Sorting Using Functions

Let’s explore sorting using functions in Python 🏅🔢—with easy explanations, step-by-step examples, and practice questions (with solutions)! 🚦

Why Sort? 🤔

Sorting helps you organize data—like arranging numbers from smallest to largest, or names alphabetically. Python has built-in ways to sort, but you can also write your own functions for custom sorting!

1️⃣ Basic Sorting with Built-in Functions

Using sorted() and .sort()

• sorted(sequence) returns a new sorted list from any sequence (list, tuple, etc.).
• .sort() sorts a list in place (modifies the original list).

numbers = [4, 2, 7, 1]
print(sorted(numbers))  # [1, 2, 4, 7]
numbers.sort()
print(numbers)          # [1, 2, 4, 7]

You can use sorted() with any sequence, but .sort() is only for lists!¹

2️⃣ Sorting with Functions ✨

You can write your own sorting function for special needs!

Example: Selection Sort (Custom Function)

def selection_sort(lst):
    result = []
    while len(lst) > 0:
        min_val = min(lst)
        result.append(min_val)
        lst.remove(min_val)
    return result

numbers = [24, 6, 32, 78, 24, 84]
print(selection_sort(numbers))  # [6, 24, 24, 32, 78, 84]

This function finds the smallest item, adds it to a new list, and removes it from the original—repeating until sorted.²

3️⃣ Sorting Dictionaries by Key or Value

Dictionaries are unordered, but you can sort them for display!

Sort by Key

d = {'b': 1, 'a': 10, 'c': 22}
for key in sorted(d):
    print(key, d[key])
# Output:
# a 10
# b 1
# c 22

This loops through keys in sorted order.³¹

Sort by Value

d = {'a': 10, 'b': 1, 'c': 22}
lst = []
for key, val in d.items():
    lst.append((val, key))  # (value, key) tuples
lst.sort(reverse=True)
for val, key in lst:
    print(key, val)
# Output:
# c 22
# a 10
# b 1

By making (value, key) tuples, you can sort by value easily!¹

4️⃣ Sorting with key Functions

The sorted() function can take a key argument—a function that tells Python how to sort.

Example: Sort Words by Length

words = ['apple', 'banana', 'kiwi', 'cherry']
sorted_words = sorted(words, key=len)
print(sorted_words)  # ['kiwi', 'apple', 'banana', 'cherry']

Here, key=len tells Python to sort by word length.

Example: Sort Tuples by Second Element

pairs = [(1, 'b'), (3, 'a'), (2, 'c')]
sorted_pairs = sorted(pairs, key=lambda x: x[^1])
print(sorted_pairs)  # [(3, 'a'), (1, 'b'), (2, 'c')]

Lambda functions are great for custom sorting!¹

5️⃣ Practice Questions with Solutions 💡

Q1. Sort a list of numbers in descending order using a function.

def sort_desc(lst):
    return sorted(lst, reverse=True)

nums = [3, 1, 4, 2]
print(sort_desc(nums))  # [4, 3, 2, 1]

Q2. Given a dictionary of students and marks, print names sorted by marks (highest first).

students = {'Anil': 85, 'Bina': 78, 'Chetan': 92, 'Deepa': 67}
lst = []
for name, marks in students.items():
    lst.append((marks, name))
lst.sort(reverse=True)
for marks, name in lst:
    print(name, marks)
# Output:
# Chetan 92
# Anil 85
# Bina 78
# Deepa 67

This uses the decorate-sort-undecorate (DSU) pattern!¹

Q3. Sort a list of words by their last letter using a function.

def sort_by_last_letter(words):
    return sorted(words, key=lambda word: word[-1])

words = ['apple', 'banana', 'kiwi', 'cherry']
print(sort_by_last_letter(words))  # ['banana', 'apple', 'cherry', 'kiwi']

6️⃣ Step-by-Step: Sorting Dictionary by Value

Goal: Print dictionary items sorted by value (highest to lowest).

Step 1: Make a list of (value, key) tuples Step 2: Sort the list in reverse order Step 3: Print key and value

d = {'a': 10, 'b': 1, 'c': 22}
lst = []
for key, val in d.items():
    lst.append((val, key))
lst.sort(reverse=True)
for val, key in lst:
    print(key, val)
# Output: c 22, a 10, b 1

¹

🧩 Try It Yourself!

Q: Write a function to sort a list of tuples by the second element.

def sort_by_second(tuples):
    return sorted(tuples, key=lambda x: x[^1])

pairs = [(5, 2), (1, 3), (4, 1)]
print(sort_by_second(pairs))  # [(4, 1), (5, 2), (1, 3)]

⭐ Key Takeaways

• Use sorted() for any sequence, .sort() for lists.
• For custom sorting, use the key argument with a function or lambda.
• To sort dictionaries, convert items to a list of tuples, then sort.
• Writing your own sorting function helps you learn how sorting works!

Keep practicing and try sorting different types of data—it’s a super useful skill! 🥇🧑‍💻