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Tuesday, July 27, 2010

Semantic Nets.

DECLARATIVE REPRESNTATIONS

The following are the four declarative mechanisms, used for representing knowledge:

(a) Semantic Nets: - These describe both objects and events in general

(b) Conceptual Dependency:- Provides a way of representing relationships among components of an action.

© Frames:- A general structure to represent complex objects from several different points of view.

(d) Scripts: A sophisticated structure use to represent common sequence of events.

All these structures share a common notion that complex entities can be describe as a collection of attributes and associated values (hence they are often called “slot-and –filter structures”) ie., these structures have the form of ordered triples

OBJECTS x ATTRIBUTES x VALUE

Information can be retrieved from the knowledge base by an associative search with VALUE.

 Semantic Nets.

it is useful to think of semantic nets using graphical notation. In this information is represented as a set nodes connected to each other by a set of labeled arcs, which represent relationships between the nodes. A typical example (with ISA and ISPART relationships) is shown below.

The knowledge in the above semantic network is represented inside a program using some kind of attribute –value structure. The following gives a LISP representation of the above semantic net.


ATOM PROPERTY LIST


CHAIR ((IS A FURNITURE))


MY-CHAIR ((IS A CAHIR)


(COLOR TAN)


(COVERING LEATHER)


(OWNER ME ))


ME ((IS A PERSON))


TAN ((IS A BROWN))


SEAT ((ISPART CHAIR))


In predicate logic the above may be represented as

IS A (chair, furniture)


IS A ( me, person)


COVERING ( my-chair, leather)


COLOR (my-chair, tan)



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