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.

Practice

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

easy

A. Returns the bottom element without removing it

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

C. Removes and returns the top element 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 at the top.

2. Step 2: Define pop operation

   The pop operation removes and returns this top element, reducing the stack size by one.

3. Final Answer:

   Removes and returns the top element of the stack -> Option C

4. Quick Check:

   Pop = remove top element [OK]

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

Common Mistakes:

• Thinking pop adds elements
• Confusing pop with peek
• Assuming pop removes bottom element

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

easy

A. pop(&stack);

B. stack.pop();

C. pop(stack);

D. stack.pop;

Solution

1. Step 1: Understand C function call for pop

   In C, stack is usually a struct or array, so pop function requires address to modify it.

2. Step 2: Identify correct syntax

   Passing &stack (address) to pop function is correct to modify stack state.

3. Final Answer:

   pop(&stack); -> Option A

4. Quick Check:

   Use & to pass stack address in C [OK]

Hint: Use & to pass stack pointer in C functions [OK]

Common Mistakes:

• Using dot notation like in C++ or Java
• Calling pop without & causing no stack change
• Missing parentheses in function call

3. What will be the output after executing the following code?

typedef struct {
  int arr[3];
  int top;
} Stack;

void pop(Stack *s) {
  if (s->top == -1) return;
  s->top--;
}

int main() {
  Stack s = {{10, 20, 30}, 2};
  pop(&s);
  printf("%d", s.arr[s.top]);
  return 0;
}

medium

A. 20

B. 30

C. 10

D. Runtime error

Solution

1. Step 1: Initial stack state

   Stack has elements [10, 20, 30] with top = 2 (pointing to 30).

2. Step 2: Perform pop operation

   Pop decreases top from 2 to 1, so top now points to 20.

3. Step 3: Print element at new top

   Printing s.arr[s.top] prints s.arr[1] which is 20.

4. Final Answer:

   20 -> Option A

5. Quick Check:

   Pop reduces top, new top element = 20 [OK]

Hint: Pop moves top down by one, print new top [OK]

Common Mistakes:

• Printing old top element after pop
• Confusing array index with top value
• Ignoring top decrement in pop

4. Identify the error in the following pop function for a stack and select the correct fix:

void pop(Stack *s) {
  if (s->top == 0) {
    printf("Stack Underflow\n");
    return;
  }
  s->top--;
}

medium

A. Add return value to pop function

B. Change condition to s->top == -1 to check empty stack

C. Change s->top-- to s->top++

D. Remove the if condition entirely

Solution

1. Step 1: Understand empty stack condition

   Stack is empty when top is -1, not 0, because top starts at -1.

2. Step 2: Fix condition to detect underflow

   Change condition to s->top == -1 to correctly detect empty stack before popping.

3. Final Answer:

   Change condition to s->top == -1 to check empty stack -> Option B

4. Quick Check:

   Empty stack top = -1, check before pop [OK]

Hint: Empty stack top is -1, not 0 [OK]

Common Mistakes:

• Checking top == 0 instead of -1
• Incrementing top on pop
• Ignoring underflow check

5. Given a stack implemented with an array of size 5 and top initially -1, after pushing 3 elements and then popping 4 times, what will be the value of top?

hard

A. 0

B. 1

C. -2

D. -1

Solution

1. Step 1: Calculate top after pushing 3 elements

   Starting top = -1, pushing 3 times increments top to 2 (0,1,2).

2. Step 2: Calculate top after popping 4 times

   Each pop decrements top by 1: 2 -> 1 -> 0 -> -1. The 4th pop tries to pop empty stack, top stays at -1.

3. Final Answer:

   -1 -> Option D

4. Quick Check:

   Pop beyond empty stack keeps top at -1 [OK]

Hint: Top never goes below -1 after pops [OK]

Common Mistakes:

• Assuming top can go below -1
• Counting pushes and pops incorrectly
• Ignoring underflow protection