Pop Operation on Stack in DSA C++ - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

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

DSA C++

stack.[1]();

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

Apush

Bempty

Cpop

Dtop

Attempts:

3 left

💡 Hint

Common Mistakes

Using push instead of pop.

Using top() which only accesses the element but does not remove it.

The pop() function removes the top element from the stack.

2fill in blank

medium

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

DSA C++

if (!stack.[1]()) {
    stack.pop();
}

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

Asize

Bempty

Ctop

Dpush

Attempts:

3 left

💡 Hint

Common Mistakes

Using size() without comparing to zero.

Using top() which returns the top element, not a boolean.

The empty() function returns true if the stack is empty.

3fill in blank

hard

Fix the error in the code to pop and print the top element safely.

DSA C++

if (!stack.empty()) {
    int topElement = stack.[1]();
    stack.pop();
    std::cout << topElement << std::endl;
}

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

Atop

Bempty

Cpush

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() to get the element, which is incorrect.

Not checking if the stack is empty before accessing top().

Use top() to access the top element before popping it.

4fill in blank

hard

Fill both blanks to pop all elements from the stack and print them.

DSA C++

while (!stack.[1]()) {
    std::cout << stack.[2]() << std::endl;
    stack.pop();
}

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

Aempty

Bsize

Ctop

Dpush

Attempts:

3 left

💡 Hint

Common Mistakes

Using size() without comparing to zero.

Using pop() to get the element instead of top().

Use empty() to check if stack is not empty and top() to access the element.

5fill in blank

hard

Fill all three blanks to pop elements from the stack only if the top is greater than 10.

DSA C++

while (!stack.[1]()) {
    if (stack.[2]() [3] 10) {
        stack.pop();
    } else {
        break;
    }
}

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

Aempty

Btop

C>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using < instead of > in the condition.

Not checking if stack is empty before accessing top().

Use empty() to check stack, top() to access element, and > to compare.

Practice

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

easy

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

B. Removes the top element from the stack

C. Returns the size 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 removed first.

2. Step 2: Define pop operation

   The pop operation removes the top element from the stack, which is the last inserted item.

3. Final Answer:

   Removes the top element from the stack -> Option B

4. Quick Check:

   pop() removes top element [OK]

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

Common Mistakes:

• Thinking pop adds elements
• Confusing pop with peek or size
• Assuming pop clears the stack

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

easy

A. stk.pop();

B. pop(stk);

C. stk.pop_element();

D. stk.remove();

Solution

1. Step 1: Recall C++ STL stack syntax

   The standard stack class in C++ uses member functions like push() and pop() called on the stack object.

2. Step 2: Identify correct pop syntax

   The correct way to remove the top element is calling pop() as stk.pop();.

3. Final Answer:

   stk.pop(); -> Option A

4. Quick Check:

   Use object.pop() to remove top [OK]

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

Common Mistakes:

• Using pop() as a free function
• Using incorrect method names like pop_element()
• Trying to remove with remove() which doesn't exist

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

std::stack<int> stk;
stk.push(10);
stk.push(20);
stk.pop();
std::cout << stk.top();

medium

A. 0

B. 20

C. 10

D. Compilation error

Solution

1. Step 1: Trace push operations

   First, 10 is pushed, then 20 is pushed on top. Stack top is now 20.

2. Step 2: Apply pop and check top

   pop() removes the top element (20), so top becomes 10. Printing top outputs 10.

3. Final Answer:

   10 -> Option C

4. Quick Check:

   After pop, top is 10 [OK]

Hint: Pop removes last pushed, top shows new top [OK]

Common Mistakes:

• Assuming top remains 20 after pop
• Confusing pop with peek
• Expecting 0 or error output

4. Identify the error in the following code snippet that uses a stack:

std::stack<int> stk;
stk.pop();
stk.push(5);

medium

A. No error, code runs fine

B. Push should be called before pop

C. Stack declaration is incorrect

D. Pop called on empty stack causes runtime error

Solution

1. Step 1: Analyze stack state before pop

   The stack is empty initially. Calling pop() on empty stack is invalid and causes runtime error.

2. Step 2: Check order of operations

   Pop is called before any push, so no element exists to remove, causing error.

3. Final Answer:

   Pop called on empty stack causes runtime error -> Option D

4. Quick Check:

   Pop on empty stack causes error [OK]

Hint: Never pop from empty stack [OK]

Common Mistakes:

• Ignoring empty stack before pop
• Thinking push order fixes error
• Assuming pop on empty is safe

5. Given a stack with elements 1, 2, 3, 4 pushed in that order (1 at bottom, 4 at top), what will be the stack content after performing two pop() operations?

hard

A. 1 -> 2 -> null

B. 3 -> 4 -> null

C. 1 -> 2 -> 3 -> null

D. 4 -> 3 -> 2 -> 1 -> null

Solution

1. Step 1: Understand initial stack order

   Elements pushed in order: 1 (bottom), 2, 3, 4 (top). Stack looks like 1 -> 2 -> 3 -> 4 -> null.

2. Step 2: Apply two pop operations

   First pop removes 4 (top), second pop removes 3 (new top). Remaining stack is 1 -> 2 -> null.

3. Final Answer:

   1 -> 2 -> null -> Option A

4. Quick Check:

   Two pops remove top two elements [OK]

Hint: Pop removes top elements one by one [OK]

Common Mistakes:

• Confusing order of elements after pop
• Listing popped elements instead of remaining
• Reversing stack order incorrectly