Sorting and reversing lists in Python - Mini Project: Build & Apply

Sorting and reversing lists

📋 What You'll Learn

💡 Why This Matters

🌍 Real World

Sorting and reversing lists is useful in many places like organizing names, products, or any data you want to show in order or backwards.

💼 Career

Many jobs require sorting data to make it easier to read or analyze, such as in sales reports, customer lists, or inventory management.

1

Create the list of book titles

Create a list called books with these exact titles in this order: 'The Hobbit', '1984', 'Pride and Prejudice', 'To Kill a Mockingbird', 'Moby Dick'.

Python

# Create the list of books
# Your code here

Hint

Use square brackets [] to create a list and separate items with commas.

2

Sort the list alphabetically

Create a new list called sorted_books that contains the items from books sorted in alphabetical order using the sorted() function.

Python

books = ['The Hobbit', '1984', 'Pride and Prejudice', 'To Kill a Mockingbird', 'Moby Dick']
# Create sorted_books using sorted(books)
# Your code here

Hint

The sorted() function returns a new sorted list without changing the original.

3

Reverse the sorted list

Create a new list called reversed_books that contains the items from sorted_books in reverse order using slicing with [::-1].

Python

books = ['The Hobbit', '1984', 'Pride and Prejudice', 'To Kill a Mockingbird', 'Moby Dick']
sorted_books = sorted(books)
# Create reversed_books by reversing sorted_books using slicing
# Your code here

Hint

Use list[::-1] to reverse a list in Python.

4

Print the sorted and reversed lists

Print the sorted_books list on one line and the reversed_books list on the next line using two separate print() statements.

Python

books = ['The Hobbit', '1984', 'Pride and Prejudice', 'To Kill a Mockingbird', 'Moby Dick']
sorted_books = sorted(books)
reversed_books = sorted_books[::-1]
# Print sorted_books and reversed_books
# Your code here

Hint

Use two print() statements, one for each list.

Practice

1. What does the list.sort() method do to a list in Python?

easy

A. It arranges the list items in ascending order.

B. It reverses the order of the list items.

C. It removes duplicate items from the list.

D. It creates a new sorted list without changing the original.

Solution

1. Step 1: Understand the purpose of list.sort()

   The list.sort() method changes the original list to arrange its items in ascending order.

2. Step 2: Differentiate from other list methods

   Unlike list.reverse(), which flips the list order, list.sort() orders items from smallest to largest.

3. Final Answer:

   It arranges the list items in ascending order. -> Option A

4. Quick Check:

   list.sort() = ascending order [OK]

Hint: Remember: sort() orders ascending by default [OK]

Common Mistakes:

• Confusing sort() with reverse()
• Thinking sort() returns a new list
• Assuming sort() removes duplicates

2. Which of the following is the correct syntax to sort a list named numbers in descending order?

easy

A. numbers.sort(reverse=True)

B. numbers.sort(reverse=1)

C. numbers.sort(descending=True)

D. numbers.reverse(sort=True)

Solution

1. Step 1: Identify correct parameter for descending sort

   The sort() method accepts reverse=True to sort in descending order.

2. Step 2: Check syntax correctness

   numbers.sort(reverse=True) uses the correct syntax: numbers.sort(reverse=True). Other options use invalid parameters or method calls.

3. Final Answer:

   numbers.sort(reverse=True) -> Option A

4. Quick Check:

   sort(reverse=True) = descending order [OK]

Hint: Use reverse=True inside sort() for descending order [OK]

Common Mistakes:

• Using reverse=1 instead of reverse=True
• Using non-existent parameters like descending=True
• Calling reverse() with parameters

3. What is the output of the following code?

items = [3, 1, 4, 2]
items.sort()
items.reverse()
print(items)

medium

A. [1, 2, 3, 4]

B. [3, 1, 4, 2]

C. [4, 3, 2, 1]

D. [2, 3, 1, 4]

Solution

1. Step 1: Apply items.sort()

   This sorts the list in ascending order: [1, 2, 3, 4].

2. Step 2: Apply items.reverse()

   This reverses the sorted list, resulting in [4, 3, 2, 1].

3. Final Answer:

   [4, 3, 2, 1] -> Option C

4. Quick Check:

   sort() then reverse() = descending list [OK]

Hint: sort() then reverse() = descending order [OK]

Common Mistakes:

• Thinking reverse() sorts the list
• Assuming print shows original list
• Confusing order of method calls

4. The following code is intended to sort the list data in descending order. What is wrong?

data = [5, 2, 9, 1]
data.reverse()
data.sort()
print(data)

medium

A. The code will cause a syntax error.

B. The reverse() call should come after sort().

C. The sort() method does not exist for lists.

D. The list is already sorted, so no change happens.

Solution

1. Step 1: Analyze method order

   The code reverses the list first, then sorts it ascending, which cancels the reverse effect.

2. Step 2: Correct method order for descending sort

   To get descending order, first sort ascending, then reverse the list.

3. Final Answer:

   The reverse() call should come after sort(). -> Option B

4. Quick Check:

   sort() then reverse() = descending order [OK]

Hint: Sort first, then reverse for descending order [OK]

Common Mistakes:

• Reversing before sorting cancels sorting effect
• Thinking reverse() sorts the list
• Assuming method order does not matter

5. You have a list of words: words = ['apple', 'banana', 'cherry', 'date']. You want to sort them in reverse alphabetical order without changing the original list. Which code snippet achieves this?

hard

A. sorted_words = words.sort(reverse=True)

B. words.sort(reverse=True)

C. words.reverse(); words.sort()

D. sorted_words = sorted(words, reverse=True)

Solution

1. Step 1: Understand difference between sort() and sorted()

   sort() changes the original list; sorted() returns a new sorted list.

2. Step 2: Choose method that sorts in reverse without changing original

   Using sorted(words, reverse=True) returns a new list sorted in reverse alphabetical order, leaving words unchanged.

3. Final Answer:

   sorted_words = sorted(words, reverse=True) -> Option D

4. Quick Check:

   sorted() returns new sorted list [OK]

Hint: Use sorted() to keep original list unchanged [OK]

Common Mistakes:

• Using sort() which changes original list
• Chaining reverse() before sort() incorrectly
• Assigning sort() result which is None