Pop Operation on Stack in DSA Javascript - Execution Trace

Concept Flow - Pop Operation on Stack

Check if stack is empty

No↓

Remove top element

↓

Update top pointer

↓

Return removed element

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 removed element. If empty, it signals underflow.

Execution Sample

DSA Javascript

stack = [10, 20, 30]
pop()
// removes 30
// stack becomes [10, 20]

This code removes the top element (30) from the stack and updates the stack to [10, 20].

Execution Table

Step	Operation	Stack Before	Top Element Removed	Stack After	Pointer Changes	Visual State
1	Check if stack is empty	[10, 20, 30]	N/A	[10, 20, 30]	Top points to 30	Bottom -> 10 -> 20 -> 30 (Top)
2	Remove top element	[10, 20, 30]	30	[10, 20]	Top updated to 20	Bottom -> 10 -> 20 (Top)
3	Return removed element	[10, 20]	30	[10, 20]	Top points to 20	Bottom -> 10 -> 20 (Top)
4	End	[10, 20]	N/A	[10, 20]	Top points to 20	Bottom -> 10 -> 20 (Top)

💡 Stack is not empty, pop removes top element 30, then operation ends.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
stack	[10, 20, 30]	[10, 20, 30]	[10, 20]	[10, 20]	[10, 20]
top	30 (index 2)	30 (index 2)	20 (index 1)	20 (index 1)	20 (index 1)
popped_element	N/A	N/A	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 prevent removing from an empty stack.

What happens to the top pointer after popping?

The top pointer moves down to the next element below the removed one. Step 2 and 3 in the execution_table show top changing from 30 to 20.

Is the popped element still in the stack after pop?

No, the popped element is removed from the stack. Step 2 shows the stack after pop no longer contains 30.

Visual Quiz - 3 Questions

Look at the execution_table, what is the stack after Step 2?

A [20, 30]

B [10, 20, 30]

C [10, 20]

D []

💡 Hint

Check the 'Stack After' column in Step 2 of the execution_table.

At which step is the top pointer updated to point to 20?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Look at the 'Pointer Changes' column in the execution_table for when top changes.

If the stack was empty at start, what would happen at Step 1?

A Pop removes an element

B Stack underflow occurs

C Top pointer moves down

D Stack becomes [10]

💡 Hint

Refer to the concept_flow where empty stack leads to underflow.

Concept Snapshot

Pop Operation on Stack:
- Check if stack is empty
- If not empty, remove top element
- Update top pointer to next element
- Return removed element
- If empty, signal underflow error

Full Transcript

The pop operation on a stack first checks if the stack is empty to avoid errors. If the stack has elements, it removes the top element, updates the top pointer to the next element below, and returns the removed element. If the stack is empty, it signals an underflow error and stops. The execution table shows each step with the stack state and pointer changes. The variable tracker follows the stack array, the top pointer, and the popped element through the steps. Key moments clarify why the empty check is needed, how the top pointer moves, and that the popped element is removed from the stack. The visual quiz tests understanding of stack state after popping, pointer updates, and behavior on empty stack.

Practice

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

easy

A. Removes and returns the top element of the stack

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

C. Returns the bottom element without removing it

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 on top.

2. Step 2: Define pop operation

   The pop operation removes and returns the top element of the stack, following LIFO.

3. Final Answer:

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

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 (which only returns)
• Assuming pop removes bottom element

2. Which of the following is the correct syntax to pop an element from a stack implemented as an array in JavaScript?

easy

A. pop(stack);

B. stack.popElement();

C. stack.pop();

D. stack.remove();

Solution

1. Step 1: Recall JavaScript array pop method

   JavaScript arrays have a built-in method pop() to remove the last element.

2. Step 2: Match correct syntax

   The correct syntax to pop from an array named stack is stack.pop();

3. Final Answer:

   stack.pop(); -> Option C

4. Quick Check:

   Use array.pop() to remove last element [OK]

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

Common Mistakes:

• Using non-existent methods like popElement()
• Calling pop as a standalone function
• Using remove() which is not valid for arrays

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

const stack = [10, 20, 30];
const popped = stack.pop();
console.log(popped);
console.log(stack);

medium

A. 20
[10, 20, 30]

B. undefined
[10, 20]

C. 30
[10, 20, 30]

D. 30
[10, 20]

Solution

1. Step 1: Understand pop() effect on array

   pop() removes the last element (30) and returns it, so popped is 30.

2. Step 2: Check stack after pop

   After pop, stack becomes [10, 20] because 30 is removed.

3. Final Answer:

   30 [10, 20] -> Option D

4. Quick Check:

   pop() returns last element and removes it [OK]

Hint: pop() returns last item and removes it from array [OK]

Common Mistakes:

• Expecting pop() to return second last element
• Assuming stack remains unchanged after pop
• Confusing pop() return value with stack content

4. Identify the error in the following code snippet that tries to pop an element from a stack:

let stack = [1, 2, 3];
let popped = stack.pop;
console.log(popped);

medium

A. pop is not a function

B. pop method is not called, missing parentheses

C. stack is not defined

D. pop returns undefined always

Solution

1. Step 1: Check how pop is used

   The code assigns stack.pop without parentheses, so it assigns the function itself, not the result.

2. Step 2: Correct usage of pop

   To pop and get the value, pop must be called with parentheses: stack.pop().

3. Final Answer:

   pop method is not called, missing parentheses -> Option B

4. Quick Check:

   Use stack.pop() to call pop method [OK]

Hint: Always use parentheses to call pop() method [OK]

Common Mistakes:

• Forgetting parentheses when calling pop
• Thinking pop is a property, not a function
• Assuming pop returns undefined without call

5. Given a stack implemented as an array stack = [5, 10, 15], what will be the state of the stack after performing two consecutive pop() operations?

hard

A. [5]

B. [10]

C. [5, 10]

D. []

Solution

1. Step 1: Perform first pop()

   First pop removes 15, stack becomes [5, 10].

2. Step 2: Perform second pop()

   Second pop removes 10, stack becomes [5].

3. Final Answer:

   [5] -> Option A

4. Quick Check:

   Two pops remove last two elements [OK]

Hint: Each pop removes one top element [OK]

Common Mistakes:

• Removing elements from the bottom instead of top
• Assuming pop does not change stack
• Confusing number of pops with elements removed