Stack (Data Structure)

Definition

A stack is a linear data structure that stores elements in a specific order called LIFO (Last In, First Out). This means the last element inserted is the first one to be removed.

It is called an Abstract Data Type (ADT) because it defines what operations can be performed, not how they are implemented internally.

Key Points

• Follows LIFO (Last In, First Out) principle

• Insertion and deletion happen only at one end, called the top

• The main operations are:

  • push() → Add an element to the top
  • pop() → Remove the top element
  • top() → View the top element (without removing)
  • size() → Get number of elements
  • empty() → Check if stack is empty

• Stack is a linear data structure

• Access is restricted → you can only access the top element

Example / Code

#include <iostream>
#include <stack>
using namespace std;

int main() {
    stack<int> s;

    // Adding elements
    s.push(10);
    s.push(20);
    s.push(30);

    // Access top element
    cout << "Top element: " << s.top() << endl;

    // Remove top element
    s.pop();

    // Size of stack
    cout << "Stack size: " << s.size() << endl;

    return 0;
}

Explanation

1. A stack s is created to store integers
2. push() adds elements: 10 → 20 → 30 (30 is on top)
3. top() returns 30 (last inserted element)
4. pop() removes 30
5. Now the stack contains 10 and 20, so size becomes 2

Output (if any)

Top element: 30
Stack size: 2

Common Mistakes

• ❌ Saying stack stores data randomly → it follows strict LIFO order
• ❌ Confusing LIFO and FIFO (FIFO is used in queues)
• ❌ Using Top() instead of correct C++ function top()
• ❌ Calling pop() or top() on an empty stack (causes error)
• ❌ Thinking you can access elements in the middle (you cannot)

Short Exam Notes (very concise revision points)

• Stack = LIFO data structure
• Operations: push, pop, top, size, empty
• Access only top element
• Implemented using arrays or linked lists
• Used in function calls, recursion, undo operations