Why Peek Top Element of Stack in DSA C++?

The Big Idea

Peek lets you see the top without touching it--saving time and avoiding mistakes!

The Scenario

Imagine you have a stack of books on your desk. You want to see the title of the book on the very top without removing it. If you try to pick up each book one by one to check, it will take a lot of time and might mess up the order.

The Problem

Manually checking the top element means removing it first, which can disturb the order and cause mistakes. It is slow and risky because you might lose track of the original stack arrangement.

The Solution

The peek operation lets you look at the top element of the stack instantly without removing it. This keeps the stack intact and saves time, just like glancing at the top book's title without moving it.

Before vs After

✗ Before

if (!stack.empty()) {
    int topElement = stack.top();
    stack.pop(); // removes the top element
    // use topElement
    stack.push(topElement); // push it back
}

✓ After

if (!stack.empty()) {
    int topElement = stack.top(); // just look, no remove
    // use topElement
}

What It Enables

Peek enables quick access to the top item without changing the stack, making your programs faster and safer.

Real Life Example

When undoing actions in a text editor, peek helps check the last action without removing it, so you can decide whether to undo or not.

Key Takeaways

Manual checking disturbs stack order and is slow.

Peek lets you see the top element without removing it.

This keeps data safe and operations efficient.

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