Knowledge Representation and Objects

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Knowledge Representation and Objects
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Representing objects

• In the procedural approach objects tend to be
  entangled in the algorithm in question and
  difficult to extract for use in other
  circumstances.
• The advantages of re-use are many and this served
  as a motivation for the development of an object
  oriented approach to both to programming and also
  to knowledge representation.

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Objects

• Objects are at the heart of the Object Oriented
  Paradigm
• What is an object?

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Objects

• We are surrounded by objects.
• In this class room there are desks, blackboards,
  lights , chairs and so on.
• Each object has specific attributes
• The desk is made of wood. The seat has a blue
  cover and so on.

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Objects have associations and relationships with
each other .

• Objects have associations and relationships with
  each other .
• Seats have desktops attached.
• All are attached to the floor
• Seats are adjacent to other seats.
• They are oriented in the same way.
• Lights are controlled by switches

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Object Roles and Functions

• Each Object in the room has a specific role or
  function or behaviour.
• Seats are to be sat on.
• Board is to be written on.
• Lights can be turned on and off
• Boards can be moved up and down and seats can be
  put up and down.

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Objects can be classified

• Within the room objects can be grouped into
  different classes
• For example we could have the furniture objects
  e.g. chairs, tables, worktop
• We could also have the electrical objects.
• Lights, projector, laptop and so on.

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The nature of Objects

• Clearly Objects constitute a complex multi
  faceted concept.
• Their definition is dependent on many elements-
  their attributes , their behaviour, their
  classification and as we will see much , much
  more
• To investigate the nature of objects , it is
  helpful to consider an important knowledge
  representation scheme which is the basis for
  theoretical frameworks of the object oriented
  paradigm
• This framework is known as semantic networks

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Semantic Nets

• Semantic networks are a popular scheme which
  elegantly reflect these ideas.
• A network consists of nodes repesenting objects,
  concepts and events and links between the nodes
  representing their interrelations.

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Example

• Using the example Birds have wings
• a typical semantic net would be

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Origin

• The development of semantic networks had its
  origins in psychology. Ross Quillian in 1968
  designed two semantic network based systems that
  were intended primariliy as psychological models
  of associateive memory.
• Semantic Networks quickly found application in
  AI. B. Raphaels SIR system, also 1968, was one of
  the first programs to use this type of
  representation scheme.
• SIR was a question Answering system and could
  answer questions requiring a variety of simple
  reasoning tasks and relationships

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Meaning of Semantic Nets

• The semantics of net structures however depends
  solely on the program that manipulates them and
  there are no fixed conventions about their
  meaning.
• A wide variety of network based systems have been
  implemented that use totally different procedures
  for making inferences.

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Another Example
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Evolving Features

• While there are no fixed conventions, a number of
  important features of Semantic nets have emerged
  , that are widely used.
• These have largely emerged because of the
  application of Semantic Nets to Object Oriented
  Theory.

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Object Definitions

• The central aspect of the object paradigm is how
  it defines objects.
• The basic mechanism of representation is the
  articulation of class hierarchies.
• Instances of Objects exist. In turn Objects
  belong to classes and these in turn can belong to
  other classes

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Example

• Consider Fido ( who is a dog)
• Fido is instance of the object dog.
• Dogs belong to the class pets, which for example
  could also include other classes such as cats.
• Pets in turn belong to a class animals and so on

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Animals
Pets
Cats
Dogs
Fido
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Labels used in Semantic Nets

• Objects and Instances
• Both Represented by Nodes linked by an
• IS_A link

Fido
Dog
IS_A
Instance
Object
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Labels used in Semantic Nets

• Objects and Classes
• Both Represented by Nodes linked by an
• Subset or a SuperSet link

Dog
Pets
Subset
Object
Class
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SuperSet links

• Objects and Classes
• Both Represented by Nodes linked by an
• SuperSet link

Dog
Pets
Subset
Object
Class
Superset
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Relationships, attributes and associations

• Relationships, attributes and associations
• Represented by a labelled link between objects
  etc

