queue data structures-linear data structure

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Page 1
QUEUE
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Page 2
Queue
• Ordered collection of homogeneous elements
• Non-primitive linear data structure.
• A new element is added at one end called rear end and the
existing elements are deleted from the other end called
front end.
• This mechanism is called First-In-First-Out (FIFO).
e.g. People standing in Queue for Movie Ticket

Page 3
Fig: Model of a Queue

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Queue as ADT(abstract data type.)
• Queue is a data structure which allows a programmer to
insert an element at one end known as “Rear” and to
delete an element at other end known as “Front”.
• Queue is an abstract data type because it not only allows
storing a elements but also allows to perform certain
operation on these elements.

Page 5
Queue as ADT(abstract data type.)
• These operations are as follows.
– Initialize()
– enqueue()
– dequeue()
– Isempty()
– Isfull()
– Display()
• Elements of queue:-
– Front
– Rear
– array

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Elements of queue:-
• Front: -
– This end is used for deleting an element from a queue.
Initially front end is set to -1. Front end is incremented
by one when a new element has to be deleted from
queue.
Rear: -
This end is used for inserting an element in a queue.
Initially rear end is set to -1. rear end is incremented by
one when a new element has to be inserted in queue.

Page 7
Algorithm to insert element
(enqueue Operation)
• Step 1: [check queue full condition]
if rear = max -1 then write “queue is full”
otherwise go to step 2
• Step 2: [increment rear point]
rear = rear + 1
• Step 3: [insert element]
q [rear] = Data
• Step 4: [check front pointer]
if front = -1 then assign front =0
• Step 5: End

Page 8
Algorithm to delete element
(dequeue Operation)
• Step 1: [check queue empty condition]
if front = -1 then write “queue is empty”
otherwise go to step 2
• Step 2: [copy data]
Data = q[front]
• Step 3: [check front and rear pointer]
if front = rear then
front = rear = -1
otherwise
front = front + 1
• Step 4: end

9
0 1 2 3 4
Front=-1
Rear=-1
A
0 1 2 3 4
Front=0
Rear=0
1
2
Insert A
A B
0 1 2 3 4
Front=0
Rear=1
3
Insert B
A B C
0 1 2 3 4
Front=0
Rear=2
4
Insert C
B C
0 1 2 3 4
Front=1
Rear=2
5
Delete
C
0 1 2 3 4
Front=2
Rear=2
6
Delete
0 1 2 3 4
Front=3
Rear=2
7
Delete
Queue is empty
Queue is empty
Entry point is called Rear &
Exit point is called Front

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‘Queue Full(Overflow)’ Condition
• Queue Full(Overflow):
– Inserting an element in a queue which is already full is known as
Queue Full condition (Rear = Max-1).
– When the queue is fully occupied and enqueue() operation is called
queue overflow occurs.
• Example: Queue Full:
– Before inserting an element in queue 1st
check whether space is
available for new element in queue. This can be done by checking
position of rear end. Array begins with 0th index position & ends
with max-1 position. If numbers of elements in queue are equal to
size of queue i.e. if rear end position is equal to max-1 then queue is
said to be full. Size of queue = 4

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‘Queue Empty(Underflow)’ Condition
• Queue Empty:
– Deleting an element from queue which is already empty is known
as Queue Empty condition (Front = Rear = -1)
– When the queue is fully empty and dequeue() operation is called
queue underflow occurs.
–
• Queue Empty:
– Before deleting any element from queue check whether there is an
element in the queue. If no element is present inside a queue &
front & rear is set to -1 then queue is said to be empty.
– Size of queue = 4
– Front = Rear = -1

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Disadvantages of linear queue
• On deletion of an element from existing
queue, front pointer is shifted to next
position.
• This results into virtual deletion of an
element.
• By doing so memory space which was
occupied by deleted element is wasted
and hence inefficient memory utilization
is occur.

Page 13
Overcome disadvantage of linear queue:
• To overcome disadvantage of linear queue,
circular queue is use.
• We can solve this problem by joining the front
and rear end of a queue to make the queue as a
circular queue .
• Circular queue is a linear data structure. It
follows FIFO principle.
• In circular queue the last node is connected back
to the first node to make a circle.

