Pop Operation on Stack in DSA Typescript - Execution Trace

Concept Flow - Pop Operation on Stack

Check if stack is empty?

No↓

Remove top element

↓

Update top pointer

↓

Return removed element

↓

Done

Yes↓

Error: Stack Underflow

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 an error.

Execution Sample

DSA Typescript

let stack = [10, 20, 30];
function pop() {
  if (stack.length === 0) {
    throw new Error('Stack Underflow');
  }
  return stack.pop();
}
pop();
// removes 30, stack becomes [10, 20]

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

Execution Table

Step	Operation	Nodes in Stack	Pointer Changes	Visual State
1	Check if stack is empty	[10, 20, 30]	top points to 30	10 -> 20 -> 30 (top)
2	Remove top element (30)	[10, 20]	top updated to 20	10 -> 20 (top)
3	Return removed element	[10, 20]	top points to 20	10 -> 20 (top)
4	Done	[10, 20]	top points to 20	10 -> 20 (top)

💡 Stack is not empty, pop removes top element 30 and updates top pointer

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
stack	[10, 20, 30]	[10, 20]	[10, 20]	[10, 20]
top	points to 30	points to 20	points to 20	points to 20
popped_element	undefined	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 (stack underflow). The execution_table row 1 shows this check to prevent removing from an empty stack.

What happens to the top pointer after popping?

The top pointer moves to the next element below the removed one. Execution_table row 2 shows 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. Execution_table row 2 shows the stack without 30, and variable_tracker confirms popped_element holds 30 separately.

Visual Quiz - 3 Questions

Look at the execution_table, what is the stack state after step 2?

A [10, 20, 30]

B [20, 30]

C [10, 20]

D []

💡 Hint

Check the 'Nodes in Stack' column at step 2 in execution_table

At which step does the top pointer update to point to 20?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

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

If the stack was empty initially, what would happen at step 1?

A Remove top element

B Error: Stack Underflow

C Update top pointer

D Return popped element

💡 Hint

Refer to concept_flow and execution_table exit_note about empty stack check

Concept Snapshot

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

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 a stack underflow error. This process ensures the stack maintains its last-in-first-out order.

Practice

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

easy

A. Clears all elements from the stack

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

C. Returns the bottom element without removing it

D. Removes and returns the top element of 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 this top element from the stack.

3. Final Answer:

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

4. Quick Check:

   pop() removes top element = A [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 TypeScript syntax to pop an element from a stack array named stack?

easy

A. pop(stack);

B. stack.remove();

C. stack.pop();

D. stack.delete();

Solution

1. Step 1: Recall array pop method in TypeScript

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

2. Step 2: Match correct syntax

   The correct syntax to pop from stack is stack.pop();. Other options are invalid methods.

3. Final Answer:

   stack.pop(); -> Option C

4. Quick Check:

   Array pop method = stack.pop() [OK]

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

Common Mistakes:

• Using non-existent methods like remove() or delete()
• Calling pop as a function with stack as argument
• Confusing pop with other array methods

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

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

medium

A. 30
[10, 20]

B. 20
[10, 20, 30]

C. 30
[10, 20, 30]

D. undefined
[10, 20]

Solution

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

   pop() removes and returns the last element. Here, 30 is removed and returned.

2. Step 2: Check console outputs

   First console.log prints popped value 30. Second prints updated stack [10, 20].

3. Final Answer:

   30 [10, 20] -> Option A

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
• Thinking pop() does not modify the array
• Confusing pop() return value with stack after pop

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

const stack: number[] = [];
const popped = stack.pop();
console.log(popped);

medium

A. pop() returns undefined when stack is empty, no error

B. Runtime error because pop() on empty array is invalid

C. Syntax error due to missing type annotation

D. pop() removes first element instead of last

Solution

1. Step 1: Check pop() behavior on empty array

   pop() returns undefined if the array is empty; it does not throw an error.

2. Step 2: Verify code correctness

   Code runs without error and prints undefined.

3. Final Answer:

   pop() returns undefined when stack is empty, no error -> Option A

4. Quick Check:

   pop() on empty array = undefined [OK]

Hint: pop() returns undefined if stack is empty, no crash [OK]

Common Mistakes:

• Expecting runtime error on empty pop()
• Confusing pop() with shift() behavior
• Thinking pop() removes first element

5. Given a stack implemented as an array stack = [5, 10, 15, 20], write a TypeScript function popTwo that pops two elements and returns them as an array. What will be the returned value and the stack after calling popTwo(stack)?

hard

A. [15, 20], stack = [5, 10]

B. [20, 15], stack = [5, 10]

C. [20, 15], stack = [10, 5]

D. [10, 5], stack = [15, 20]

Solution

1. Step 1: Understand pop() order and stack state

   pop() removes last element first. First pop returns 20, second pop returns 15.

2. Step 2: Determine returned array and stack after two pops

   Returned array is [20, 15]. Stack after pops is [5, 10].

3. Final Answer:

   [20, 15], stack = [5, 10] -> Option B

4. Quick Check:

   Two pops remove last two elements in order [20, 15] [OK]

Hint: Pop twice returns last two elements in reverse order [OK]

Common Mistakes:

• Returning popped elements in wrong order
• Assuming stack order changes after pop
• Mixing up stack contents after pops