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241-320 Design Architecture and Engineeringfor Intelligent System

Suntorn Witosurapot

Contact Address:Phone: 074 287369 or

Email: [email protected]

November 2010

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Lecture 11:

Knowledge Representationand Reasoning Part 2

(Semantic Networks, Frames)

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241-320 Design Architecture &Engineering for Intelligent System

Knowledge Representation andReasoning - part 2

3

Preview:

KR can be classified into 3 categories :

1. Logic-based representation () Propositional Logic2. Object-based representation ()

Semantic Networks, Frames

3. Rule-based representation ()

So far, we have learned how to use propositional logicas a knowledge representation language

We can write sentences in this language (syntax) withsome logical structure

We can define the interpretations of these sentencesusing truth tables (semantics)

This lecture

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Knowledge Representation andReasoning - part 2 4

Preview:

In this lecture, we will learned about 2 methods ofobject-based KR (semantic networks & frames)

Semantic nets are a simple way of representing the

relationships between entitities and concepts.

Frames can do the things that semantics networksdo, but take a more object-oriented type approach.

They allow procedures (called demons) to be attached to

their slots increasing greatly the power of this knowledgerepresentation method.

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Outiline

Semantic Networks

Comparison with Database System

Properties of Semantic Nets

Semantic network operation

Disadvantages

Frame-based Approach

Examples Advantages & Disadvantages

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Semantic networks at a glance

(network) Meaning of an object is defined in terms of

a network of associations with other objects

in a mind or a knowledge base

Semantic nets,

(Arc)

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Example 1: KR in Semantic Nets

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Example 2: KR in Semantic Nets

Correct:

Height ofThree-Finger

Brown is195cm

Incorrect:

Height ofThree-

FingerBrown is178cm

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Correct: marital status(J J ADAMS) is married

Incorrect: marital status(J J ADAMS) is divorced

An InstanceA record structure: or the same as directed graph:

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Comparison with Database System(cont.)

Properties of traditional database systems:

Only aspects of entities&their simplerelationships can be represented.

Reasoning is very simple: reasoning = lookup

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Examples of Stronger relationships inSemantic Nets

Network representation ofproperties of snow & ice

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Examples of Stronger relationships inSemantic Nets (cont.)

, the third year student of primary school feedsthe pigeon whose name is .

Relations:-

is_a(somchai, third year student)

is_a(third year student, primary school)

go(primary, school)

owns(, )

is_a(santiphab, pigeon) color(pigeon, grey)

is_a(pigeon, bird)

has_part(bird, feather)

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Examples of Stronger relationships inSemantic Nets (cont.)

Q: Can you describe the relationships below?

State: I own a tan leather chair.

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Allows to structure the knowledge to reflect the structureof that part of the universe which is being represented.

Strong representation facilities, and consume lessmemory through the use of property inheritance

() Ex: if is_a(x,y) and is_a(y,z) then is_a(x,z).

if has_part(x,y) and has_part(y,z)

then has_part(x,z).

Note: the notion explained up to now quite general andmust be much refined when implementation,especiallyclear syntax, but clear semantics has to be worked out.

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Example of Reasoning in Semantic Nets

(pattern matching) 2

:Whats the color of , the pegion owned by ?

, color(,X)

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Assume the following network is coded in a computer.

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CASE 1: A simple question to node

e.g. How does a bird travel?

The node will start to find from the information attachedto it and just return the answer, if found

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241-320 Design Architecture &Engineering for Intelligent System Knowledge Representation andReasoning - part 2 18

CASE 2: A more complex question to node

e.g. How does the tweety travel?

If the node is unable to locate the answer via a localarc, then it searches for an answer via its inheritance

links (IS-A relation).

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Detour: IS-A Hierachy

is-a corresponds mathematically to the subset relation,

instance corresponds math. to the membership relation,

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Question:

To find the value of a property of e1, first look at e1

If the property is not attached to that node, "climb"the isalink to the node's parent and search there.

Repeat, using isa/akolinks, until the property is foundor the inheritance hierarchy is exhausted.

How many legsdoes the e1have?

Hint: using inheritance

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Disadvantage of Semantic Network

Incomplete (no explicit operational/procedural knowledge)

Lack of standards, ambiguity in node/link descriptions

Not temporal (i.e. doesn't represent time or sequence)

Finally

Semantic networks are mainly used as an aid toanalysis to visually represent parts of the problemdomain. The `knowledge' can be transformed intorules or frames for implementation

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Outiline

Semantic Networks Introduction in Brief




Disadvantages

Frame-based Approach Examples

Advantages & Disadvantages

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Introduction to Frame-based Approach

A frameis a data structure containing typicalknowledge about a concept or object and canencompass both semantic & procedural knowledge.

A frame represents knowledge about real world

things (or entities) by containing both descriptions ofattributes and procedural details

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Frames

A frame consists of a collection of slots which can befilled by values or pointers to other frames.

A simple scenario:

We can know the meaning of Birthday party, sincewe have already known the meaning of Party

Childs birthday partyMeaning of should be

approximated bya partyassembled tocelebrate a birthday

peopleassembled tocelebrate a birthday

partyMeaning ofLink to

not approx. by

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Frames (cont.)

Children all know the meaning of Childs birthdayparty and plus more default assignments:

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Why Frames?

Frame is a data-structure for representing a stereotypedsituation, like [...] going to a childs birthday party.

Attached to each frame are several kinds of information Some of this info. is about how to use the frame Some is about what one can expect to happen next Some is about what to do if these expectations are not confirmed

A definition:

Frame is a remembered framework for representing

knowledgethat can be adapted to fit reality bychanging details as necessary.

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Frames What and Why?

The other definition:

A Frame is a collection of questions to be asked abouta hypothetical situation: it specifies issues to be raised

and methods to be used in dealing with them.

To understand a situation, questions can be asked like: What caused it (agent)? What was the purpose (intention)? What are the consequences (effects)? Whom does it affect (recipient)? How is it done (instruments)?

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Ex: Representation of a View of a Cube

Consider conceptional object like a view of a cube asa composed object with relations.

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Different Views of a Cube

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Example (cont.)

Part of a tabular representation of the frame for one view

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Part of the Frame Descriptionof a Hotel Room

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Procedural Attachment

This is a way to balance between:

the expressiveness of a KR formalism and

the efficiency to manipulate the represented knowledge

Idea of frames: Structure the knowledge, this meansalso restricting the expressive power in some parts, andextending in others.

The Procedural Attachment allows functions to bewritten in a programming language to be stored instead

of the value of some slot.

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Procedural Attachment (cont.)

Instead of writing explicit values, the values of theslots area, circumference are calculated by need.Consider update of length from 5cm to 6cm!

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Frames: Advantages & Disadvantages

Disadvantages complex reasoning (inferencing) is difficult

explanation is difficultAdvantages

knowledge domain can be naturally structured [asimilar motivation as for the O-O approach].

handles both semantic/procedural knowledge. easy to include the idea of default values, detect

missing values, include specialised procedures andto add further slots to the frames

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Conclusion

Semantic Nets:

Structured Representation.

Inheritance are ways of achieving some

commonsense reasoning by default Frames:

organize knowledge around concepts considered tobe of interest (like cube & hotel room in the example code).

Also allow procedural attachment attached to slotsfor yielding flexibility in uses

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Final Notes

There is no single most adequate knowledgerepresentation formalism for everything.

Main points for selecting a representation formalism:

what should be represented, how should theknowledge be processed.

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241 320 Design Architecture & Knowledge Representation and 37

Reading

5 (Knowledge Representation)

5.5.3 5.5.4

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