Peek Top Element of Stack in DSA C++ - Build from Scratch

Peek Top Element of Stack

📖 Scenario: You are building a simple program to manage a stack of books. You want to see which book is currently on top without removing it.

🎯 Goal: Create a stack of books, then peek at the top book to see its title.

📋 What You'll Learn

Create a stack of strings named bookStack with these books in order: "Math", "Science", "History"

Create an integer variable stackSize to store the number of books in the stack

Use the top() method to get the title of the top book and store it in a string variable topBook

Print the value of topBook

💡 Why This Matters

🌍 Real World

Stacks are used in many real-world applications like undo features in text editors, browser history, and managing function calls.

💼 Career

Understanding stack operations like peek is essential for software development roles that involve data structure manipulation and algorithm design.

1

Create the stack with books

Create a stack of strings called bookStack and push these books in order: "Math", "Science", "History"

DSA C++

#include 
#include 
#include 

int main() {
    // Create stack and push books
    // Your code here

Hint

Use std::stack<std::string> bookStack; to create the stack. Use bookStack.push("BookName") to add books.

2

Get the size of the stack

Create an integer variable called stackSize and set it to the size of bookStack using the size() method

DSA C++

#include 
#include 
#include 

int main() {
    std::stack bookStack;
    bookStack.push("Math");
    bookStack.push("Science");
    bookStack.push("History");
    // Create stackSize and assign size of bookStack
    // Your code here

Hint

Use int stackSize = bookStack.size(); to get the number of books.

3

Peek the top book

Create a string variable called topBook and assign it the top element of bookStack using the top() method

DSA C++

#include 
#include 
#include 

int main() {
    std::stack bookStack;
    bookStack.push("Math");
    bookStack.push("Science");
    bookStack.push("History");
    int stackSize = bookStack.size();
    // Get the top book title
    // Your code here

Hint

Use std::string topBook = bookStack.top(); to peek the top book.

4

Print the top book

Print the value of topBook using std::cout

DSA C++

#include 
#include 
#include 

int main() {
    std::stack bookStack;
    bookStack.push("Math");
    bookStack.push("Science");
    bookStack.push("History");
    int stackSize = bookStack.size();
    std::string topBook = bookStack.top();
    // Print the top book
    // Your code here
}

Hint

Use std::cout << topBook << std::endl; to print the top book.

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