Stacks in C

After learning linked lists, we are now ready to study one of the most important data structures: the Stack.

Stacks are used everywhere in computer science, even when you run a program.

What Is a Stack?

A stack is a linear data structure that follows the principle:

LIFO – Last In, First Out

This means the element inserted last will be removed first.

Real-World Example

Think of a stack of plates:

• You place a plate on top
• You remove the top plate first

You cannot remove a plate from the middle.

Basic Stack Operations

• Push – Insert an element
• Pop – Remove the top element
• Peek – View the top element

Stack Implementation Using Array

This is the simplest way to understand stacks.

#include <stdio.h>
#define SIZE 5

int stack[SIZE];
int top = -1;

void push(int value) {
    if (top == SIZE - 1) {
        printf("Stack Overflow\n");
        return;
    }
    stack[++top] = value;
}

void pop() {
    if (top == -1) {
        printf("Stack Underflow\n");
        return;
    }
    printf("Popped: %d\n", stack[top--]);
}

int main() {
    push(10);
    push(20);
    push(30);
    pop();
    pop();
    return 0;
}

Stack Overflow and Underflow

• Overflow – Stack is full, cannot push
• Underflow – Stack is empty, cannot pop

Stack Using Linked List

Linked list implementation removes size limitation.

#include <stdio.h>
#include <stdlib.h>

struct Node {
    int data;
    struct Node *next;
};

struct Node *top = NULL;

void push(int value) {
    struct Node *newNode = malloc(sizeof(struct Node));
    newNode->data = value;
    newNode->next = top;
    top = newNode;
}

void pop() {
    if (top == NULL) {
        printf("Stack Underflow\n");
        return;
    }
    struct Node *temp = top;
    printf("Popped: %d\n", temp->data);
    top = temp->next;
    free(temp);
}

int main() {
    push(5);
    push(15);
    pop();
    pop();
    return 0;
}

Where Stacks Are Used

• Function calls (call stack)
• Undo / Redo operations
• Expression evaluation
• Backtracking algorithms

Mini Practice

• Create a stack using array
• Implement push and pop
• Display top element

Quick Quiz

Q1. What does LIFO mean?

Last In, First Out

Q2. What operation inserts an element?

Push

Q3. What is stack overflow?

Trying to push into a full stack

Q4. Which stack implementation removes size limitation?

Linked list

Q5. Are stacks used in real programs?

Yes