Example with Case Insensitivity and Reverse Order
# List of strings with mixed case
strings = ["apple", "Banana", "cherry", "Kiwi"]
# Sort case-insensitively and in reverse order
strings.sort(key=str.lower, reverse=True)
print("Case-insensitive and reverse order sort:", strings)
"""
Output:
Case-insensitive and reverse order sort: ['Kiwi', 'cherry', 'Banana', 'apple']
"""
Here, strings.sort(key=str.lower, reverse=True) sorts the list in a case-insensitive manner and then reverses the order.
Reverse Order Sorting with Nested Data Structures
Apply reverse order sorting to more complex data structures, such as lists of tuples.
Example with Tuples
Suppose you have a list of tuples and want to sort by a specific element in reverse order:
# List of tuples (name, age)
people = [("Alice", 30), ("Bob", 25), ("Charlie", 35), ("Dave", 20)]
# Sort by age in reverse order
people.sort(key=lambda x: x[1], reverse=True)
print("Reverse order sort by age:", people)
"""
Output:
Reverse order sort by age: [('Charlie', 35), ('Alice', 30), ('Bob', 25), ('Dave', 20)]
"""
Here, people.sort(key=lambda x: x[1], reverse=True) sorts the tuples by age from highest to lowest.
Using Reverse Order in Conjunction with Other Sorting Techniques
You can use reverse order sorting alongside other sorting techniques, such as sorting by multiple criteria or custom functions.
Example with Multiple Criteria and Reverse Order
# List of tuples (name, score)
data = [("Alice", 85), ("Bob", 95), ("Charlie", 85), ("Dave", 90)]
# Sort by score in reverse order, then by name in reverse order
data.sort(key=lambda x: (-x[1], x[0]))
print("Sorted by score and name in reverse order:", data)
"""
Output:
Sorted by score and name in reverse order: [('Bob', 95), ('Dave', 90), ('Alice', 85), ('Charlie', 85)]
"""
In this example, data.sort(key=lambda x: (-x[1], x[0])) first sorts by score in descending order and then by name in ascending order when scores are the same.
Summary
- Basic Reverse Order Sorting with sort(): Sorts the list in place from largest to smallest using reverse=True.
- Reverse Order Sorting with sorted(): Creates a new list sorted in reverse order without modifying the original.
- Custom Key Function: Combine reverse order with a custom key function to sort by specific criteria.
- Case Insensitivity: Combine case-insensitive sorting with reverse order for customized results.
- Nested Data Structures: Apply reverse order sorting to complex data structures like lists of tuples.
- Combined Techniques: Use reverse order sorting alongside other sorting techniques for advanced use cases.