Introduction
Of
Queue

Introduction
 A queue is a non-primitive linear data structure.
 It is an homogeneous collection of elements in
which new elements are added at one end called the
Rear end and the existing elements are deleted from
other end called the Front end.
 A queue is logically a First In First Out (FIFO)
type of list.
 In our everyday life we come across many situation
where we are wait for the desired service, there we
have to get into a waiting line for our turn to get
serviced.

Introduction
 This waiting queue can be thought of as a Queue.
 This Queue means a Line.
 For example:
 At the railway reservation booth, we have to get
into the reservation queue.
 Here, the important feature of the queue is when
new customers got into the queue from the rear
end, whereas the customers who get their sets
reserved leave the queue from the front end.

Introduction
 It means the customers are serviced in the order
in which they arrive the service center.
 This same characteristics apply to our Queue.
 The next slide figures show queue graphically
during insertion and deletion operations.

Introduction
 The following figures show queue graphically during
insertion operation's=Front & R=Rear
10 20
10
(a) Empty Queue
F=-1 & R=-1 0 1 2 3 4
(b) One element inserted in queue
F=0 & R=0 0 1 2 3 4
0 1 2 3 4
(b) Second element inserted in queue
F=0 & R=1
F R
F R
F R

Introduction
 The following figures show queue graphically during
deletion operation's=Front & R=Rear
10 20
20
(a) Empty Queue
F=-1 & R=-1
0 1 2 3 4
(b) One element inserted in queue
F=1 & R=1
0 1 2 3 4
0 1 2 3 4
(b) Second element inserted in queue
F=0 & R=1
F R
F R
F R

Queue Implementation
 There are two ways to implement a Queue:
 Static implementation (using array)
 Dynamic implementation
(using Pointer/Linked list)

Static Implementation (using array)
 If queue is implemented using arrays, we must be
sure about the exact number of elements .
 We want to store in the queue, because we have to
declare the size of the array at design time or before
the processing starts.
 In this case, the beginning of the array will become
the front for the queue and the last location of the
array will act as rear for the queue.

Static Implementation (using array)
 The following relation gives the total number of
elements present in the queue, when implemented
using arrays:
Total no of elem.=front-rear+1
 Also note that if rear<front then there will be no
element in the queue or queue is always be empty.

Dynamic implementation
(using Pointer/Linked list)
 Implementing queues using pointers, the main
disadvantage is that a node in a linked
representation occupies more memory space then a
corresponding element in the array representation.

Types of Queue
 There are many different way to implement a
Queue:
 Simple Queue/Linear Queue
 Circular Queue
 Double Ended Queue (Deque)
 Priority Queue

Simple Queue
 In the simple queue, the beginning of the array will
become the front for the queue and the last location
of the array will act as rear for the queue.
 The following relation gives the total number of
elements present in the queue, when implemented
using arrays:
Total no of elem.=front-rear+1

Simple Queue
 Also note that if rear<front then there will be no
element in the queue or queue is always be empty.
 Bellow show a figure a empty simple queue Q[5]
which can accommodate five elements.
Q[0] Q[1] Q[2] Q[3] Q[4]
Way of deletion Way of insertion
Fig: Simple Queue

Limitation of Simple Queue
 There are certain problems associated with a simple
queue when queue is implemented using array.
 Consider an example of a simple queue Q[5], which
is initially empty.
 We can only five elements insertion in queue.
 If we attempt to add more elements, the elements
can’t be inserted because in a simple queue rear
increment up to size-1.

Limitation of Simple Queue
 At that time our program will flash the message
“Queue is full”.
 But if the queue is too large of say 5000 elements it
will be a tedious job to do and time consuming too.
 To remove this problem we use circular queue.

Circular Queue
 A circular queue is one in which the insertion of a
new element is done at the very first location of the
queue if the last location of the queue is full.
 In other words if we have a queue Q of say n
elements, then after inserting an element last (n-1th)
location of the array the next elements will be
inserted at the very first location (0) of the array.
 It is possible to insert new elements, if and only if
those location (slots) are empty.

Circular Queue
 We can say that a circular queue is one in which the
first element comes just after the last element.
 It can be viewed as a mesh or loop of wire, in which
the two ends of the wire are connected together.
 A circular queue overcomes the problem of
unutilized space in linear queues implemented as
arrays.
 A circular queue also have a Front and Rear to keep
the track of the elements to be deleted and inserted
and therefore to maintain the unique characteristic
of the queue.

Circular Queue
 Bellow show a figure a empty circular queue Q[5]
which can accommodate five elements.
`
Q[0] Q[1]
Q[2]
Q[3]
Q[4]
Fig: Circular Queue

Double Ended Queue (Deque)
 It is also a homogeneous list of elements in which
insertion and deletion operations are performed
from both the ends.
 That is, we can insert elements from the rear end or
from the front ends.
 Hence it is called double-ended queue. It is
commonly referred as a Deque.
 There are two types of Deque. These two types are
due to the restrictions put to perform either the
insertions or deletions only at one end.

Double Ended Queue (Deque)
 There are:
 Input-restricted Deque.
 Output-restricted Deque.
 Bellow show a figure a empty Deque Q[5] which
can accommodate five elements.
Q[0] Q[1] Q[2] Q[3] Q[4]
deletion
deletion
insertion
insertion
Fig: A Deque

Double Ended Queue (Deque)
 There are:
 Input-restricted Deque: An input restricted
Deque restricts the insertion of the elements at
one end only, the deletion of elements can be
done at both the end of a queue.
10 20 30 40 50
Q[0] Q[1] Q[2] Q[3] Q[4]
deletion
deletion
insertion
Fig: A representation of an input-restricted Deque
F R

Double Ended Queue (Deque)
 There are:
 Output-restricted Deque: on the contrary, an
Output-restricted Deque, restricts the deletion of
elements at one end only, and allows insertion to
be done at both the ends of a Deque.
10 20 30 40 50
Q[0] Q[1] Q[2] Q[3] Q[4]
insertion
deletion
insertion
Fig: A representation of an Output-restricted Deque
F R

Double Ended Queue (Deque)
 The programs for input-restricted Deque and
output-restricted Deque would be similar to the
previous program of Deque except for a small
difference.
 The program for the input-restricted Deque would
not contain the function addqatbeg().
 Similarly the program for the output-restricted
Deque would not contain the function delatbeg().

Priority Queue
 A priority queue is a collection of elements where
the elements are stored according to their priority
levels.
 The order in which the elements should get added or
removed is decided by the priority or the element.
 Following rules are applied to maintain a priority
queue.
 The element with a higher priority is processes
before any element of lower priority.

Priority Queue
 If there are elements with same priority, then the
element added first in the queue would get
processed
 Here, smallest number that is most highest priority
and greater number that is less priority.
 Priority queues are used for implementing job
scheduling by the operating system.
 Where jobs with higher priorities are to be processed
first.
 Another application of priority queue is simulation
systems where priority corresponds to event times.

Applications of Queues
 Round robin technique for processor scheduling is
implemented using queues.
 All types of customer service (like railway ticket
reservation) center software’s are designed using
queues to store customers information.
 Printer server routines are designed using queues. A
number of users share a printer using printer server
the printer server then spools all the jobs from all the
users, to the server’s hard disk in a queue. From here
jobs are printed one-by-one according to their
number in the queue.