Pop Operation on Stack in DSA Typescript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to remove the top element from the stack.

DSA Typescript

function pop(stack: number[]): number | undefined {
  if (stack.length === 0) {
    return undefined;
  }
  return stack.[1]();
}

Drag options to blanks, or click blank then click option'

Apop

Bshift

Cpush

Dunshift

Attempts:

3 left

💡 Hint

Common Mistakes

Using push() instead of pop()

Using shift() which removes from the front

Using unshift() which adds to the front

The pop() method removes the last element from the array, which represents the top of the stack.

2fill in blank

medium

Complete the code to check if the stack is empty before popping.

DSA Typescript

function safePop(stack: number[]): number | null {
  if (stack.length [1] 0) {
    return null;
  }
  return stack.pop();
}

Drag options to blanks, or click blank then click option'

A>

B===

C<

D!==

Attempts:

3 left

💡 Hint

Common Mistakes

Using > or < which do not correctly check for empty

Using !== which checks for not equal

We check if the stack length is exactly zero to know if it's empty.

3fill in blank

hard

Fix the error in the pop function to correctly remove the top element.

DSA Typescript

function popElement(stack: number[]): number | undefined {
  if (stack.length === 0) {
    return undefined;
  }
  return stack.[1]();
}

Drag options to blanks, or click blank then click option'

Apush

Bdelete

Cremove

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using non-existent methods like remove or delete

Using push which adds instead of removes

The correct method to remove the last element is pop(). Methods like remove or delete do not exist on arrays.

4fill in blank

hard

Fill both blanks to pop the top element and update the stack length.

DSA Typescript

function popAndCount(stack: number[]): { popped: number | undefined; length: number } {
  const popped = stack.[1]();
  const length = stack.[2];
  return { popped, length };
}

Drag options to blanks, or click blank then click option'

Apop

Bpush

Clength

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using push instead of pop

Using size instead of length

pop() removes the top element, and length gives the current size of the stack.

5fill in blank

hard

Fill both blanks to pop the top element, check if stack is empty, and return the result.

DSA Typescript

function popIfNotEmpty(stack: number[]): number | null {
  if (stack.[1] === 0) {
    return null;
  }
  const popped = stack.[2]();
  return popped === undefined ? null : popped;
}

Drag options to blanks, or click blank then click option'

Alength

Bpop

Cpush

Dshift

Attempts:

3 left

💡 Hint

Common Mistakes

Using push or shift instead of pop

Checking stack emptiness incorrectly

We check length to see if stack is empty, then use pop() to remove the top element. push is a distractor here.

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