Data Structure- Queue

1. - By Sonali Soni

2. Introduction to Queue.
Operations on Queue.
Representation of Queue in memory.
Application of Queue.

3. Queue is an ordered group of homogeneous
items of elements.
A queue has two ends - front end & rear
end.
Insertion can take place at rear end &
deletion can take place at front end.

4. Queue is also known as First In First
Out(FIFO) data structure.
Insertion & Deletion operations are known
as Enqueue & Dequeue respectively.
Front Rear

5. The following operations are performed on
queue:-
1. CreateQueue(q)- To create q as an empty queue.
2. Enqueue(q, i)- To insert an element i in the
queue.
3. Dequeue(q)- To access & remove an element of
the queue.
4. Peek(q)- To access the first element of the
queue without removing it.
5. IsFull(q)- to check whether the queue is full.
6. IsEmpty(q)- To check whether the queue is
empty.

6. Queue can be represented in memory using linear
array or linear linked list.
A queue represented using a linked list is called a
linked queue.
The overflow condition for array implementation is
front is equal to zero & rear is equal to (size of
array)-1.
The underflow condition for array implementation is
front is equal to -1.

7. typedef struct nodetype
{
int info;
struct nodetype *next;
}node;
typedef struct
{
node *front;
node *rear;
}queue;

8. We define two datatypes- node &
queue.
The datatype queue, a structure
whose first element front holds the
address of first element of queue &
second element rear holds the
address of last element of the
queue.

9. front rear
5 10 20 0

10. CREATING AN EMPTY QUEUE
void CreateQueue(queue *pq)
{
pq->front=pq->rear=NULL;
}
The front and rear end of the queue are
initialized to NULL.

11. typedef enum {true,false}boolean;
boolean IsEmpty(queue *pq)
{
if(pq->front==NULL)
return true;
else
return false;
}

12. void Enqueue(queue *pq, int item)
{
node *ptr;
ptr=(node *)malloc(sizeof(node));
ptr->info=item;
ptr->next=NULL;
if(pq->rear==NULL)
pq->front=pq->rear=ptr;
else
{
(pq->rear)->next=ptr;
pq->rear=ptr;
}
}

13. int Dequeue(queue *pq)
{
int temp;
node *ptr;
temp=(pq->front)->info;
ptr=pq->front;
if(pq->front==pq->rear)
pq->front=pq->rear=NULL;
else
pq->front=(pq->front)->next;
free(ptr);
return temp;
}

14. Queue is used to access files from a
disk system.
In a multiprogramming environment,
queue is used for CPU scheduling or
job scheduling of operating system.
In ticket reservation system, queue
can be used for issuing tickets to the
customers.