Peek Top Element of Stack in DSA C++ - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of peeking the top element after these stack operations?

Consider a stack where you push 10, then 20, then 30. You then peek the top element without popping it. What is the output?

DSA C++

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

A 10

B 30

C 20

D Stack is empty

💡 Hint

Remember, the top element is the last one pushed onto the stack.

The last element pushed is 30, so peeking the top returns 30 without removing it.

❓ Predict Output

intermediate

What happens when you peek the top element of an empty stack?

Given an empty stack, what will happen if you call the top() method?

DSA C++

std::stack<int> s;
int topElement = s.top();
std::cout << topElement << std::endl;

A Undefined behavior or runtime error

B Throws std::out_of_range exception

C 0

D Prints 'Stack is empty'

💡 Hint

Think about what happens if you try to access an element that does not exist.

Calling top() on an empty stack leads to undefined behavior or runtime error because there is no element to return.

🔧 Debug

advanced

Identify the error in peeking the top element of a stack

What is wrong with this code snippet that tries to peek the top element?

DSA C++

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

A No error, code works fine

B pop() removes the element but returns the top element correctly

C top() should be called before pop() to get the top element

D pop() returns void, so assigning it to topElement causes a compile error

💡 Hint

Check the return type of pop() in the stack API.

The pop() method does not return the top element; it only removes it. Assigning its result to a variable causes a compile error.

❓ Predict Output

advanced

What is the output after peeking and popping elements from the stack?

Given the following operations on a stack, what is printed?

DSA C++

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

A

3
2

B

2
1

C

3
3

D

1
2

💡 Hint

Remember that pop() removes the top element, so the next top() shows the new top.

First top is 3, after popping 3, the new top is 2.

🧠 Conceptual

expert

Which statement about peeking the top element of a stack is TRUE?

Choose the correct statement about the top() operation on a stack.

A <code>top()</code> removes and returns the top element of the stack.

B <code>top()</code> returns the top element without removing it and throws an exception if the stack is empty.

C <code>top()</code> returns the top element without removing it but causes undefined behavior if the stack is empty.

D <code>top()</code> returns the bottom element of the stack.

💡 Hint

Think about what happens if you call top() on an empty stack in C++ STL.

In C++ STL, top() returns the top element without removing it but calling it on an empty stack causes undefined behavior (no exception is thrown).

Practice

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

easy

A. Returns the top element without removing it

B. Removes the top element from the stack

C. Adds a new element to the top of the stack

D. Checks if the stack is empty

Solution

1. Step 1: Understand stack operations

   A stack is a collection where elements are added or removed from the top only.

2. Step 2: Define peek operation

   The peek operation returns the top element without removing it, unlike pop which removes it.

3. Final Answer:

   Returns the top element without removing it -> Option A

4. Quick Check:

   peek() = top element [OK]

Hint: Peek shows top without removal [OK]

Common Mistakes:

• Confusing peek with pop
• Thinking peek adds elements
• Assuming peek checks emptiness

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

easy

A. stk.pop();

B. stk.peek();

C. stk.top();

D. stk.getTop();

Solution

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

   The standard method to access the top element is top().

2. Step 2: Identify correct syntax

   stk.top(); returns the top element without removing it.

3. Final Answer:

   stk.top(); -> Option C

4. Quick Check:

   stk.top() = peek top element [OK]

Hint: Use .top() to peek in C++ STL stack [OK]

Common Mistakes:

• Using .peek() which is not in C++ STL
• Using .pop() which removes element
• Using non-existent .getTop() method

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

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

medium

A. 30

B. 10

C. 20

D. Compilation error

Solution

1. Step 1: Trace push operations

   Elements pushed in order: 10, then 20, then 30. Stack top is last pushed element 30.

2. Step 2: Check top() output

   stk.top() returns 30 without removing it.

3. Final Answer:

   30 -> Option A

4. Quick Check:

   Last pushed = top = 30 [OK]

Hint: Top is last pushed element [OK]

Common Mistakes:

• Confusing top with bottom element
• Assuming top removes element
• Thinking output is 20 or 10

4. Identify the error in the following code snippet that tries to peek the top element of a stack named stk:

int topElement = stk.peek();

medium

A. stk should be declared as a queue

B. peek() is not a valid method in C++ STL stack

C. topElement should be a pointer

D. stk.pop() should be used instead

Solution

1. Step 1: Check method availability in C++ STL stack

   C++ STL stack does not have a method named peek().

2. Step 2: Correct method to peek top element

   The correct method is top(), so stk.top() should be used.

3. Final Answer:

   peek() is not a valid method in C++ STL stack -> Option B

4. Quick Check:

   Use stk.top() not stk.peek() [OK]

Hint: Use .top(), not .peek() in C++ STL [OK]

Common Mistakes:

• Using peek() method which doesn't exist
• Confusing pop() with peek()
• Thinking topElement must be pointer

5. Given a stack stk with elements [5, 10, 15] pushed in that order, which code snippet correctly prints the top element without removing it and then prints the stack size?

hard

A. std::cout << stk.pop() << std::endl; std::cout << stk.size() << std::endl;

B. std::cout << stk.top() << std::endl; stk.pop(); std::cout << stk.size() << std::endl;

C. std::cout << stk.peek() << std::endl; std::cout << stk.size() << std::endl;

D. std::cout << stk.top() << std::endl; std::cout << stk.size() << std::endl;

Solution

1. Step 1: Understand peek without removal

   To peek, use stk.top() which returns top element without removing it.

2. Step 2: Check stack size after peek

   Since peek does not remove, stk.size() remains 3.

3. Final Answer:

   std::cout << stk.top() << std::endl; std::cout << stk.size() << std::endl; -> Option D

4. Quick Check:

   top() no removal + size() correct [OK]

Hint: Use top() to peek, size() unchanged [OK]

Common Mistakes:

• Using pop() which removes element
• Using peek() which is invalid in C++ STL
• Printing size after pop() expecting no change