Queue Implementation

                                    Queue Implementation in C Language

/**
 * Queue implementation using array.
 */

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

// Queue capacity
#define CAPACITY 100

/**
 * Global queue declaration.
 */
int queue[CAPACITY];
unsigned int size  = 0;
unsigned int rear  = CAPACITY - 1;   // Initally assumed that rear is at end
unsigned int front = 0;



/* Function declaration for various operations on queue */
int enqueue(int data);
int dequeue();
int isFull();
int isEmpty();
int getRear();
int getFront();



/* Driver function */
int main()
{
    int ch, data;


    /* Run indefinitely until user manually terminates */
    while (1)
    {
        /* Queue menu */
        printf("--------------------------------------\n");
        printf("  QUEUE ARRAY IMPLEMENTATION PROGRAM  \n");
        printf("--------------------------------------\n");
        printf("1. Enqueue\n");
        printf("2. Dequeue\n");
        printf("3. Size\n");
        printf("4. Get Rear\n");
        printf("5. Get Front\n");
        printf("0. Exit\n");
        printf("--------------------------------------\n");
        printf("Select an option: ");

        scanf("%d", &ch);

     
        /* Menu control switch */
        switch (ch)
        {
            case 1:
                printf("\nEnter data to enqueue: ");
                scanf("%d", &data);

                // Enqueue function returns 1 on success
                // otherwise 0
                if (enqueue(data))
                    printf("Element added to queue.");
                else
                    printf("Queue is full.");

                break;

            case 2:
                data = dequeue();

                // on success dequeue returns element removed
                // otherwise returns INT_MIN
                if (data == INT_MIN)
                    printf("Queue is empty.");
                else
                    printf("Data => %d", data);

                break;

            case 3:

                // isEmpty() function returns 1 if queue is emtpy
                // otherwise returns 0
                if (isEmpty())
                    printf("Queue is empty.");
                else
                    printf("Queue size => %d", size);

                break;

            case 4:

                if (isEmpty())
                    printf("Queue is empty.");
                else
                    printf("Rear => %d", getRear());

                break;

            case 5:

                if (isEmpty())
                    printf("Queue is empty.");
                else
                    printf("Front => %d", getFront());

                break;

            case 0:
                printf("Exiting from app.\n");
                exit(0);
     
            default:
                printf("Invalid choice, please input number between (0-5).");
                break;
        }

        printf("\n\n");
    }
}



/**
 * Enqueue/Insert an element to the queue.
 */
int enqueue(int data)
{
    // Queue is full throw Queue out of capacity error.
    if (isFull())
    {
        return 0;
    }

    // Ensure rear never crosses array bounds
    rear = (rear + 1) % CAPACITY;

    // Increment queue size
    size++;

    // Enqueue new element to queue
    queue[rear] = data;

    // Successfully enqueued element to queue
    return 1;
}


/**
 * Dequeue/Remove an element from the queue.
 */
int dequeue()
{
    int data = INT_MIN;

    // Queue is empty, throw Queue underflow error
    if (isEmpty())
    {
        return INT_MIN;
    }

    // Dequeue element from queue
    data = queue[front];

    // Ensure front never crosses array bounds
    front = (front + 1) % CAPACITY;

    // Decrease queue size
    size--;

    return data;
}


/**
 * Checks if queue is full or not. It returns 1 if queue is full,
 * overwise returns 0.
 */
int isFull()
{
    return (size == CAPACITY);
}


/**
 * Checks if queue is empty or not. It returns 1 if queue is empty,
 * otherwise returns 0.
 */
int isEmpty()
{
    return (size == 0);
}


/**
 * Gets, front of the queue. If queue is empty return INT_MAX otherwise
 * returns front of queue.
 */
int getFront()
{
    return (isEmpty())
            ? INT_MIN
            : queue[front];
}


/**
 * Gets, rear of the queue. If queue is empty return INT_MAX otherwise
 * returns rear of queue.
 */
int getRear()
{
    return (isEmpty())
            ? INT_MIN
            : queue[rear];

}