Semantic Networks and Frames - Class room slides and self taught

CSE Dept., JNV University

Semantic Networks and Frames

KR Chowdhary, Associate Professor,

Department of Computer Science & Engineering, MBM Engineering College, JNV University, Jodhpur,

[email protected], Page:http://www.krchowdhary.com


Knowledge Representation as a medium for human expression

� An intelligent system must have knowledge representations that can be interpreted by humans.

� We need to be able to encode information in the knowledge base without significant effort.

� We need to be able to understand what the system knows and how it draws its conclusions.


Knowledge Representation

� Logic (prepositional, predicate)

� Network representation

� Semantic nets

� Structured representation

� Frames

� Issues in KR

� Hierarchies, inheritance, exceptions

� Advantages and disadvantages


KNOWLEDGE REPRESENTATION

The Farmer, The Fox, The Goose and The Grain

The farmer must get a fox, a goose and a sack of grain across a river, however his boat is small and he can only carry one thing at a time. His problem is that if he leaves the fox with the goose the goose will be eaten, and if he leaves the goose with the grain, the grain will be eaten. . . .


The Farmer, The Fox, The Goose and The Grain

A good representation makes it easier for us to solve the problem:

1. Draw possible safe combinations in a diagram.2. Arrange appropriate combinations in order.3. Link appropriate arrangements to represent

boat trips.4. Problem is solved!

KNOWLEDGE REPRESENTATION


Grain

Fox

Farmer

Goose

Farmer

Goose

Grain

Fox

Farmer

Fox

Goose

Grain

Fox

Grain

Farmer

Goose

Farmer

Fox

Grain

Goose

Goose

Farmer

Fox

Grain

Farmer

Goose

Fox

Grain

Farmer

Goose

Fox

Grain

Fox

Farmer

Goose

Grain

Farmer

Fox

Goose

Grain


KNOWLEDGE REPRESENTATION: 1960’S NETWORKS & MEANING

•Ross Quillian (1966 and 1968) was among the early AI

workers to develop a computational model which

represented 'concepts' as hierarchical networks.

•This model was amended with some additional

psychological assumptions to characterise the structure

of [human] semantic memory.


Semantic Networks

� First introduced by Quillian back in the late-60s M. Ross Quillian. "Semantic Memories", In M. M. Minsky, editor, Semantic

Information Processing, pages 216-270. Cambridge, MA: MIT Press, 1968

� Semantic network (also called Associative Network) is simple representation scheme that uses a graph of labeled nodes and labeled directed arcs to encode knowledge

� Nodes are: objects, concepts, events

� Arcs are: relationships between nodes

� Graphical depiction associated with semantic networks is a big reason for their popularity


Nodes and Arcs

� Arcs define binary relations which hold between objects denoted by the nodes.

Sue John 5

Max34

mother age

fatherage

wifehusband

mother (john, sue).

age (john, 5).

wife (sue, max).

age (max, 34).

…age

Corresponding Predicates:


Non-binary relations

� We can represent the generic GIVE event as a relation involving three things: � A giver

� A recipient

� An object

Mary GIVE John

book

recipient giver

object


Inheritance

� Inheritance is one of the main kind of reasoning done in semantic nets

� The ISA (is a) relation is often used to link a class and its super-class.

� Some links (e.g. haspart) are inherited along ISA paths

� The semantics of a semantic net can be relatively informal or very formal (Often defined at the implementation level)

Bird

Robin

Rusty

isa

Red

isaisa

Animal

isa

WingshasPart


NETWORKS & MEANING

•Concepts can be represented as hierarchies of interconnected concept nodes (e.g. animal, bird, canary)

•Any concept has a number of associated attributes at a given level( e.g. animal --> has skin; eats etc.)

• Some concept nodes are super-ordinates of other nodes (e.g. animal >bird) and some are subordinates (canary< bird)

•For reasons of cognitive economy, subordinates inherit all the attributes of their super-ordinate concepts

• Some instances of a concept are excepted from the attributes that help

[humans] to define the super-ordinates (e.g. ostrich is excepted from flying)

•Various [psychological] processes search these hierarchies for information

about the concepts represented


Parts of a semantic representation

� 4 parts

1. Lexical-which symbols are allowed in the representation’s vocabulary

2. Structural-describes constraints on how the symbols can be arranged

3. Procedural-specifies the access procedures (to create, modify, answer questions)

4. Semantic-establishes the way of associating the meaning


Semantic nets convey meaning

� Lexical parts – nodes to denote objects, links denote relation between objects, link-labels denote particular relations

� Structural-nodes are connected to each other by links.

