Queue In Data Structure And Algorithm Using C, Code, Explanation,

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

struct queue{
    int size;
    int f;
    int r;
    int * arr;
};

int isEmpty(struct queue *q){
    if(q->r==q->f){
        return 1;
    }
    return 0;
}

int isFull(struct queue *q){
    if(q->r==q->size-1){
        return 1;
    }
    return 0;
}

void enqueue(struct queue *q, int val){
    if(isFull(q)){
        printf("This Queue is full\n");
    }
    else{
        q->r++;
        q->arr[q->r]=val;
    }
}

int dequeue(struct queue *q){
    int a = -1;
    if(isEmpty(q)){
        printf("This Queue is Empty\n");
    }
    else{
        q->f++;
        a = q->arr[q->f];
    }
    return a;
}


int main()
{
    int O, val;
    struct queue q;
    q.size = 100;
    q.f = q.r = -1;
    q.arr = (int*)malloc(q.size*sizeof(int));
    printf("1: For Enqueue\n");
    printf("2: For Dequeue\n");
    printf("3: To check empty\n");
    printf("4: To check full\n");
    printf("5: Display\n");
    printf("6: To exit\n");

    while(q.r != q.size-1){
        printf("Enter the operation you want to perform\n");
        scanf("%d", &O);
        switch(O)
        {
        case 1:
            printf("Enter the element you want to store\n");
            scanf("%d",&val);
            enqueue(&q, val);
            printf("Element is %d\n",q.arr[q.r]);
            break;
        case 2:
            printf("Dequeueing element %d\n", dequeue(&q));
            break;
        case 3:
            if(isEmpty(&q)){
            printf("Queue is Empty\n");
            }
            else
                printf("Not Empty\n");
            break;
        case 4:
            if(isFull(&q)){
            printf("Queue is Full\n");
            }
            else
                printf("Not full\n");
            break;
        case 5:
            printf("Display All Values Stored\n");
            while(q.f!=q.r)
            {
                q.f++;
                printf("Element are %d\n",q.arr[q.f]);
            }
            break;
        default:
            exit(0);
        }
    }

   return 0;
}

1: For Enqueue
2: For Dequeue
3: To check empty
4: To check full
5: Display
6: To exit
Enter the operation you want to perform
1
Enter the element you want to store
22
Element is 22
Enter the operation you want to perform
1
Enter the element you want to store
44
Element is 44
Enter the operation you want to perform
1
Enter the element you want to store
66
Element is 66
Enter the operation you want to perform
4
Not full
Enter the operation you want to perform
3
Not Empty
Enter the operation you want to perform
2
Dequeueing element 22
Enter the operation you want to perform
5
Display All Values Stored
Element are 44
Element are 66
Enter the operation you want to perform