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Data Structures: A Pseudocode Approach with C 1

Chapter 4Chapter 4

Objectives

Upon completion you will be able to:

Ex

plain the design, use, and operation of a queue Implement a queue using a linked list structure

Understand the operation of the queue ADT

Write application programs using the queue ADT

Explain categorizing data and queue simulation

QueuesQueues

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What is a queue ?

A queue is a linear list in which data canbe inserted at one end, called the rear,

and deleted from the other end, called thefront.

It is a first in first out (FIFO) restricteddata structure.

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4-1 Queue Operations

This section discusses t he four basic queue operations. Using This section discusses t he four basic queue operations. Using

diagrammatic figures, it shows how each of t hem work. It diagrammatic figures, it shows how each of t hem work. It

concludes wit h a comprehensive example t hat demonstrates each concludes wit h a comprehensive example t hat demonstrates each

operation.operation.

Enqueue

Dequeue

Queue Front Queue Rear

Queue Example

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Queue operations

nqueue

Dequeue

Queue front Queue rear

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nqueue

nqueue inserts an element at the rear of thequeue.

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Dequeue

Dequeue deletes an element at the front of thequeue.

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Queue front

Queue front retrieves the element at the front of the queue.

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Queue rear

Queue rear retrieves the element at the rear of the queue.

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(Continued)

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4-2 Queue Linked List Design

W e first discuss t he data structure for a linked-list implementation.

W e t hen develop t he eig ht algorit hms required to implement a queue.

Data Structure Queue

Algorithms

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Data structure

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pointer

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Queue algorithms

Create queue

Enqueue

Dequeue

Queuefront

Queuerear

Empty queue

Full queue

Queue count

Destroy queue

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(Continued)

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Create queue

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enqueue

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dequeue

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4-3 Queue ADT

This section develops t he data structures and C code toThis section develops t he data structures and C code to

implement a Queue ADT . The first program containsimplement a Queue ADT . The first program contains

t he data structure declarations and a list of t he prototypest he data structure declarations and a list of t he prototypes

for all of t he functions. W e t hen develop t he C code for for all of t he functions. W e t hen develop t he C code for t he algorit hms discussed in Section 4.2t he algorit hms discussed in Section 4.2

Queue Structure

Queue ADT Algorithms

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(

)

dequeue (queue, (void*)&dataPtr)

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queueFront (queue, (void*)&dataPtr)

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queueRear (queue, (void*)&dataPtr)

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4-4 Queuing Theory

Queuing t heory is a field of applied mat hematics t hat is

used to predict t he performance of queues. In t his

section we review a few basic queuing t heory concepts.

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Queuing theory

Queuing theory is a field of appliedmathematics that is used to predict theperformance of queues.

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Queue types

Single server queue

Provide service to only one customer at atime.

Multiserver queue

Provide service to many customers at a time.

Multiqueues, multiple single-server queues

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Common queue elements

One or more customers

A customer is any person or thing needingservice.

Service

The service is any activity needed toaccomplish the required result.

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Affected factors

Arrival rate

The rate at which customers arrive in thequeue for service.

Service time

The average time required to complete theprocessing of a customer request.

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Response time

The average time from the point at whichcustomers enter the queue until the moment they leave the server.

queue time + average service time

Queue time

The average length of time customers wait in queue Average service time

The average time required to complete the processing of acustomer request.

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4-5 Queue Applications

W e develop two queue applications. The first shows how to use a

queue to categorize data. The second is a queue simulator,

which is an excellent tool to simulate t he performance and to

increase our understanding of its operation.

Categorizing Data

Queue Simulation

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Categorizing data

It is often necessary to rearrange datawithout destroying their basic sequence.

Example

3 22 12 6 10 34 65 29 9 30 81 4 5 19 20 57 44 99

We want the list rearranged as shown below.

Less than 10 : 3 6 9 4 5

Between 10 and 19 : 12 10 19 Between 20 and 29 : 22 29 20

30 and greater : 34 65 30 81 57 44 99

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Categorizing data design

Category queues

Fill category queues

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(Continued)

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(Continued)

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Queue simulation

We build a model of a single-server queue.

the store is open 8 hours per day, 7 days a week.

To simulate a day, we build a model that runsfor 480 minutes(8×60).

window

clerk

New customer

queue

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Queue simulation

The simulation use a digital clock that events start and stop in 1-minute intervals.

In each minute of operation, three eventswill be checked:

The arrival of customers

The start of customer processing

The completion of customer processing

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Events

newCustomer

svcFree (svcStart)

svcComplete

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Data structures

Queue head Queue node

Customer status

Simulation statistics

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Flag moreCusts

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Queue node

custNum

arriveTime

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custStatus

custNum

arriveTime

startTime

svcTime

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simStatus

numCust

totSvcTime

totWaitTime

maxQueueSize

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An example

Four customers arrive at time 1, 2, 3 and5.

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An example

clocks

events1 2 3 4 5 6 7 8

newCustomer 1 2 3 4

svcFree

(svcStart)

YES

1

YES

2

YES

3

YES

4

svcComplete YES

1

YES

2

YES

3

YES

4

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