My Circular Queue implementation is not working properly - c

Question

I have created a program to implement a Circular Queue with insert, delete and display. The insertion is working fine and the deletion too but once I try to enter numbers after deletion, nothing is displayed. Here is my source code:
#include<stdio.h>
#include<conio.h>
#define SIZE 5
int front = -1;
int rear = -1;
int queue[SIZE];
void enqueue(int item);
int dequeue();
void display();
void main()
{
int item, choice, cont = 1;
clrscr();
while(cont == 1)
{
printf("\n1.Enqueue into queue.\n");
printf("\n2.Dequeue from queue.\n");
printf("\n3.display quesue elements\n");
printf("\nEnter your choice: ");
scanf("%d",&choice);
switch(choice)
{
case 1:
printf("\nEnter the value of item: ");
scanf("%d",&item);
enqueue(item);
break;
case 2:
item = dequeue();
if(item != NULL)
{
printf("\nItem dequeued: %d\n",item);
}
break;
case 3:
display();
break;
default:
printf("\nInvalid choice.\n");
break;
}
printf("\nDo you want to continue (1/0): ");
scanf("%d",&cont);
}
getch();
}
void enqueue(int item)
{
if(front==0 && rear==SIZE-1)
printf("\n Queue OverFlow Occured");
else if(front==-1 && rear==-1)
{
front=rear=0;
queue[rear]=item;
}
else if(rear==SIZE-1 && front!=0)
{
rear=0;
queue[rear]=item;
}
else
{
rear++;
queue[rear]=item;
}
}
int dequeue()
{
int item = NULL;
if(front == -1 && rear == -1)
{
printf("\nQueue is empty. Dequeue not possible.\n");
}
else
{
item = queue[front];
queue[front] = NULL;
if(front == rear)
{
front = -1;
rear = -1;
}
else
{
front = front + 1;
}
}
return(item);
}
void display()
{
int i;
if(front==-1)
printf("\n No elements to display");
else
{
printf("\n The queue elements are:\n ");
for(i=front;i<=rear;i++)
{
printf("\t %d",queue[i]);
}
}
}

Answer 1

Check this code its working fine.
Few suggestions don't use conio.h [clrscr(), and getch()] these are not standards.
Try the code below it worked for me fine.
CODE
#include<stdio.h>
#define SIZE 5
int front = -1;
int rear = -1;
int queue[SIZE];
void enqueue(int item);
int dequeue();
void display();
int main()
{
int item, choice, cont = 1;
while(cont == 1)
{
printf("\n1.Enqueue into queue.\n");
printf("\n2.Dequeue from queue.\n");
printf("\n3.display quesue elements\n");
printf("\nEnter your choice: ");
scanf("%d",&choice);
switch(choice)
{
case 1:
printf("\nEnter the value of item: ");
scanf("%d",&item);
enqueue(item);
break;
case 2:
item = dequeue();
if(item != NULL)
{
printf("\nItem dequeued: %d\n",item);
}
break;
case 3:
display();
break;
default:
printf("\nInvalid choice.\n");
break;
}
printf("\nDo you want to continue (1/0): ");
scanf("%d",&cont);
}
printf("");
return 0;
}
void enqueue(int item)
{
if(front==0 && rear==SIZE-1)
printf("\n Queue OverFlow Occured");
else if(front==-1 && rear==-1)
{
front=rear=0;
queue[rear]=item;
}
else if(rear==SIZE-1 && front!=0)
{
rear=0;
queue[rear]=item;
}
else
{
rear++;
queue[rear]=item;
}
}
int dequeue()
{
int item = NULL;
if(front == -1 && rear == -1)
{
printf("\nQueue is empty. Dequeue not possible.\n");
}
else
{
item = queue[front];
queue[front] = NULL;
if(front == rear)
{
front = -1;
rear = -1;
}
else
{
front = front + 1;
}
}
return(item);
}
void display()
{
int i;
if(front==-1)
printf("\n No elements to display");
else
{
printf("\n The queue elements are:\n ");
for(i=front;i<=rear;i++)
{
printf("\t %d",queue[i]);
}
}
}
OUTPUT
Do you want to continue (1/0): 1
1.Enqueue into queue.
2.Dequeue from queue.
3.display quesue elements
Enter your choice: 3
The queue elements are:
5 5
Do you want to continue (1/0): 1
1.Enqueue into queue.
2.Dequeue from queue.
3.display quesue elements
Enter your choice: 1
Enter the value of item: 43
Do you want to continue (1/0): 1
1.Enqueue into queue.
2.Dequeue from queue.
3.display quesue elements
Enter your choice: 3
The queue elements are:
5 5 43
Do you want to continue (1/0): 1
1.Enqueue into queue.
2.Dequeue from queue.
3.display quesue elements
Enter your choice: 2
Item dequeued: 5
Do you want to continue (1/0): 1
1.Enqueue into queue.
2.Dequeue from queue.
3.display quesue elements
Enter your choice: 3
The queue elements are:
5 43
Do you want to continue (1/0):

