Stack operations (push, pop, peek) in Data Structures Theory - Mini Project: Build & Apply

Stack operations (push, pop, peek)

📖 Scenario: Imagine you have a stack of books on your desk. You can only add a book on top, remove the top book, or look at the top book without removing it. This is how a stack data structure works in computer science.

🎯 Goal: You will build a simple stack using a list and perform the basic stack operations: push (add a book), pop (remove the top book), and peek (see the top book).

📋 What You'll Learn

Create a list called stack with initial books

Create a variable called new_book to add to the stack

Write code to push new_book onto stack

Write code to pop the top book from stack and store it in top_book

Write code to peek at the top book of stack and store it in peek_book

💡 Why This Matters

🌍 Real World

Stacks are used in real life for things like stacking plates, undo features in software, and managing tasks in order.

💼 Career

Understanding stack operations is important for programming jobs, especially in areas like software development, algorithms, and system design.

1

Create the initial stack

Create a list called stack with these exact books in order: 'Math', 'Science', 'History'.

Data Structures Theory

# Create the initial stack list with the books
# Your code here

Hint

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

2

Add a new book to the stack

Create a variable called new_book and set it to the string 'English'.

Data Structures Theory

stack = ['Math', 'Science', 'History']
# Create a variable new_book with value 'English'
# Your code here

Hint

Assign the string 'English' to the variable new_book.

3

Push the new book onto the stack

Use the append() method on stack to add the new_book to the top of the stack.

Data Structures Theory

stack = ['Math', 'Science', 'History']
new_book = 'English'
# Add new_book to stack using append()
# Your code here

Hint

Use stack.append(new_book) to add the book on top.

4

Pop and peek the stack

Remove the top book from stack using pop() and store it in top_book. Then, get the current top book without removing it by accessing the last item of stack and store it in peek_book.

Data Structures Theory

stack = ['Math', 'Science', 'History']
new_book = 'English'
stack.append(new_book)
# Remove the top book and store in top_book
# Get the current top book and store in peek_book
# Your code here

Hint

Use pop() to remove the last item and stack[-1] to peek at the last item.

Practice

1. What does the pop operation do in a stack?

easy

A. Shows the bottom item without removing it

B. Adds a new item to the bottom of the stack

C. Removes and returns the top item from the stack

D. Clears all items from the stack

Solution

1. Step 1: Understand the pop operation

   The pop operation removes the last item added to the stack, which is the top item.

2. Step 2: Confirm the behavior of pop

   It returns this top item and removes it from the stack, following last-in, first-out order.

3. Final Answer:

   Removes and returns the top item from the stack -> Option C

4. Quick Check:

   pop = remove top item [OK]

Hint: Pop always removes the last item added [OK]

Common Mistakes:

• Thinking pop adds an item
• Confusing pop with peek
• Assuming pop removes bottom item

2. Which of the following is the correct way to add an item 5 to a stack named s using the push operation?

easy

A. s.pop(5)

B. s.push(5)

C. s.peek(5)

D. s.add(5)

Solution

1. Step 1: Identify the push syntax

   Push adds an item to the top of the stack, usually called with push(item).

2. Step 2: Match the correct method call

   Only s.push(5) correctly adds 5 to the stack.

3. Final Answer:

   s.push(5) -> Option B

4. Quick Check:

   push = add item [OK]

Hint: Push means add, so look for push() method [OK]

Common Mistakes:

• Using pop instead of push
• Using peek to add items
• Using add which is not standard for stacks

3. Given the following stack operations:

s.push(10)
s.push(20)
s.pop()
s.push(30)
top = s.peek()

What is the value of top after these operations?

medium

A. 30

B. 20

C. None

D. 10

Solution

1. Step 1: Trace the stack changes step-by-step

   After push(10), stack = [10]. After push(20), stack = [10, 20]. After pop(), top 20 is removed, stack = [10]. After push(30), stack = [10, 30].

2. Step 2: Determine the top item with peek

   Peek returns the top item without removing it, so top = 30.

3. Final Answer:

   30 -> Option A

4. Quick Check:

   Last pushed after pop is 30 [OK]

Hint: Track stack after each operation carefully [OK]

Common Mistakes:

• Forgetting pop removes top
• Assuming peek removes item
• Mixing order of operations

4. Consider this code snippet for a stack:

stack = []
stack.push(1)
stack.push(2)
item = stack.pop()

What is the error in this code?

medium

A. stack is not initialized correctly

B. pop() does not return the top item

C. stack.pop() removes the bottom item, not top

D. push() is not a method of list in Python

Solution

1. Step 1: Check stack initialization

   The stack is initialized as an empty list, which is correct in Python.

2. Step 2: Verify method usage on list

   Python lists use append() to add items, not push(). Using push() causes an error.

3. Final Answer:

   push() is not a method of list in Python -> Option D

4. Quick Check:

   Python list uses append(), not push() [OK]

Hint: Remember Python list uses append(), not push() [OK]

Common Mistakes:

• Assuming push() works on Python lists
• Confusing pop() behavior
• Thinking stack initialization is wrong

5. You want to reverse a list [1, 2, 3] using a stack. Which sequence of stack operations correctly reverses the list?

hard

A. Push all items in order, then pop all items to get reversed list

B. Pop all items first, then push them back in order

C. Peek all items, then push them again

D. Push items in reverse order, then pop all items

Solution

1. Step 1: Understand stack reversal logic

   Pushing items in original order stores them with last item on top. Popping all items removes them in reverse order.

2. Step 2: Apply push then pop to reverse list

   Push 1, then 2, then 3. Popping returns 3, 2, 1, which is the reversed list.

3. Final Answer:

   Push all items in order, then pop all items to get reversed list -> Option A

4. Quick Check:

   Push then pop reverses order [OK]

Hint: Push in order, pop all to reverse list [OK]

Common Mistakes:

• Pushing in reverse order first
• Using peek to reverse
• Popping before pushing