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Knowledge Representation. Structured Objects. Structural knowledge is important e.g. viral meningitis is meningitis Representation is analogous to graphs or records. Motivation: Grouping of knowledge intuitively All knowledge about an entity is stored together

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knowledge representation

Knowledge Representation

Structured Objects

slide2
Structural knowledge is important
    • e.g. viral meningitis is meningitis
  • Representation is analogous to graphs or records.
  • Motivation:
    • Grouping of knowledge intuitively
      • All knowledge about an entity is stored together
    • Having intuitive access paths
  • Inferencing via:
    • Generalisations of situations
    • Properties and interrelationships
    • Inheritance of properties
      • Same benefits of object-oriented design and programming e.g. Saves on storage
semantic networks
Semantic Networks
  • Concentrate on categories of objects and the relations between them

BIRTHDAY-PARTY

ELEMENT-OF

JOHN’S-B-P

food

date

CAKE

AUGUST-3

guest

place

JOHN’S-B-P

MARY

slide5

Subset

LinkType SemanticsExample

A B A  B Cats  Mammals

A B A  B Bill  Cats

A B R(A,B) Bill 12

A B x x  A  R(x,B) Birds 2

A B x  y x  A  Birds Birds

y  B  R(x,y)

Member

R

Age

R

Legs

Parent

R

example
Example:

Person

member

member (is-a)

profession

Mary

lecturer

married-to

Joe

profession

engineer

lives-in

lives-in

is-a

Kingston

city

characteristics of semantic nets
Characteristics of Semantic Nets
  • Limited Expressiveness
    • Cannot express negation or disjunction
    • Quantification: Complex, using partitioned nets
  • Simple and easy to understand.
  • Syntax is not clear and consistent.
  • Semantics are intuitive and dependent on implementation
  • Inheritance is captured
  • Inferencing
    • Intersection Search
      • e.g. “What is the relationship between Joe and Mary?”
frames
Frames
  • A data structure representing a stereotype situation.
    • The pulling together of procedural and declarative knowledge.
  • Has slot names and slot fillers
  • Usually arranged in a hierarchy
    • Frames lower down inherit slot fillers from frames higher up.
    • Properties high up are fixed
    • Properties with values lower down overwrite information higher up
slide9

Animals

Alive:

T

Flies:

F

Subset

Subset

Birds

Mammals

Legs:

2

Legs:

4

Flies:

T

Subset

Subset

Subset

Penguins

Cats

Bats

Legs:

2

Flies:

F

Flies:

T

Member

Member

Member

Opus

Bill

Pat

Name: Pat

Name: Opus

Name: Bill

Friend:

Friend:

types of slots
Types of Slots
  • Attribute-value slots
    • Primitive data types

<name Jumbo>

    • Pointers to other frames

<owner e56>

  • Attribute slots with value restrictions

<owner (a person)>

<mother (an elephant with <sex female>) >

  • Object hierarchy slots
    • super-class/subclass slots
    • member-of/instance slots
  • Procedure slots
    • Used for calculations
      • instead of storing the value e.g. salary
    • Used to propagate changes when a slot value is changed
slide11
Example:

< e1 <member-of american-person

dog-owner>

<name “Mickey Mouse”>

<address “Disneyland”>

<owns e3>

<personality unpleasant> >

< e3 <member-of dog>

<name “Pluto”>

<owned-by e1> >

< dog-owner <superclass person>

<owns (a dog) >

<must-have (a dog-licence) >

<personality pleasant> >

<dog <superclass pet

carnivore >

< address (Get address of owned-by) >

reasoning in frame systems
Reasoning in Frame Systems
  • Matching
    • Find a matching frame
    • Difficult because an instance frame may have values from several class frames.
    • Potential inefficiency
  • Inheritance
    • If the value of the slot is not found in the instance frame, search up the hierarchy.
      • E.g. Does Bill fly? No
    • Allows for Default Reasoning
      • e.g. Does Opus fly?
        • No, since instance value overrides class value.
slide13
Allowing multiple inheritance can cause conflicts in reasoning.
  • Example:
    • Does Tweety fly?

Bird

Move: Fly

Ostrich

Move: Walk,

Not fly

Cartoon Bird

Tweety

    • Depends on the path taken for the search.
  • Strategy 1: Use path length (BFS)
    • Tweety does not fly.
slide14

Bird

Move: Fly

IS A

  • Problem with BFS: Does Tweety fly?

IS A

Ostrich

Move: Walk,

Not fly

Cartoon Bird

IS A

Plumed Ostrich

IS A

White-Plumed

Ostrich

instance

instance

Tweety

  • Strategy 2: Use Inferential Distance
    • Tweety does not fly.
slide15

Republican

Pacifist : false

  • Can still get a contradiction

IS A

Quaker

Pacifist : true

Conservative-Republican

Instance

Instance

Dick

Pacifist : ?

advantages
Advantages
  • More knowledge about the nature of the entities involved.
    • More than logic or production rules
  • Can represent highly structured knowledge
  • Easy
    • To maintain
    • To add new objects
  • Default Reasoning Possible
    • “The drawing of plausible inferences on the basis of less than conclusive evidence in the absence of evidence to the contrary.” (Moore 1985)
disadvantages
Disadvantages
  • Precise notion of meaning is absent
    • Translation work has been done, but has difficulties with default reasoning and procedural attachment.
  • Hard to represent rules (p,q  r)
problems
Problems
  • Draw a semantic net for the following information: Tweety is a canary who is a bird and all birds are animals. All animals breathe, typically birds have wings and their method of travel is by flying. A penguin is a bird whose method of travel is by walking. Discus how the queries “How does Tweety travel?”, “Does Tweety have wings?” and “What is the link between “Tweety and Penguins?” will be executed.
slide19
Birds are usually, covered with feathers, fly and reproduce by laying eggs. There are three groups of birds, flightless birds, songbirds and scavengers. Flightless birds do not fly. Song birds eat bugs and seeds while scavengers eat meat. Sparrows and canaries are both flightless birds, the difference between them being that canaries are found in tropical countries, while sparrows are found in North America. Penguins are flightless birds; they eat fish and are found in the South Pole. Opus, Tweety and Beaky are all birds; Opus is a penguin, Tweety is a canary and Beaky is a rather unusual bird as he is a mix between a penguin and a canary.

Represent the information above using frames. What would be the results of the following queries?

  • “How does Opus reproduce?”
  • “Does Beaky fly?”
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