Fido
Black and white
colour
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Component Parts

• Object components
• Both Represented by Nodes linked by an
• HAP (has as part) link
• A dog has a tail

Dog
Tail
HAP
Object
Object
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Inheritance

• Attributes of classes are inherited by subclasses
  and instances of objects
• Because we know dogs have tails and Fido is a dog
  we know Fido has a tail since this is inherited
  from the parent class

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Inheritance

• A dog has a tail and Fido is a dog

Dog
Tail
HAP
ISA
Object
Object
Fido
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Bigger Example

• Exercise
• What does the following Semantic net represent

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Building
subset
Purpose
Habitation
House
No. Of Storeys
subset
brick
Bungalow
1
Made_of
ISA
Colour
HAP
HAP
My House
Roof
walls
Red
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Advantages of Semantic Nets

• Easy to visualize Graphical in nature easy
  for humans to interpret
• Expressive power equal to or exceeding that of
  First Order Predicate Logic
• Formal definitions of semantic networks have been
  developed for use
• Related knowledge is easily clustered logically
  and physically close
• Efficient in space requirements
• Objects represented only once
• Relationships handled by pointers
• Other schemes are limited to True or False
  answers where as Semantic Nets are more
  informative and flexible
• Not limited to only binary representation can
  also represent action concepts

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Disadvantages of Semantic Nets

• Inheritance (particularly from multiple sources
  and when exceptions in inheritance are wanted)
  can cause problems such as conflicts
• Facts placed inappropriately cause problems
• No standards about node and arc values in spite
  of the generic formal definitions

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Frames

• The representation of knowledge about objects and
  events typical to specific situations has been
  the focus of research into a concept called
  frames.
• Frames were originally proposed in 1975 by
  Minsky, as a basis for understanding visual
  perception, natural language dialogues and other
  complex behaviours.

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How does a frame work?

• Attached to each frame are several different
  kinds of knowledge.
• Some knowledge is how to use the frame, some is
  about the different elements of the frame, some
  is about what to expect in given situations and
  some is what to do if the specified conditions do
  not materialise.
• Frames contain a representation mechanism called
  a slot.
• Knowledge which is relevant to part of a given
  situation can be entered into a slot. thus the
  composite pieces of knowledge that go to make up
  the larger entity can be represented as slots
  within the larger structure of a frame.

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What are slots again?

• A frame is a composite data structure which
  consists of a number of slots which correspond to
  various aspects of the object being represented.
  For example slots in the frame could contain
  information such as

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Frame elements

• 1 frame identification information.
• 2 relationship of this frame to other frames.
• 3 descriptors of requirements for frame match.
  for example the chair frame might specify a flat
  base on four vertical supports with a back
  support at 90 degrees to the base.

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More Frame elements

• 4 procedural information on the use of the
  structure called demons.
• 5 frame default information for example as
  default a chair has four legs.
• 6 new instance information. Many frame slots are
  left unspecified until a particular instance of
  the structure is identified. For example the
  colour of the chair might not be filled in until
  a particular instance is identified.

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Inheritance

• Frames support inheritance
• Class types are represented by frames which omit
  the detail of objects of the class, for example
  the frame representing a three legged chair
  object could inherit the slots of a class frame
  representing an abstract chair and augment these
  with its own particular detail to describe the
  new object.
• 8 The big problem with frames is controlling
  their invocation particularly when a number of
  them are applicable to a situation. e.g.
• The man broke a leg when he fell onto the chair.
  Would the man frame or the chair frame take
  precedence.

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In Summary

• Frames then are data structures for representing
  composite entities.
• The top levels of a frame are fixed and represent
  things that are always true about a supposed
  situation. The top levels therefore give a high
  level abstract view of the situation. Only the
  details are missing.
• The lower levels have many slots that must be
  filled by specific instances or data. That is the
  details of the specific situation can be added at
  these levels.

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Adding new knowledge

• The slot mechanism is a modular concept. New
  knowledge pertaining to a frame can be added
  simply by creating new slots.
• Conversely redundant knowledge may be deleted by
  removing slots from a frame.