� Procedural- procedures are : constructor procedure, reader procedure, writer procedure, and erasure procedure

� Semantic-nodes and links denote application specific entities


a n im a ls k i n

f i s h

s w im m in g

b i r d

f l y i n g

f e a t h e r s

p e n g u in c a n a r y r o b i no s t r i c h

w a lk i n g

O p u s

T w e e t y

y e l l o w

r e d

w h i t e

c o v e r e d _ b y

t r a v e l s _ b y

i s ai s a

i s a i s a i s a i s a

c o v e r e d _ b y

t r a v e l s _ b y

t r a v e l s _ b y

t r a v e l s _ b y

i n s t a n c e _ o f

i n s t a n c e _ o f

c o l o u r

c o l o u r

c o l o u r


A Hierarchical Network

birdcan fly, has wings,

has feathers

salmonlays eggs; swims upstream,

is pink, is edible

ostrichruns fast, cannot fly,

is tall

canarycan sing, is yellow

fishcan swim, has fins, has gills

animalcan breathe, can eat,

has skin

is-a

is-a

is-a

is-a

is-a


•From the above taxonomic organisation of knowledge about a number of

different animals, one can conclude, by ‘inheriting properties down the

taxonomy’, that canaries, ostriches and salmon all have skin and can breathe.

•But we as humans can also make exceptions to inherited properties in that we

can represent an non-flying bird in a (sub-) hierarchy of birds by simply noting

the exception, 'can't fly'. Like, in the case of Ostrich.


has skin


has feathers


is pink, is edible


is tall



is-a

is-a

is-a

is-a

is-a


Collins and Quillian carried out a number of tests on human subjects and found that

individuals recognise propositions lower down the hierarchy (canary is a yellow bird)

more readily than propositions higher up the hierarchy (canary has skin).


has skin


has feathers


is pink, is edible


is tall



is-a

is-a

is-a

is-a

is-a


A semantic network is a structure for representing knowledge as a pattern of

interconnected nodes and arcs. Nodes in the net represent concepts of entities,

attributes, events, values. Arcs in the network represent relationships that hold between the concepts.


has skin


has feathers


is pink, is edible


is tall



is-a

is-a

is-a

is-a

is-a


Multiple Inheritance

� A node can have any number of super-classes that contain it, enabling a node to inherit properties from multiple parent nodes and their ancestors in the network. Some times it may cause conflicting inheritance.

RQ

N

P ? !P


Advantages of Semantic nets

� Easy to visualize

� Formal definitions of semantic networks have been developed.

� Related knowledge is easily clustered.

� Efficient in space requirements

� Objects represented only once

� Relationships handled by pointers


Disadvantages of Semantic nets

� Inheritance (particularly from multiple sources and when exceptions in inheritance are wanted) can cause problems.

� Facts placed inappropriately cause problems. � No standards about node and arc values


Knowledge Representation

using Frames


Frames� Devised by Marvin Minsky, 1974.

� Incorporates certain valuable human thinking characteristics:

� Representation of stereotypes situations

� The essence of this form of knowledge representation is typicality, with exceptions, rather than definition.


� Frames are semantic net with properties

� A frame represents an entity as a set of slots (attributes) and associated values

� A frame can represent a specific entry, or a general concept

� Frames are implicitly associated with one another because the value of a slot can be another frame

� The idea of frame hierarchies is very similar to the idea of class hierarchies found in object-orientated programming.

Frames and Inheritance


Organization of frames

� A frame system is a hierarchy of frames

� Each frame has:� a name.

� slots: these are the properties of the entity that has the name, and they have values. A particular value may be:

� a default value� an inherited value from a higher frame� a procedure, called a daemon, to find a value� a specific value, which might represent an exception.


Organization..

� In the higher levels of the frame hierarchy, typical knowledge about the class is stored.

