Peek Top Element of Stack in DSA C++ - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to peek the top element of the stack.

DSA C++

int topElement = stack.[1]();

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

Atop

Bpop

Cpush

Dempty

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() instead of top() causes the element to be removed.

Using push() or empty() does not return the top element.

The top() function returns the top element of the stack without removing it.

2fill in blank

medium

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

DSA C++

if (!stack.[1]()) {
    int topElement = stack.top();
}

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

Aempty

Btop

Cpop

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using top() to check emptiness causes errors if stack is empty.

Using pop() removes elements, not checks emptiness.

The empty() function checks if the stack has no elements.

3fill in blank

hard

Fix the error in the code to correctly peek the top element safely.

DSA C++

if (!stack.[1]()) {
    int topElement = stack.top();
}

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

Apop

Bempty

Ctop

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Checking if stack.top() returns true causes errors if stack is empty.

Using pop() in condition removes elements unexpectedly.

We must check if the stack is not empty before accessing the top element. Using empty() returns true if empty, so condition should be negated.

4fill in blank

hard

Fill both blanks to create a function that returns the top element or -1 if stack is empty.

DSA C++

int peekTop(std::stack<int>& stack) {
    if (stack.[1]()) {
        return -1;
    }
    return stack.[2]();
}

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

Aempty

Btop

Cpop

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() instead of top() removes the element.

Not checking if stack is empty causes runtime errors.

The function checks if the stack is empty using empty(). If empty, it returns -1. Otherwise, it returns the top element using top().

5fill in blank

hard

Fill all three blanks to create a safe peek function that prints the top or a message if empty.

DSA C++

void printTop(std::stack<int>& stack) {
    if (stack.[1]()) {
        std::cout << "Stack is empty" << std::endl;
    } else {
        int topElement = stack.[2]();
        std::cout << "Top element: " << topElement << std::endl;
    }
    stack.[3]();
}

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

Aempty

Btop

Cpop

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Not checking if stack is empty before accessing top.

Using pop() before top() causes wrong output.

The function first checks if the stack is empty using empty(). If not empty, it gets the top element with top(), prints it, and then removes it with pop().

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