C program to implement queue data structure

This is a simple queue implementation using array.

#include <stdio.h>
#include <stdlib.h>
#define MAX 5   // maximum size of the queue

int queue[MAX];
int front = -1, rear = -1;

// Function to check if queue is full
int isFull() {
    return (front == (rear + 1) % MAX);
}

// Function to check if queue is empty
int isEmpty() {
    return (front == -1);
}

// Function to insert an element in the queue
void enqueue(int value) {
    if (isFull()) {
        printf("Queue Overflow! Cannot insert %d\n", value);
        return;
    }
    if (front == -1) {
        front = 0;
    }
    rear = (rear + 1) % MAX;
    queue[rear] = value;
    printf("%d enqueued into the queue.\n", value);
}

// Function to delete an element from the queue
int dequeue() {
    if (isEmpty()) {
        printf("Queue Underflow! No elements to dequeue.\n");
        return -1;
    }
    int value = queue[front];
    if (front == rear) {
        // Queue becomes empty
        front = rear = -1;
    } else {
        front = (front + 1) % MAX;
    }
    printf("%d dequeued from the queue.\n", value);
    return value;
}

// Function to display the queue
void display() {
    if (isEmpty()) {
        printf("Queue is empty.\n");
        return;
    }
    printf("Queue elements: ");
    int i = front;
    while (1) {
        printf("%d ", queue[i]);
        if (i == rear) break;
        i = (i + 1) % MAX;
    }
    printf("\n");
}

int main() {
    int choice, value;
    while (1) {
        printf("\n--- Queue Menu ---\n");
        printf("1. Enqueue\n2. Dequeue\n3. Display\n4. Exit\n");
        printf("Enter your choice: ");
        scanf("%d", &choice);

        switch (choice) {
        case 1:
            printf("Enter value to enqueue: ");
            scanf("%d", &value);
            enqueue(value);
            break;
        case 2:
            dequeue();
            break;
        case 3:
            display();
            break;
        case 4:
            exit(0);
        default:
            printf("Invalid choice! Try again.\n");
        }
    }
    return 0;
}

Code Walkthrough

1. Circular Queue: Uses modulo (% MAX) to reuse array space.
2. Prevents overflow and underflow.
3. Menu-driven (enqueue, dequeue, display, exit).
4. Uses front and rear indices to manage the queue.