Python: Sorting Techniques: Sorting How To

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Python lists have a built-in list.sort() method that modifies the list in-place. There is also a sorted() built-in function that builds a new sorted list from an iterable.

We explore the various techniques for sorting data using Python.

Sorting Basics

list.sort()

Sorts the elements of a list in place, meaning it modifies the original list directly.

Syntax:

list.sort(key=None, reverse=False)

Parameters:

key (optional): A function that takes a single element from the list and returns a key to be used for sorting. This allows for custom sorting criteria. reverse (optional): A boolean value. If True, sorts the list in descending order. Defaults to False (ascending order).

Python Code Example: Sorting a List in Ascending and Descending Order

# Creating a list of numbers
numbers = [5, 2, 9, 1, 5, 6]

# Sorting the list in ascending order
numbers.sort()
print("Sorted in ascending order:", numbers)

# Sorting the list in descending order
numbers.sort(reverse=True)
print("Sorted in descending order:", numbers)

Output:

Sorted in ascending order: [1, 2, 5, 5, 6, 9]
Sorted in descending order: [9, 6, 5, 5, 2, 1]

Sorting a List of Strings:

# List of names
names = ["John", "Alice", "Bob", "Charlie"]

# Sorting alphabetically
names.sort()
print("Alphabetically sorted:", names)

# Sorting in reverse alphabetical order
names.sort(reverse=True)
print("Reverse alphabetical order:", names)

Example:

• Video: List sort() Function

sorted Function

The sorted() function in Python is used to return a new sorted list from an iterable (like a list, tuple, or string) without modifying the original list.

Syntax:

sorted(iterable, key=None, reverse=False)

Parameters:

iterable: The iterable object to be sorted (e.g., a list, tuple, string). key (optional): A function that takes a single element from the iterable and returns a key to be used for sorting. This allows for custom sorting criteria. reverse (optional): A boolean value. If True, sorts the iterable in descending order. Defaults to False (ascending order).

Python Code Example 1: Sorting a List of Numbers

# Original list
numbers = [5, 2, 9, 1, 5, 6]

# Sorting in ascending order
sorted_numbers = sorted(numbers)
print("Sorted in ascending order:", sorted_numbers)

# Sorting in descending order
sorted_numbers_desc = sorted(numbers, reverse=True)
print("Sorted in descending order:", sorted_numbers_desc)

# Original list remains unchanged
print("Original list:", numbers)

Output:

Sorted in ascending order: [1, 2, 5, 5, 6, 9]
Sorted in descending order: [9, 6, 5, 5, 2, 1]
Original list: [5, 2, 9, 1, 5, 6]

Python Code Example 2: Sorting a List of Strings

# List of names
names = ["John", "Alice", "Bob", "Charlie"]

# Sorting alphabetically
sorted_names = sorted(names)
print("Alphabetically sorted:", sorted_names)

# Sorting in reverse alphabetical order
sorted_names_desc = sorted(names, reverse=True)
print("Reverse alphabetical order:", sorted_names_desc)

Python Code Example 3: Sorting with a Custom Key

You can use the key parameter to sort based on custom criteria.

# Sorting words by length
words = ["apple", "banana", "kiwi", "grape"]

# Sorting by word length (shortest to longest)
sorted_words = sorted(words, key=len)
print("Sorted by word length:", sorted_words)

Output:

Sorted by word length: ['kiwi', 'grape', 'apple', 'banana']

Examples:

• Video: sorted() Function

Key Functions

Both list.sort() and sorted() have a key parameter to specify a function (or other callable) to be called on each list element prior to making comparisons.

Examples:

• Video: Sorting a List of Colors by Length using Python’s sorted() Function
• Video: How to Sort a List of Fruit Tuples
• Video: Sorting a List of Dictionaries by Age using Lambda Function

Operator Module Functions

Ascending and Descending

Both list.sort() and sorted() accept a reverse parameter with a boolean value. This is used to flag descending sorts.

Sort Stability and Complex Sorts

Decorate-Sort-Undecorate

Comparison Functions

True/False (Mark T for True and F for False)

• The list.sort() method modifies the original list. True or Fasle
• The sorted() function returns a new sorted list. True or False
• The key parameter is used to specify a custom sorting function. True or False
• The reverse parameter is used to reverse the sorting order. True or False
• The sorted() function can only sort lists. True or False

Multiple Choice (Select the best answer)

Which of the following statements sorts a list in descending order?

