This document discusses queues as an abstract data type (ADT) in data structures. It defines queues as first-in, first-out (FIFO) lists where elements are inserted at one end and removed from the other. The primary operations for queues are enqueue, which adds an element to the end, and dequeue, which removes an element from the front. Circular queues are also described, which connect the last queue position back to the first to form a circle. Common applications of queues include printer jobs, customer calls, and file server access control.

1. QUEUE ADT IN DATA STRUCTURES
BY,
SUBHALAKSHMI.S
M.Sc(Computer Science)
Nadar saraswathi college of arts and science.

2. CONTENTS:
Queue ADT
Queue ADT Operations
Circular Queue
Linked List Based Implementation
Applications of Queues

3. Queue Abstract Data Type(ADT):
 It Insert at one end of List and remove at the other end.
 Queues are “FIFO” (first in first out).

4. Operations:
 The Primary operations are Enqueue and Dequeue.
 Enqueue - add an entry at the end of the queue (also called “rear” or “tail”).
 Dequeue - remove the entry from the front of the queue.

5. CIRCULAR QUEUE:
 Circular Queue is a linear data structure in which the operations are performed
based on FIFO(First In First Out).
 The last position is connected back to the first position to make a circle.
 It is also called ’Ring Buffer’.
 This function is used to insert an element into the circular queue.

7. Basic Features:
1.Incase of a circular queue, head pointer will always point to
front of the queue and tail pointer will always point to the end of the queue.
2.Initially, the head and tail pointers will be pointing to the
location, this would mean that the queue is empty.

8. 3.New data is always added to the location pointed by the tail pointer, and
once the data is added tail pointer is incremented to point to the next available location.
4.In a circular queue, data is not actually removed from the queue. Only the
head pointer is incremented by one position when dequeue, is executed. As the queue
data is only the data between head and tail, hence the data left outside is not a part of
the queue anymore, hence removed.

9. 5.The head and the tail pointer will get reinitialized to 0 every time they reach the end
of the queue.

10. 6.Also, the head and tail pointers can across each other. In other words, head pointer
can be greater than the tail.
Linked List Based Implementation:
Pointer‐Based Implementation
 Can be implemented using linear linked list or circular linked list.
 Linear linked list
Need two external pointer (front and back)

11. • Circular linked list
Need only one pointer, that point at back.
Need 2 structure
 Declaration of the node
struct nodeQ {
char item;
nodeQ * next;
}

12.  Declaration of the queue
class queue
{public:
nodeQ *backPtr, * frontPtr;
// operations for queue
};

13. APPLICATIONS OF QUEUES:
 Printer Queue: Jobs submitted to a printer are printed in order of
arrival.
 Phone calls made to customer service hotlines are usually placed in a
queue.
 File servers: Users needing access to
their files on a shared file server machine are given access on a FIFO Basis.