5. queue - data structures using c++ by varsha patil

1Oxford University Press © 2012

Data Structures Using C++ by Dr Varsha Patil

5. QUEUE



Objectives Learn about restricted linear list, the ‘Queue’ Implement Queue using arrays Learn and implement the Circular Queue Study use of Queue in simulations



The Concept A queue is an ordered list in which all insertions

are done at one end, called rear and deletions at another end called front

Queue when implemented using arrays has some drawbacks which can be avoided by circular queue

Queue is used in many applications such as simulation, priority queue, job queue etc



Queue One of the most common data processing

structures Frequently used in most of the system software's

like operating systems, Network and Database implementations and in many more other areas

Very useful in time-sharing and distributed computer systems where many widely distributed users share the system simultaneously



Queue As An Adt

Create : Create creates an empty queue, Q Add (i,Q) : Add adds the element i to the

rear end of queue, Q and returns the new queue

Delete (Q) : Delete takes out an element i from the front end of queue and returns the resulting queue

Front(Q) : Front returns the element i which is at the front position of queue

Is_Empty_Q(Q) : Is_Empty_Q returns true if queue is empty otherwise returns false



Drawbacks of linear queue using array

The linear queue is of fixed size. So the user does not have flexibility to dynamically change the size of the queue

Arbitrarily declared maximum size of queue leads to poor utilization of memory

We need to write suitable code to make Front regularly catch up with Rear and resetting of both queue Front and Rear. It leads to queue full state even though actually queue is not full

To avoid this, when queue full is signaled, we need to rewind the entire queue to the original start location (if there are vacant empty locations) so that the first element is at 0th location and Front is set to –1. Such movement of data is efficient way to avoid this drawback.



Circular Queue

Fig 1: The queue is said to be full only when there are n

elements in the queue.



If more number of queues is required to be implemented, then efficient data structure to handle multiple queues is required

It is possible to utilize all the available space in a single array

Multi-queue



The word deque is a short form of double-ended queue and defines a data structure in which items can be added or deleted at either the front end or rear end, but no changes can be made else where in the list. It is pronounced as ‘deck’

Thus a deque is a generalization of both a stack and a queue

It is sequential container that is optimized for fast index-bccess and efficient insertion at either of its ends.

Deque



Following figure shows the representation of a deque

Fig 2: Representation of a deque



Priority queue is a collection of finite number of prioritized items

Priority queues are the queues in which we can insert items or delete items from any position based on some fundamental ordering of the elements

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 of the element

Priority Queue



The element with a higher priority is processed before any element of lower priority

If there were elements with the same priority, then the element added first in the queue would get processed first

Priority queues are used for implementing job scheduling by the operating system where jobs with higher priority are to be processed first.

Another application of priority queues is simulation systems where priority corresponds to event times

There are two ways to implement priority queue

Following rules are applied to maintaina priority queue:



An example of a priority queue is a list of patients in an emergency room; each patient might be given a ranking that depends on the severity of the patient’s illness

Another example of priority queue is a list of background jobs on a multi-user computer; each background job is given a priority level

Common applications



Initialize PQ to be the empty priority queue Determine if PQ is empty Determine if PQ is full. If PQ is not full, insert an item X into PQ If PQ is not empty, remove an item X of the

highest priority

Operations on a Priority Queue, Priority :



Structure for priority queue as follows :

struct priority queue{

int data;int priority;

}



After inserting 81 with priority 3, the updated queue is as follows :



Hence two ways to Implement a Priority Queue :

Sorted list a) Advantage : Deletion is easy; items are stored by

priority, so just delete from the beginning of the list b) Disadvantage : Insertion is hard; it is necessary to

find the proper location for insertion c) A linked list is convenient for this implementation

Unsorted list a) Advantage : Insertion is easy; just add item to the end

of the list b) Disadvantage : Deletion is hard; it is necessary to find

the item first c) An array is convenient for this implementation



Array Implementation of a Priority Queue

where data Type would be data type of item like int, char, etc.

Priority would be the priority number of the element and order would be the priority number of the element in which the element has been added to the queue

struct data{<data Type> item;int priority;int order;} PQ[20];



Queue Applications

Josephus Problem Job Scheduling Simulation



Queue Applications

A powerful tool that can be used to study the behavior of systems is simulation

“Simulation is the process of forming an abstract model of a real world in order to understand the effect of modifications and the effect of introducing various strategies on the situation”



End of

Chapter 5 …!

5. queue - data structures using c++ by varsha patil

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