� The value in a slot may be a range or a condition.

� In the lower levels, the value in a slot may be a specific value, to override the value which would otherwise be inherited from a higher frame.

� An instance of an object is joined to its class by an 'instance_of' relationship.

� A class is joined to its superclass by a 'subclass_of' relationship.


Organization …

� Frames may contain both procedural and declarative knowledge.

� Slot values normally amount to declarative knowledge, but a daemon is in effect a small program. So a slot with a daemon in it amounts to procedural knowledge.

� A frames system may allow multiple inheritance but, if it does so, it must make provision for cases when inherited values conflict.


Frames: some examples

� We start with a simple piece of information: there is a category of things called cars.

� Given this information, we can start to build a frame:

Name: car Subclass of: thing


Name: car Subclass of: thing

Slots:

Name: Value: Restrictions:

wheels 4

moved by engine

fuel ? petrol or diesel

car subclass_of

thing

with

wheels: 4,

moved_by:

engine,

fuel:

[value:

unknown,

type:

[petrol,diesel]].

� Add More information: a car has 4 wheels, is moved by an engine, and runs on petrol or diesel.

� Now add three slots to the frame.

� Last of these has a restriction rather than a specific value.


Name: VW Subclass of: car

Slots:


made in Germany

‘VW’ subclass_of

car

with

made_in:

‘Germany’.

� We can add a second frame to our system, with one slot. We do not need to repeat the slots and values in the previous frame: they will be inherited.

� Add More information: there is a particular type of car called a VW, manufactured in Germany.


Name: Golf Subclass of: VW

Slots:


top sunroof

‘Golf’ subclass_of

VW

with

top: sunroof.

� More information: there is a particular type of VW called a Golf, which has a sun-roof.

� We can add a third frame to our system, with one slot. Once again, we don’t repeat the slots in the previous frames, becausethey will be inherited.


Add 4th frame: “there is a particular type of Golf called a TDi, which runs on diesel, has 4 cylinders,

and has a 1.8 litre engine.”

Name: TDi Subclass of: Golf

Slots:


fuel diesel

engine

capacity 1.8 litres

cylinders 4

‘TDi’ subclass_of

‘Golf’

with

fuel: diesel,

engine_capacity:

1.8,

cylinders: 4.


�Title � AI. A modern Approach

�Author � Russell & Norvig

�Year � 2003

Slot � Filler

Book Frame

frame name

attributes (slots)

values (fillers: list of values, range, string, or procedures etc.)

components of a frame


Inheritance� Similar to Object-Oriented programming paradigm

�what � room

�where �hotel

�contains�

�hotel chair

�hotel phone

�hotel bed

Hotel Room �what � chair

�height �20-40cm

�legs � 4

Hotel Chair

�what � phone

�billing � guest

Hotel Phone

�what � bed

�size �king

�part �mattress

Hotel Bed

�price �INR.1000

Mattress


Summary:Features of Frame Representation

� More natural support of values then semantic nets (each slots has constraints describing legal values that a slot can take)

� Frame may describe instances or classes

� A special slot, the Is-a slot, short for is-a-member-of-the-class, ties instances to the class that they are member of.

� Another special slot, ako slot, short of a-kind-of, ties classes together.

� Can be easily implemented using object-oriented programming techniques

� Inheritance is easily controlled


Frames have access procedures

� Needed to manipulate instances and classes

� A class constructor: makes a frame that has one direct super-class

� An instance constructor: makes instance frames. Input is name of the class to which the instance belongs. New instance is connected to class by is-a slot.

� A slot writer: installs slot values. Its input is the name of the frame, the name of slot, and value.

� A slot reader: retrieves slot values. Its input is frame name, slot name, and output corresponding value.


Advantages of Frames

� Makes programming easier by grouping related knowledge

� Easily understood by non-developers

� Expressive power

� Easy to set up slots for new properties and relations

� Easy to include default information and detect missing values


Drawbacks of Frames

� No standards (slot-filler values)

� More of a general methodology than a specific representation:

� Frame for a class-room will be different for a professor and for a maintenance worker

� No associated reasoning/inference mechanisms

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Semantic Networks and Frames - Class room slides and self taught

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