Knowledge Representation

Introduction

• Domain specific knowledge is needed to solve some problems.

• Knowledge base – representation.• Inference techniques• Use to prove facts.• Use to answer queries

Knowledge Representation Schemes

• Logical representation schemes – logical proofs, e.g. predicate logic.

• Procedural representation schemes – procedural list of instructions, e.g. production rule systems.

• Network representation schemes – graphical representation, e.g. semantic networks and conceptual graphs.

• Structured representation schemes –extension of network schemes, e.g. scripts and frames.

• Structures for incomplete or inconsistent knowledge: Bayesian reasoning, Dempster-Shafer, fuzzy logic.

.

Semantic Networks

• Consists of nodes and arcs.• Nodes represents objects.• Arcs represents relationships between

the objects.• Relationships:

– is-a – Instance– has/has-a– Others

.

Example 1Clyde is an elephant.

.

Mammal

Elephant

Clyde

is-a

instance

Example 2

Tail Dog 4 Legs

GreatDane

Barks

Tall

Pluto Spots

has has

is-a communicates

heightinstance

has

Example 3

Mammal

Person Nose

Pee-Wee BrooklynBlueReese Dodgers

is-a

has-a

instance

teamuniformcolour

Example 4

John’s height is 72.

John is taller than Bill.

John 72height

John

H1

Bill

H2

height height

greaterthan

Example 5

John gives Mary the book.

Give

Book32

recipient

object

giverMary John

Using Case Frames

• Problems with semantic networks• Attempts to standardize semantic networks

led to the development of case frames.• Each sentence is represented by a case

frame.• Each case frame represents an actions.• Case frames define case relationships:

agent, object, instrument, location and time..

Example: Case Frame

glue

Fix

instrument

time

objectagentSarah

past

chair

Exercises

Construct semantic networks for the following statements:• Pompeian(Marcus), Blacksmith(Marcus). • Mary gave the green coloured vase to

her favorite cousin.• John went downtown to deposit his

money in a bank.

Conceptual Graphs

• Is a connected bipartite graph.• Conceptual relation nodes represent

relations between concepts. • The arcs connecting nodes are not

labeled.• Concepts are represented by boxes.• Relations are represented by arcs.

Conceptual Graph Relations

bird flies

dog colour brown

child parents

mother

father

1-ary relation

2-ary relation

3-ary relation

Example

Mary gave John the book.

Maryagent

agent

give object

book

person:

person:John

Unique Markers

• If an individual object is unknown a unique marker can be used in place of the name of the object.

• A unique marker is comprised of a hash symbol (#) followed by a number.

• Each object has its own unique marker.

Example 1: Unique Markers

Emmacolour brown dog:

#1352colour brown dog:

Example 2: Unique Marker

name ”emma”

#1352colour brown dog:

Example Her name was McGill and she called herself Lil,

but everyone knew her as Nancy.

name ”Nancy”

#941name

“McGill”

person: name

“Lil”

Generic Markers

• A generic marker is used to represent an unspecified individual of a type.

• A generic marker is represented by an asterisk *.

• Name variables can also be used, e.g., *X to indicate an unspecified individual.

Example: Generic MarkerThe dog is scratching its ear with its paw.

objectagent scratch

instrument

dog: *X

paw

ear

dog: *X

part

part

Display Form

John is going to Boston by bus.

agentperson:John go destination

city:Boston

instance

bus

Linear Form

[Go] –(Agnt) → [Person: John](Dest) → [City: Boston](Inst) → [Bus]

John is going to Boston by bus.

Exercises

Construct conceptual graphs for the following statements:• Jane gave Tom an ice cream.• Basketball players are tall.• Paul cut down the tree with an axe.• Place all the ingredients in a bowl and

mix thoroughly.