02 Semantic Nets - Semanticnetjg1

8/18/2019 02 Semantic Nets - Semanticnetjg1

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Objects

• Objects are at the heart of the Object

Oriented Paradigm

•What is an object?


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Objects

• We are surrounded by objects.

• In this class room there are desks,

blackboards, lights , chairs and so on.• Each object has specific attributes

• he desk is made of !ood. he seat has a

blue co"er and so on.


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Objects ha"e associations and

relationships !ith each other .

• Objects ha"e associations and

relationships !ith each other .

•#eats ha"e desktops attached.

• $ll are attached to the floor

• #eats are adjacent to other seats.

• hey are oriented in the same !ay.• %ights are controlled by s!itches


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Object &oles and 'unctions

• Each Object in the room has a specific

role or function or beha"iour.

•#eats are to be sat on.

• (oard is to be !ritten on.

• %ights can be turned on and off

• (oards can be mo"ed up and do!n andseats can be put up and do!n.


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Objects can be classified

• Within the room objects can be grouped

into different classes

•'or e)ample !e could ha"e the furnitureobjects e.g. chairs, tables, !orktop

• We could also ha"e the electrical objects.

• %ights, projector, laptop and so on.


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he nature of Objects

• *learly Objects constitute a comple) multifaceted concept.

• heir definition is dependent on many elements+their attributes , their beha"iour, theirclassification and as !e !ill see much , muchmore

• o in"estigate the nature of objects , it is helpfulto consider an important kno!ledge

representation scheme !hich is the basis fortheoretical frame!orks of the object orientedparadigm

• his frame!ork is kno!n as semantic net!orks


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#emantic ets

• #emantic net!orks are a popular scheme

!hich elegantly reflect these ideas.

• $ net!ork consists of nodes repesentingobjects, concepts and e"ents and links

bet!een the nodes representing their

interrelations.


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E)ample

• -sing the e)ample Birds have wings

• a typical semantic net !ould be


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Origin

• he de"elopment of semantic net!orks had itsorigins in psychology. &oss uillian in /012designed t!o semantic net!ork based systemsthat !ere intended primariliy as psychological

models of associatei"e memory.• #emantic et!orks 3uickly found application in

$I. (. &aphaels #I& system, also /012, !as oneof the first programs to use this type of

representation scheme.• #I& !as a 3uestion $ns!ering system and couldans!er 3uestions re3uiring a "ariety of simplereasoning tasks and relationships


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4eaning of #emantic ets

• he semantics of net structures ho!e"er

depends solely on the program that

manipulates them and there are no fi)ed

con"entions about their meaning.

• $ !ide "ariety of net!ork based systems

ha"e been implemented that use totally

different procedures for makinginferences.


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$nother E)ample

Dog bone

likes


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E"ol"ing 'eatures

• While there are no fi)ed con"entions, a

number of important features of #emantic

nets ha"e emerged , that are !idely used.

• hese ha"e largely emerged because of

the application of #emantic ets to Object

Oriented heory.


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Object 5efinitions

• he central aspect of the object paradigm

is ho! it defines objects.

• he basic mechanism of representation is

the articulation of class hierarchies.

• Instances of Objects e)ist. In turn Objectsbelong to classes and these in turn can

belong to other classes


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E)ample

• *onsider 'ido 6 !ho is a dog7

• 'ido is instance of the object 8dog9.

• 5ogs belong to the class pets, !hich fore)ample could also include other classes

such as cats.

• Pets in turn belong to a class animals andso on


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'ido

5ogs

Pets

$nimals

*ats


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%abels used in #emantic ets

• Objects and Instances

• (oth &epresented by odes linked by an

• I#:$ link

'ido 5ogI#:$

InstanceObject


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%abels used in #emantic ets

• Objects and *lasses


• #ubset or a #uper#et link

5og Pets#ubset

Object*lass


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#uper#et links

• Objects and *lasses


• #uper#et link

5og Pets#ubset

Object*lass#uperset


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&elationships, attributes and

associations

&elationships, attributes and associations

&epresented by a labelled link bet!een

objects etc

'ido(lack and

!hite

colour


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*omponent Parts

• Object components


• ;$P 6has as part7 link• $ dog has a tail

5og ail;$P

ObjectObject


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Inheritance

• $ttributes of classes are inherited by

subclasses and instances of objects

• (ecause !e kno! dogs ha"e tails and

'ido is a dog !e kno! 'ido has a tail

since this is inherited from the parent class


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Inheritance

• $ dog has a tail and 'ido is a dog

5og ail;$P

ObjectObject

'ido

I#$


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(igger E)ample

• E)ercise

• What does the follo!ing #emantic net

represent


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4y ;ouse

(ungalo!

;ouse

(uilding

&oof

/

brick

!alls&ed

;abitation

o. Of

#toreys

Purpose

*olour ;$P ;$P

subset

subset

I#$4ade:of


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Advantages of Semantic Nets

• Easy to "isualiraphical in nature = easy for humans tointerpret

• E)pressi"e po!er e3ual to or e)ceeding that of 'irst OrderPredicate %ogic

• 'ormal definitions of semantic net!orks ha"e been de"eloped for

use• &elated kno!ledge is easily clustered = logically and physicallyclose

• Efficient in space re3uirements = Objects represented only once = &elationships handled by pointers

• Other schemes are limited to rue or 'alse ans!ers !here as#emantic ets are more informati"e and fle)ible

• ot limited to only binary representation can also represent actionconcepts


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Disadvantages of Semantic Nets

• Inheritance 6particularly from multiplesources and !hen e)ceptions in

inheritance are !anted7 can causeproblems such as conflicts

• 'acts placed inappropriately cause

problems• o standards about node and arc "alues =

in spite of the generic formal definitions

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02 Semantic Nets - Semanticnetjg1

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