Pop Operation on Stack in DSA C++ - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output after popping from the stack?

Consider the following C++ code that uses a stack. What will be printed after the pop operation?

DSA C++

std::stack<int> s;
s.push(10);
s.push(20);
s.push(30);
s.pop();
std::cout << s.top();

A Compilation error

B 30

C 20

D 10

💡 Hint

Remember that pop removes the top element, then top returns the new top.

Initially, the stack has 10 at bottom, then 20, then 30 at top. After pop(), 30 is removed, so top() returns 20.

❓ Predict Output

intermediate

What happens if you pop from an empty stack?

What will happen if you execute the following C++ code?

DSA C++

std::stack<int> s;
s.pop();

A Runtime error (undefined behavior)

B Prints 0

C Compilation error

D Does nothing safely

💡 Hint

Check what happens when pop is called on an empty stack in C++ STL.

Calling pop on an empty stack causes undefined behavior and may crash the program at runtime.

🔧 Debug

advanced

Identify the error in this pop operation code

What error will this code produce when compiled or run?

DSA C++

std::stack<int> s;
s.push(5);
int x = s.pop();
std::cout << x;

A Compilation error: pop() returns void, cannot assign

B Runtime error: pop on empty stack

C Prints 5

D Compilation error: missing #include <stack>

💡 Hint

Check the return type of pop() in C++ STL stack.

pop() in C++ STL stack returns void, so assigning it to a variable causes a compilation error.

❓ Predict Output

advanced

What is the stack content after multiple pops?

Given the code below, what will be the printed output?

DSA C++

std::stack<int> s;
s.push(1);
s.push(2);
s.push(3);
s.pop();
s.pop();
std::cout << s.top();

A 3

B 1

C 2

D Compilation error

💡 Hint

Pop removes the top element each time.

After pushing 1,2,3, the stack top is 3. Two pops remove 3 and 2, leaving 1 at the top.

❓ Predict Output

expert

What is the output after popping all elements and pushing new ones?

Analyze the code and determine the final output printed.

DSA C++

std::stack<int> s;
s.push(100);
s.push(200);
s.pop();
s.push(300);
s.pop();
s.pop();
s.push(400);
std::cout << s.top();

A Runtime error (empty stack)

B 300

C 200

D 400

💡 Hint

Track the stack after each operation carefully.

Operations remove and add elements leaving 400 at the top at the end.

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