Stack Concept and LIFO Principle in DSA C++ - Build from Scratch

Stack Concept and LIFO Principle

📖 Scenario: Imagine you are managing a stack of books on a table. You can only add or remove the top book. This is called Last In, First Out (LIFO) because the last book you put on the stack is the first one you take off.

🎯 Goal: You will create a stack using a list, add books to it, remove the top book, and then show the remaining books in order from top to bottom.

📋 What You'll Learn

Create a stack as a list with exact book titles

Add a new book to the top of the stack

Remove the top book from the stack

Print the stack from top to bottom showing the order

💡 Why This Matters

🌍 Real World

Stacks are used in real life when you pile plates or books, where you only take the top one first.

💼 Career

Understanding stacks helps in programming tasks like undo features, expression evaluation, and managing function calls.

1

Create the initial stack

Create a list called stack with these exact book titles in order: "Book A", "Book B", "Book C".

DSA C++

# Create the stack list with the given books
# Your code here

Hint

Use std::vector<std::string> to create the stack list with the books inside curly braces.

2

Add a new book to the stack

Add the book "Book D" to the top of the stack using the push_back method.

DSA C++

std::vector stack = {"Book A", "Book B", "Book C"};
// Add "Book D" to the top of the stack
# Your code here

Hint

Use stack.push_back("Book D") to add the new book on top.

3

Remove the top book from the stack

Remove the top book from the stack using the pop_back method.

DSA C++

std::vector stack = {"Book A", "Book B", "Book C"};
stack.push_back("Book D");
// Remove the top book from the stack
# Your code here

Hint

Use stack.pop_back() to remove the last book added.

4

Print the stack from top to bottom

Use a for loop with an integer i starting from stack.size() - 1 down to 0 to print each book in stack[i] on its own line, showing the top book first.

DSA C++

std::vector stack = {"Book A", "Book B", "Book C"};
stack.push_back("Book D");
stack.pop_back();
// Print the stack from top to bottom
# Your code here

Hint

Start from the last index stack.size() - 1 and go down to 0, printing each book.

Practice

1. What does the LIFO principle mean in the context of a stack?

easy

A. Last In, First Out - the last element added is removed first

B. First In, First Out - the first element added is removed first

C. Elements are removed randomly

D. Elements are removed based on their value

Solution

1. Step 1: Understand LIFO principle

   LIFO means the last element added to the stack is the first one to be removed.

2. Step 2: Match with stack behavior

   Stacks follow LIFO, so the last pushed element is popped first.

3. Final Answer:

   Last In, First Out - the last element added is removed first -> Option A

4. Quick Check:

   LIFO = Last In, First Out [OK]

Hint: Remember stack removes newest item first [OK]

Common Mistakes:

• Confusing LIFO with FIFO
• Thinking stack removes oldest element first
• Assuming random removal order

2. Which of the following is the correct way to push an integer 10 onto a stack named s in C++ STL?

easy

A. s.add(10);

B. s.insert(10);

C. push(s, 10);

D. s.push(10);

Solution

1. Step 1: Recall C++ STL stack push syntax

   In C++ STL, the method to add an element to stack is push() called on the stack object.

2. Step 2: Verify correct method usage

   Using s.push(10); correctly pushes 10 onto stack s.

3. Final Answer:

   s.push(10); -> Option D

4. Quick Check:

   Stack push method = push() [OK]

Hint: Use s.push(value) to add in C++ STL stack [OK]

Common Mistakes:

• Using non-existent methods like add or insert
• Calling push as a free function
• Confusing vector methods with stack

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

std::stack<int> s;
s.push(5);
s.push(10);
s.push(15);
int x = s.top();
s.pop();
int y = s.top();
std::cout << x << " " << y << std::endl;

medium

A. 15 10

B. 5 10

C. 10 5

D. 15 5

Solution

1. Step 1: Trace stack push operations

   Elements pushed in order: 5 (bottom), 10, 15 (top).

2. Step 2: Check top and pop operations

   First, x = s.top() gives 15. Then s.pop() removes 15. Next, y = s.top() gives 10.

3. Final Answer:

   15 10 -> Option A

4. Quick Check:

   Top after push = 15, after pop = 10 [OK]

Hint: Top shows last pushed; pop removes it [OK]

Common Mistakes:

• Confusing order of elements after pop
• Assuming pop returns value
• Mixing bottom and top elements

4. Identify the error in the following C++ code snippet using stack:

std::stack<int> s;
s.pop();
s.push(20);

medium

A. Syntax error in stack declaration

B. Cannot pop from an empty stack; causes runtime error

C. push should come before pop always

D. No error; code runs fine

Solution

1. Step 1: Analyze stack operations order

   The code calls pop() on an empty stack before pushing any element.

2. Step 2: Understand pop behavior on empty stack

   Calling pop() on empty stack is undefined and causes runtime error.

3. Final Answer:

   Cannot pop from an empty stack; causes runtime error -> Option B

4. Quick Check:

   Pop empty stack = runtime error [OK]

Hint: Never pop before pushing any element [OK]

Common Mistakes:

• Ignoring empty stack before pop
• Assuming pop returns value
• Thinking push order fixes pop error

5. Given a stack with elements pushed in this order: 3, 7, 9, 2, 5. Which sequence of pop() operations will produce the output 5 2 9 when printing popped elements?

hard

A. Pop twice, push 1, then pop once

B. Pop once, push 4, then pop twice

C. Pop three times consecutively

D. Pop once, pop again, then push 6 and pop

Solution

1. Step 1: Understand stack order after pushes

   Stack top to bottom: 5 (top), 2, 9, 7, 3 (bottom).

2. Step 2: Pop three times consecutively

   First pop: 5, second pop: 2, third pop: 9. This matches output sequence.

3. Final Answer:

   Pop three times consecutively -> Option C

4. Quick Check:

   Pop order matches output 5 2 9 [OK]

Hint: Pop in order to get last pushed elements first [OK]

Common Mistakes:

• Adding extra pushes changes output
• Popping fewer times than needed
• Confusing push/pop order