1. list.sort(reverse=True)
2. sorted(list, reverse=True)
3. list.reverse()
4. sorted(list, ascending=False)

What is the output of the following code?

numbers = [3, 1, 4, 2]
numbers.sort(key=lambda x: -x)
print(numbers)

1. [1, 2, 3, 4]
2. [4, 3, 2, 1]
3. [1, 4, 2, 3]
4. [3, 4, 1, 2]

How would you sort a list of tuples by the second element of each tuple?

1. list.sort(key=lambda x: x[1])
2. sorted(list, key=lambda x: x[1])
3. list.sort(key=operator.itemgetter(1))
4. All of the above

Fill in the Blanks

1. The ___ method sorts a list in place.
2. The ___ function returns a new sorted list.
3. The ___ parameter is used to specify a custom sorting function.
4. The ___ parameter is used to reverse the sorting order.

#49 How can you sort a list of dictionaries based on a specific key?

people = [ {‘name’: ‘Ali’, ‘age’: 25}, {‘name’: ‘Ahmad’, ‘age’: 30}, {‘name’: ‘Hamza’, ‘age’: 20} ]

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a. sorted_people = sorted(people, key=’name’) b. sorted_people = sorted(people, key=’age’) c. sorted_people = sorted(people, key=lambda x[‘name’]) d. sorted_people = sorted(people, key=lambda x: x[‘age’])

#48 Which of the following is an example of using the sorted() function to sort a list of strings in reverse alphabetical order?

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https://youtube.com/shorts/Ic58XdR9TS0?feature=share

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a. sorted_fruits = sorted(fruits) b. sorted_fruits = sorted(fruits, reverse=True) c. sorted_fruits = sorted(fruits, key=len) d. sorted_fruits = sorted(fruits, key=lambda x: x[-1], reverse=True

#45 What is the difference between the sort() and sorted() functions in Python?

a. sort() is a built-in function that returns a new sorted list, while sorted() is a method that sorts the list in-place.

b. sort() can only be used on any iterable, while sorted() can be used on lists.

c. sort() is a method that sorts the list in-place, while sorted() is a built-in function that returns a new sorted list.

d. sort() returns a new list, while sorted() returns None.

Which of the following is an example of using the sort() method to sort a list of integers in descending order? a. nums.sort(reverse=True) b. nums = sort(nums, reverse=True) c. sorted(nums, reverse=True) d. nums = nums.sort(reverse=True)

Which of the following is an example of using the sorted() function to sort a list of strings in reverse alphabetical order? a. sorted_fruits = sorted(fruits) b. sorted_fruits = sorted(fruits, reverse=True) c. sorted_fruits = sorted(fruits, key=len) d. sorted_fruits = sorted(fruits, key=lambda x: x[-1], reverse=True)

How can you sort a list of tuples based on the second element of each tuple? a. sorted_items = sorted(items, key=lambda x: x[2]) b. sorted_items = sorted(items, key=lambda x: x[1]) c. sorted_items = sorted(items, key=lambda x: x[-1]) d. sorted_items = sorted(items, key=lambda x: x[0])

Exercises

1. Sorting Numbers:

• Create a list of numbers and sort them in ascending order using both list.sort() and sorted().
• Sort a list of numbers in descending order.
• Sort a list of numbers in ascending order, but keep the original list intact.
• Sort a list of numbers based on their absolute values.

2. Sorting Strings:

• Sort a list of strings alphabetically.
• Sort a list of strings in reverse alphabetical order.
• Sort a list of strings by their length. solution
• Sort a list of strings alphabetically, but case-insensitively.

3. Sorting Tuples:

• Sort a list of tuples by their first element.
• Sort a list of tuples by their second element.
• Sort a list of tuples by the length of their second element.

4. Sorting Dictionaries:

• Sort a list of dictionaries by their keys.
• Sort a list of dictionaries by their values.
• Sorting a List of Dictionaries by Age using Lambda Function Solution

5. Custom Sorting:

• Sort a list of objects based on a custom attribute.
• Sort a list of words based on the number of vowels in each word.
• Sort a list of employees based on their salary, then by their name if salaries are equal.

6. Sorting Algorithms:

• Implement the bubble sort algorithm.
• Implement the insertion sort algorithm.
• Implement the selection sort algorithm.
• Implement the merge sort algorithm.
• Implement the quick sort algorithm.

7. Advanced Sorting:

• Sort a large list of numbers efficiently using a suitable algorithm.
• Sort a list of items based on multiple criteria.
• Handle sorting of mixed data types (e.g., numbers, strings, tuples).
• Implement a stable sorting algorithm.

Review Questions

• Explain the difference between list.sort() and sorted().
• What is the purpose of the key parameter in sorting functions?

References and Bibliography

• Sorting HOW TO: Python - Documentation