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Static versus variable slots

• The idea of having fixed higher and variable
  lower levels is entirely suitable for
  representing different versions of the one
  context.
• This is so because different instances of the
  same context differ only in detail.
• For example any room has walls, a ceiling and a
  floor but one instance of a room differs from
  another in what it contains and how these
  contents are arranged i.e. in the specific
  details of that room.

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Demons

• Procedures called demons can be attached to slots
  of a frame.
• Thus algorithmic knowledge (or Behaviour) can be
  incorporated into a frame based system.
• One interesting function of these demons is to
  activate other appropriate frames.

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Frames and stereotyped situations

• Many everyday situations are stereotyped or
  conform to a familiar pattern. waking or opening
  a door are examples of stereotyped situations.
• Most of the events associated with these
  situations are expected. The slots of a frame
  are therefore filled with default values to
  mirror these expected events.
• However if we are trying to represent new or
  uncertain situations then default values tend not
  to work very well. More sophisticated inference
  is generally needed.
• The notion of using stereotypes would reflect
  thinking among psychologists that we use previous
  experiences to deal with situations that we have
  never met before.

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Problems

• There are three major problems involved in the
  use of frames.
• 1 When should a frame be activated i.e. when
  does it become relevant.
• 2 When should a frame be de-activated i.e. when
  does it become irrelevant.
• 3 What happens if a situation arises in which no
  frames are relevant.

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Applications of Frames

• When Minsky first published his ideas on frames,
  it started a flurry of research into its use.
• A side development but one that is very similar
  is the development of Object Oriented Orogramming
  Paradigms a huge area in its own right.
• Modern AI research is considering the notion of
  flavors i.e. a frame based structure which
  introduces concepts by presenting a stereotypical
  flavor of it.
• One major applications Minsky proposed for frame
  based structures is the idea of scripts.

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Scripts

• A script is a structured representation
  describing a stereotyped sequence of events in a
  particular context. The script was originally
  deigned by Schank and Ableson 1977.
• A typical script is the restaraunt script.
• The customer recognizes the restaraunt by a sign
  on the door. They are met at the door and
  directed to an appropriate table by the waiter.
• They are brought menus. They order the meal which
  is brought by the waiter. They eat the meal, pay
  and leave.
• This is fairly typical scenario and comfortably
  describes the average trip to restaurant.

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Script Representation

• To represent scenarios like this a script has a
  number of components.
• 1 Entry Conditions or descriptors of the world
  thaty must be true for the script to be called.
  In a restaraunt script these might be for example
  a restaurant that is open and a customer who is
  hungry.
• 2 Results or facts that are true when the script
  has terminated, for example the customer is full
  and the money has been paid.
• 3 Props or the things that make up the content
  of the script including cast for example menu,
  money, tables, waiters, customers and so on.

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More

• 4 Roles are the actions that the individual
  participants perform, for example the waiter
  shows the customer to their seat, brings the menu
  and food and accepts the money at the end.
• 5 Scenes These provide a temporal partitioning
  of the script for example in the restaurant the
  scenes and their order might be entering,
  ordering, eating, paying , leaving.

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Applications of scripts

• Scripts can be used to answer questions about
  particular situations. For example it would be
  easy to answer questions like Who brought the
  food, With what did the customer pay etc.
• They can be used to resolve referential
  ambiguities because each prop has a specific role.

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• Of the disadvantages of scripts the biggest
  problem is when two or more scripts are
  applicable to a given situation.
• Consider for example the text
• "John visited his favourite restaurant on the way
  to the concert. He was pleased by the bill
  because he liked Mozart"
• In this case which should take precedence a
  concert script or a restaurant script.

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Ambiguity in Natural Language

• Exact meaning can still present problems because
  of the idiosyncratic ambiguities in English.
• For example the following text will present
  problems.
• "John was eating dinner at his favourite
  restaraunt when a large piece of plaster fell
  from the ceiling and landed on his date"
• Was the date on his plate or was he gazing into
  her eyes.