Pop Operation on Stack in DSA C - Execution Trace

Concept Flow - Pop Operation on Stack

Check if stack is empty?

No↓

Access top element

↓

Remove top element

↓

Update top pointer

↓

Return popped element

Yes↓

Stack Underflow Error

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

Execution Sample

DSA C

stack = [10, 20, 30]
top = 2
popped = stack[top]
top = top - 1
print(popped)

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

Execution Table

Step	Operation	Top Index	Stack Before	Stack After	Popped Element	Visual State
1	Check if stack is empty (top == -1)	2	[10, 20, 30]	[10, 20, 30]	N/A	10 -> 20 -> 30 -> null
2	Access top element stack[top]	2	[10, 20, 30]	[10, 20, 30]	30	10 -> 20 -> 30 -> null
3	Remove top element by decrementing top	2	[10, 20, 30]	[10, 20, 30]	30	10 -> 20 -> 30 -> null
4	Update top to top - 1	1	[10, 20, 30]	[10, 20, 30]	30	10 -> 20 -> null
5	Return popped element	1	[10, 20, 30]	[10, 20, 30]	30	10 -> 20 -> null
6	End of pop operation	1	[10, 20, 30]	[10, 20, 30]	30	10 -> 20 -> null

💡 Pop ends after removing top element and updating top pointer.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
top	2	2	1	1	1
popped	N/A	30	30	30	30
stack	[10, 20, 30]	[10, 20, 30]	[10, 20, 30]	[10, 20, 30]	[10, 20, 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 prevent invalid removal.

Does the stack array change physically after pop?

No, the array elements remain but the 'top' pointer moves down. Steps 3 and 4 show the top index decreasing, effectively ignoring the popped element.

Why do we return the element before updating the top pointer?

Because the top pointer points to the current top element. Step 2 accesses the element before Step 4 updates the top to exclude it.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'top' after Step 4?

A 1

B 2

C -1

D 0

💡 Hint

Check the 'Top Index' column in Step 4 of the execution_table.

At which step is the popped element accessed from the stack?

A Step 1

B Step 2

C Step 3

D Step 5

💡 Hint

Look at the 'Operation' column describing 'Access top element'.

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

A Pop proceeds normally

B Top pointer increments

C Stack Underflow error occurs

D Popped element is returned as null

💡 Hint

Refer to the concept_flow where empty stack leads to underflow.

Concept Snapshot

Pop Operation on Stack:
- Check if stack is empty (top == -1)
- Access element at stack[top]
- Decrement top to remove element
- Return popped element
- If empty, signal 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 not empty, it accesses the element at the current top index, stores it as the popped element, then decrements the top pointer to remove it from the stack. Finally, it returns the popped element. If the stack is empty, the operation signals an underflow error and does not remove any element.