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DSA SERIES — Chapter 5 STACKS (IN C)

1. INTRODUCTION TO STACK

A stack organizes data such that the last inserted element is the first one to be removed.
Example: A stack of plates — the last plate placed on top is the first to be taken out.

CHECK LIST OF REAL LIFE EXAMPLES FOR STACKS

LIFO(Last In First Out) Rule:

Push → Insert element
Pop → Remove most recently inserted element

2. STACK REPRESENTATION IN C

We can implement stacks in two ways:

Using Arrays
Using Linked Lists

In this post, we focus on Array-based Stack first.

3. STACK STRUCTURE IN C (Array-Based)

struct Stack {
    int arr[100];   // Stores the actual stack elements
    int top;        // Points to index of the top element
    int size;       // Maximum capacity of the stack
};

Explanation of Structure Members:

arr[100] → Fixed-size array to store stack data.
top →
- Initially -1 (means stack is empty)
- After push: increases
- After pop: decreases
size → Total allowed capacity.
- Helps check overflow (top == size - 1)

4. INITIALIZING A STACK

void initStack(struct Stack *s, int capacity) {
    s->top = -1;
    s->size = capacity;
}

5. CHECK OPERATIONS

Check if Stack is Full

int isFull(struct Stack *s) {
    return s->top == s->size - 1;
}

Check if Stack is Empty

int isEmpty(struct Stack *s) {
    return s->top == -1;
}

6. PUSH OPERATION (INSERTION)

Adds element to the top of the stack.

void push(struct Stack *s, int value) {
    if (isFull(s)) {
        printf("Stack Overflow! Cannot push %d\n", value);
        return;
    }
    s->arr[++(s->top)] = value;
    printf("%d pushed to stack\n", value);
}

Detailed Explanation:

isFull() prevents overflow error
++(s->top) increments the top pointer first
Stores value at the new top position

7. POP OPERATION (DELETION)

Removes and returns the top element.

int pop(struct Stack *s) {
    if (isEmpty(s)) {
        printf("Stack Underflow!\n");
        return -1;
    }
    return s->arr[(s->top)--];
}

Explanation:

Checks for empty condition
Returns current top value
Decrements top pointer

8. PEEK / TOP ELEMENT

Returns top element without removing it.

int peek(struct Stack *s) {
    if (isEmpty(s)) {
        printf("Stack is Empty\n");
        return -1;
    }
    return s->arr[s->top];
}

9. DISPLAY STACK

void display(struct Stack *s) {
    if (isEmpty(s)) {
        printf("Stack is Empty\n");
        return;
    }
    
    printf("Stack elements (Top to Bottom):\n");
    for (int i = s->top; i >= 0; i--) {
        printf("%d ", s->arr[i]);
    }
    printf("\n");
}

10. COMPLETE PROGRAM

#include <stdio.h>

struct Stack {
    int arr[100];
    int top;
    int size;
};

void initStack(struct Stack *s, int capacity) {
    s->top = -1;
    s->size = capacity;
}

int isFull(struct Stack *s) {
    return s->top == s->size - 1;
}

int isEmpty(struct Stack *s) {
    return s->top == -1;
}

void push(struct Stack *s, int value) {
    if (isFull(s)) {
        printf("Stack Overflow! Cannot push %d\n", value);
        return;
    }
    s->arr[++(s->top)] = value;
    printf("%d pushed to stack\n", value);
}

int pop(struct Stack *s) {
    if (isEmpty(s)) {
        printf("Stack Underflow!\n");
        return -1;
    }
    return s->arr[(s->top)--];
}

int peek(struct Stack *s) {
    if (isEmpty(s)) {
        printf("Stack is Empty\n");
        return -1;
    }
    return s->arr[s->top];
}

void display(struct Stack *s) {
    if (isEmpty(s)) {
        printf("Stack is Empty\n");
        return;
    }
    printf("Stack elements (Top to Bottom):\n");
    for (int i = s->top; i >= 0; i--) {
        printf("%d ", s->arr[i]);
    }
    printf("\n");
}

int main() {
    struct Stack s;
    initStack(&s, 100);

    push(&s, 10);
    push(&s, 20);
    push(&s, 30);

    display(&s);

    printf("Top element: %d\n", peek(&s));
    printf("Popped: %d\n", pop(&s));

    display(&s);

    return 0;
}