Circular Queue Operations Using Array - c

Question

Code shows basic operations on Circular Queue.
#define maxsize 10
typedef struct queue
{
int data[maxsize];
int f,r;
}myQueue;
myQueue q;
void init(myQueue *q);
int full(myQueue *q);
int empty(myQueue *q);
void enqueue(myQueue *q,int num);
void dequeue(myQueue *q);
void print(myQueue *q);
void main()
{
init(&q);
int op;
do
{
printf("\nCircular queue operations: Press:\n");
printf("1 for enqueue\n");
printf("2 for dequeue\n");
printf("3 to print Circular Queue\n");
int num,choice;
printf("\nEnter choice:\n");
scanf("%d",&choice);
switch(choice)
{
case 1: printf("Enter number to insert :\n");
scanf("%d",&num);
if(full(&q))
{
printf("\nQueue is full\n");
exit(0);
}
enqueue(&q,num);
break;
case 2: if(empty(&q))
{
printf("\nQueue is empty\n");
exit(0);
}
dequeue(&q);
break;
case 3: printf("Printing current queue: \n");
print(&q);
break;
default:break;
}
printf("Press 1 to continue or 0 to exit:\n");
scanf("%d",&op);
}
while(op);
}
void init(myQueue *q)
{
q->f=-1;
q->r=-1;
}
int full(myQueue *q)
{
if((q->r+1)%maxsize==q->f)
{
return 1;
}
else
return 0;
}
int empty(myQueue *q)
{
if(q->r==-1)
{
return 1;
}
else
return 0;
}
enqueue function is used to add the elements into queue.
void enqueue(myQueue *q,int num)
{
if(empty(&q))
{
q->f=0;
q->r=0;
}
else
{
q->r=(q->r+1)%maxsize;
}
q->data[q->r]=num;
printf("\n%d is enqueued\n",q->data[q->r]);
}
Dequeue function is used to delete elements from the stack.
void dequeue(myQueue *q)
{
int del_num;
del_num=q->data[q->f];
if(q->f==q->r)
{
init(&q);
}
else
{ //To move front to the next position in the circular array.
q->f=(q->f+1)%maxsize;
}
printf("\n%d is dequeued\n",del_num);
}
void print(myQueue *q)
{
int i;
for(i=q->f;i!=q->r;i=(i+1)%maxsize)
{
printf("%d\n",q->data[i]);
}
printf("%d\n",q->data[q->r]);
}
Issue: Circular queue is automatically enqueuing the 0 element in it initially.
However, rest of the operations are working just fine.
I am not able to identify, why it is automatically inserting 0 in the circular queue, without me enqueuing it.

Answer 1

Your print() function always prints q->data[q->r] as its last operation. This makes no sense when your queue is empty. Maybe you can avoid it like this:
void print(myQueue *q)
{
if (empty(q))
return;
int i;
for(i=q->f;i!=q->r;i=(i+1)%maxsize)
{
printf("%d\n",q->data[i]);
}
printf("%d\n",q->data[q->r]);
}
Anyway there are many more problems with your code, which I doubt it is compiling correctly. Just as an example, the function enqueue() receives a parameter of type myQueue*. Then it provides function empty with the address of it, but this is wrong. you have to pass the function the pointer itself, so q instead of &q. The same mistake is repeated over and over again.

Answer 2

1st Code Snippet requiring change:
void enqueue(myQueue *q,int num)
{
The below line is changed.
if(empty(q)==1)
{
q->f=0;
q->r=0;
}
else
{
q->r=(q->r+1)%maxsize;
}
q->data[q->r]=num;
printf("\n%d is enqueued\n",q->data[q->r]);
}
2nd code snippet requiring changed:
void dequeue(myQueue *q)
{
int del_num;
del_num=q->data[q->f];
if(q->f==q->r)
{
Line below is changed.
init(q);
}
else
{ //To move front to the next position in the circular array.
q->f=(q->f+1)%maxsize;
}
printf("\n%d is dequeued\n",del_num);
}
3rd snippet requiring change:
void print(myQueue *q)
{
int i;
Line below is changed.
if(empty(q))
{
printf("Queue empty");
exit(0);
}
else
{
printf("Printing current queue: \n");
for(i=q->f;i!=q->r;i=(i+1)%maxsize)
{
printf("%d\n",q->data[i]);
}
printf("%d\n",q->data[q->r]);
}
}
That makes it perfectly alright. :)

Answer 3

#include <stdio.h>
#define SIZE 5
int items[SIZE];
int front = -1, rear = -1;
int isFull() {
if ((front == rear + 1) || (front == 0 && rear == SIZE - 1)) return 1;
return 0;
}
int isEmpty() {
if (front == -1) return 1;
return 0;
}
void enQueue(int element) {
if (isFull())
printf("\n Queue is full!! \n");
else {
if (front == -1) front = 0;
rear = (rear + 1) % SIZE;
items[rear] = element;
printf("\n Inserted -> %d", element);
}
}
int deQueue() {
int element;
if (isEmpty()) {
printf("\n Queue is empty !! \n");
return (-1);
} else {
element = items[front];
if (front == rear) {
front = -1;
rear = -1;
}
else {
front = (front + 1) % SIZE;
}
printf("\n Deleted element -> %d \n", element);
return (element);
}
}
void display() {
int i;
if (isEmpty())
printf(" \n Empty Queue\n");
else {
printf("\n Front -> %d ", front);
printf("\n Items -> ");
for (i = front; i != rear; i = (i + 1) % SIZE) {
printf("%d ", items[i]);
}
printf("%d ", items[i]);
printf("\n Rear -> %d \n", rear);
}
}
int main() {
deQueue();
enQueue(1);
enQueue(2);
enQueue(3);
enQueue(4);
enQueue(5);
enQueue(6);
display();
deQueue();
display();
enQueue(7);
display();
enQueue(8);
return 0;
}