Answer 2

Thats a rather confusing code. Since the idea is for the queue to be circular, trying to determine if the queue is full by checking the position of front and rear can be really tricky, aswell as trying to make decisions based on front being higher than rear etc. You can avoid all that. A third variable keeping track of the length of the queue will make your life a whole lot easier.
Try this implementation instead:
#define SIZE 5
int queue[SIZE];
int read = 0, write = 0, size = 0;
void enqueue(int item)
{
if (size >= SIZE)
{
printf("Queue is full");
return;
}
queue[write] = item;
write = (write + 1) % SIZE;
size++;
}
int dequeue()
{
if (size == 0)
{
printf("Queue is empty");
return 0;
}
read %= SIZE;
size--;
return queue[read++];
}

Answer 3

Here is your Fully Functional updated code. I pasted code on Ideone.com Link is link to code
If you don't understand anything just ask. I tried to explain it using comments. And now it is not having any problem that you were facing .
#include<stdio.h>
#include<conio.h>
#define SIZE 5
int front = -1;
int rear = -1;
int queue[SIZE];
void enqueue(int item);
int dequeue();
void display();
void main()
{
int item, choice, cont = 1;
clrscr();
while(cont == 1)
{
printf("\n1.Enqueue into queue.\n");
printf("\n2.Dequeue from queue.\n");
printf("\n3.display quesue elements\n");
printf("\nEnter your choice: ");
scanf("%d",&choice);
switch(choice)
{
case 1:
printf("\nEnter the value of item: ");
scanf("%d",&item);
enqueue(item);
break;
case 2:
item = dequeue();
if(item != NULL)
{
printf("\nItem dequeued: %d\n",item);
}
break;
case 3:
display();
break;
default:
printf("\nInvalid choice.\n");
break;
}
printf("\nDo you want to continue (1/0): ");
scanf("%d",&cont);
}
getch();
}
void enqueue(int item)
{
int temp = (rear+1)%SIZE; //EDIT HERE
if(temp == front){
printf("\n Queue OverFlow Occured");
return;
}
else if(front==-1 )
{
front=rear=0;
queue[rear]=item;
return;
}
else{
rear = (rear+1)%SIZE; // EDIT HERE
queue[rear%SIZE]=item;
}
}
int dequeue()
{
int item = NULL;
if(front == rear) // modified condition
{
printf("\nQueue is empty. Dequeue not possible.\n");
}
else
{
item = queue[front];
queue[front] = NULL;
front++; // front must be incremented
if(front > rear)
{
front = -1;
rear = -1;
}
}
return(item);
}
void display()
{
int i;
if(front==-1)
printf("\n No elements to display");
else
{
printf("\n The queue elements are:\n ");
for(i=front;i<=rear;i++)
{
printf("%d\t",queue[i]);
}
}
}