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Overcome disadvantage of linear queue:
• It is also called as “Ring buffer”.
• Items can inserted and deleted from a queue
in O(1) time.

Page 15
Representation Of
Queues
1.Using an array
2.Using linked list

Page 16
Types Of Queue
1. Circular Queue
2. Dequeue (Double Ended Queue)
3. Priority Queue

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CIRCULAR QUEUE
• A queue, in which the last node is connected back to
the first node to form a cycle, is called as circular
queue.
• Circular queue are the queues implemented in circular
form rather than in a straight line.
• Circular queues overcome the problem of unutilized
space in linear queue implemented as an array.
• The main disadvantage of linear queue using array is
that when elements are deleted from the queue, new
elements cannot be added in their place in the queue,
i.e. the position cannot be reused.

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CIRCULAR QUEUE

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CIRCULAR QUEUE IMPLEMENTATION
• After rear reaches the last position, i.e. MAX-1
in order to reuse the vacant positions, we can
bring rear back to the 0th position, if it is
empty, and continue incrementing rear in same
manner as earlier.
• Thus rear will have to be incremented
circularly.
• For deletion, front will also have to be
incremented circularly..

Page 20
Enqueue(Insert) operation on
Circular Queue:
• Step 1: Check for queue full
• If rear=max–1 and front=0 or if front=rear+1
then circular queue is full and insertion operation is
not possible. otherwise go to step 2
• Step 2: Check position of rear pointer
If rear=max–1
then set rear=0 otherwise increment rear by 1.
rear=(rear+1)%MAX
• Step 3: Insert element at the position pointer by rear
pointer.
q[rear]=Data
• Step 4: Check the position of front pointer
If front=–1 then set front as 0.

Page 21
Dequeue (Delete) operation on
Circular Queue:
• Step 1: Check for queue empty if (front = -1)
then circular queue is empty and deletion operation
is not possible. otherwise go to step 2
• Step 2: Check for position of front and rear pointers.
if front = rear then
Data = q[front];
set front=rear=-1
• Step 3: Check position of front
if front = Max-1
Data = q[front];
then set front=0;
otherwise
Data = q[front];
front = (front+1)%MAX

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PRIORITY QUEUE
• A priority Queue is a collection of elements where
each element is assigned a priority and the order in
which elements are deleted and processed is
determined from the following rules:
• 1) An element of higher priority is processed
before any element of lower priority.
• 2) Two elements with the same priority are
processed according to the order in which they are
added to the queue.

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The priority queue implementation
• The priority queue is again implemented in two
way
1. array/sequential representation.
2. Dynamic/ linked representation.
In priority queue node is divided into three parts

Page 24
PRIORITY QUEUE
• An example where priority queue are used is in
operating systems.
• The operating system has to handle a large
number of jobs.
• These jobs have to be properly scheduled.
• The operating system assigns priorities to each
type of job.
• The jobs are placed in a queue and the job with
the highest priority will be executed first.

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PRIORITY QUEUE
• Advantages:-
– Preferences to the higher priority process are
added at the beginning.
– Keep the list sorted in increasing order.

Page 26
Applications of Queue
• Queue, as the name suggests is used whenever we
need to have any group of objects in an order in which
the first one coming in, also gets out first while the
others wait for there turn, like in the following
scenarios :
– Serving requests on a single shared resource, like a printer,
CPU task scheduling etc.
– In real life, Call Center phone systems will use Queues, to
hold people calling them in an order, until a service
representative is free.
– Handling of interrupts in real-time systems. The interrupts are
handled in the same order as they arrive, First come first
served.

Page 27
Distinguish between stack and queue
Sr.No STACK QUEUE
1
It is LIFO(Last In First Out) data
structure
It is FIFO (First In First Out) data structure.
2
Insertion and deletion take place
at only one end called top
Insertion takes place at rear and deletion
takes place at front.
3 It has only one pointer variable It has two pointer variables.
4 No memory wastage Memory wastage in linear queue
5
Operations:
1.push() 2.pop()
Operations:
1.enqueue() 2.dequeue()
6
In computer system it is used in
procedure calls
In computer system it is used time/resource
sharing
7.
Plate counter at marriage
reception is an example of stack
Student standing in a line at fee counter is an
example of queue.
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queue data structures-linear data structure

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