Pop Operation on Stack in DSA C++ - Execution Trace

Concept Flow - Pop Operation on Stack

Check if stack is empty

No↓

Remove top element

↓

Update top pointer

↓

Return popped element

↓

Done

Yes↓

Stack Underflow - Cannot pop

The pop operation first checks if the stack is empty. If not, it removes the top element, updates the top pointer, and returns the popped element. If empty, it reports underflow.

Execution Sample

DSA C++

stack = [10, 20, 30]
top = 2
pop()
// removes 30, top becomes 1
// stack is now [10, 20, 30]

This code removes the top element (30) from the stack and updates the top pointer.

Execution Table

Step	Operation	Top Before	Top After	Popped Element	Stack State	Pointer Changes
1	Check if stack is empty (top == -1?)	2	2	N/A	[10, 20, 30]	No change
2	Remove element at top	2	2	30	[10, 20, 30]	No change yet
3	Update top pointer (top = top - 1)	2	1	30	[10, 20, 30]	top moves from 2 to 1
4	Return popped element	1	1	30	[10, 20, 30]	Stack logically has 2 elements now
5	End of pop operation	1	1	30	[10, 20, 30]	No change

💡 Pop ends after removing top element and updating top pointer

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
top	2	2	2	1	1	1
stack	[10, 20, 30]	[10, 20, 30]	[10, 20, 30]	[10, 20, 30]	[10, 20, 30]	[10, 20, 30]
popped_element	N/A	N/A	30	30	30	30

Key Moments - 3 Insights

Why do we check if the stack is empty before popping?

Because popping from an empty stack causes an error (underflow). Step 1 in the execution_table shows this check to avoid removing from an empty stack.

Does the element get physically removed from the stack array?

No, the element remains in the array but is considered removed because the top pointer moves down. Step 4 shows the stack logically has two elements after pop.

Why do we update the top pointer after removing the element?

Updating the top pointer marks the new top of the stack. Step 3 shows top changing from 2 to 1, which means the previous top element is no longer accessible.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'top' after Step 3?

A 2

B -1

C 1

D 0

💡 Hint

Check the 'Top After' column in Step 3 of the execution_table.

At which step does the popped element get identified?

A Step 2

B Step 4

C Step 1

D Step 3

💡 Hint

Look at the 'Popped Element' column in the execution_table rows.

If the stack was empty (top = -1), what would happen at Step 1?

A Top pointer increases

B Stack underflow error occurs

C Pop proceeds normally

D Element is removed anyway

💡 Hint

Refer to the concept_flow and Step 1 condition check in the execution_table.

Concept Snapshot

Pop Operation on Stack:
- Check if stack is empty (top == -1)
- If not empty, remove element at top
- Decrement top pointer by 1
- Return popped element
- If empty, report underflow error

Full Transcript

The pop operation on a stack first checks if the stack is empty by verifying if the top pointer is -1. If the stack is not empty, it removes the element at the current top position, then updates the top pointer by decreasing it by one. This effectively removes the top element from the stack. The popped element is then returned. If the stack is empty, the operation cannot proceed and an underflow error is reported. The execution table shows each step with the top pointer and stack state changes. The variable tracker follows the top pointer, stack contents, and popped element through the operation.

Practice

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

easy

A. Adds a new element to the bottom of the stack

B. Removes the top element from the stack

C. Returns the size of the stack

D. Clears all elements from the stack

Solution

1. Step 1: Understand stack behavior

   A stack follows Last In First Out (LIFO) principle, so the last added element is removed first.

2. Step 2: Define pop operation

   The pop operation removes the top element from the stack, which is the last inserted item.

3. Final Answer:

   Removes the top element from the stack -> Option B

4. Quick Check:

   pop() removes top element [OK]

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

Common Mistakes:

• Thinking pop adds elements
• Confusing pop with peek or size
• Assuming pop clears the stack

2. Which of the following is the correct syntax to pop an element from a stack named stk in C++ STL?

easy

A. stk.pop();

B. pop(stk);

C. stk.pop_element();

D. stk.remove();

Solution

1. Step 1: Recall C++ STL stack syntax

   The standard stack class in C++ uses member functions like push() and pop() called on the stack object.

2. Step 2: Identify correct pop syntax

   The correct way to remove the top element is calling pop() as stk.pop();.

3. Final Answer:

   stk.pop(); -> Option A

4. Quick Check:

   Use object.pop() to remove top [OK]

Hint: Use object.pop() to remove top element [OK]

Common Mistakes:

• Using pop() as a free function
• Using incorrect method names like pop_element()
• Trying to remove with remove() which doesn't exist

3. What will be the output of the following C++ code?

std::stack<int> stk;
stk.push(10);
stk.push(20);
stk.pop();
std::cout << stk.top();

medium

A. 0

B. 20

C. 10

D. Compilation error

Solution

1. Step 1: Trace push operations

   First, 10 is pushed, then 20 is pushed on top. Stack top is now 20.

2. Step 2: Apply pop and check top

   pop() removes the top element (20), so top becomes 10. Printing top outputs 10.

3. Final Answer:

   10 -> Option C

4. Quick Check:

   After pop, top is 10 [OK]

Hint: Pop removes last pushed, top shows new top [OK]

Common Mistakes:

• Assuming top remains 20 after pop
• Confusing pop with peek
• Expecting 0 or error output

4. Identify the error in the following code snippet that uses a stack:

std::stack<int> stk;
stk.pop();
stk.push(5);

medium

A. No error, code runs fine

B. Push should be called before pop

C. Stack declaration is incorrect

D. Pop called on empty stack causes runtime error

Solution

1. Step 1: Analyze stack state before pop

   The stack is empty initially. Calling pop() on empty stack is invalid and causes runtime error.

2. Step 2: Check order of operations

   Pop is called before any push, so no element exists to remove, causing error.

3. Final Answer:

   Pop called on empty stack causes runtime error -> Option D

4. Quick Check:

   Pop on empty stack causes error [OK]

Hint: Never pop from empty stack [OK]

Common Mistakes:

• Ignoring empty stack before pop
• Thinking push order fixes error
• Assuming pop on empty is safe

5. Given a stack with elements 1, 2, 3, 4 pushed in that order (1 at bottom, 4 at top), what will be the stack content after performing two pop() operations?

hard

A. 1 -> 2 -> null

B. 3 -> 4 -> null

C. 1 -> 2 -> 3 -> null

D. 4 -> 3 -> 2 -> 1 -> null

Solution

1. Step 1: Understand initial stack order

   Elements pushed in order: 1 (bottom), 2, 3, 4 (top). Stack looks like 1 -> 2 -> 3 -> 4 -> null.

2. Step 2: Apply two pop operations

   First pop removes 4 (top), second pop removes 3 (new top). Remaining stack is 1 -> 2 -> null.

3. Final Answer:

   1 -> 2 -> null -> Option A

4. Quick Check:

   Two pops remove top two elements [OK]

Hint: Pop removes top elements one by one [OK]

Common Mistakes:

• Confusing order of elements after pop
• Listing popped elements instead of remaining
• Reversing stack order incorrectly