ARTIFICIAL INTELLIGENCE [INTELLIGENT AGENTS PARADIGM] Professor Janis Grundspenkis Riga Technical University Faculty of Computer Science and Information Technology Institute of Applied Computer Systems Department of Systems Theory and Design KNOWLEDGE REPRESENTATION SCHEMAS

Knowledge Representation Schemas Logical schemas –First-order logic –Higher-order logic Procedural schemas –Rule-based systems Network schemas –Semantic networks –Conceptual graphs Structured schemas –Frames –Scripts

Semantic Networks Definition: Semantic network (Quillian, 1967) is a knowledge representation schema that captures knowledge as a graph. The nodes denote objects or concepts, their properties and corresponding values. The arcs denote relationships between the nodes. Both nodes and arcs are generally labelled (arcs have weights).

Example of Semantic Network headanimal part of bird is a fly travel feathers covering fish is a wings part of ostrich is a walk travel penguin travel is a color has value brown canary color has value yellow sound sing robin covering skin is a sound swim travel tweety is a color white has value color has value red opus is a

Semantic Networks Typical relationships –IS-A –PART-OF –HAS –VALUE –LINGUISTIC

Semantic Networks Typical relationships (continued) –IS-A Supertype – type (superclass – class) Type – subtype (class – subclass) Subtype – instance (subclass – instance) –PART-OF Supertype – type (superclass – class) Type – subtype (class – subclass)

Semantic Networks Typical relationships (continued) –HAS Object – property –VALUE Property – value –LINGUISTIC Examples: likes, owns, travel, made of, …

Semantic Networks NETWORK EXTENSION –Addition of new nodes and their relationships –Three ways of node addition: Similar object More specific object More general object

Semantic Networks REASONING –Query node –Answer search Answer localization at the query node Path construction following labels of arcs

Semantic Networks INHERITANCE –Definition: Inheritance is a process by which the local information of a superclass node is assumed by a class node, a subclass node, and an instance node

Semantic Networks INHERITANCE (continued) –“+” 1) Provides a natural tool for representing taxonomically structured knowledge. 2) Provides economical means of expressing properties common to a class of objects. 3) Reduces the size of a knowledge base. 4) Provides more compact code –“–” 1) Exception handling by local priority. 2) Additional workload of knowledge engineer who must decide at which node to define common properties

Conceptual Graphs DEFINITION: A conceptual graph (John Sowa, 1984) is a finite, connected, bipartite graph The nodes of the graph denote either concepts or conceptual relations Conceptual graphs do not use labelled arcs; instead the conceptual relation nodes represent relations between concepts Concepts can only have arcs to conceptual relations, and vice versa Concepts are represented as boxes and conceptual relations as ellipses

Conceptual Graphs Concept nodes represent: 1.Concrete concepts (objects), for instance, cat, telephone, book, etc. These concepts are characterised by our ability to form an image of them in our minds. Concrete concepts include generic concepts such as cat or book along with concepts of specific cats and books 2.Abstract concepts, for instance, beauty, loyalty, and love that do not correspond to images in our minds

Conceptual Graphs Conceptual relation nodes indicate a relation involving one or more concepts. Some special relation nodes, namely, agent, recipient, object, experiencer, are used to link a subject and the verb Conceptual graphs can represent relations of any arity A relation of arity n is represented by a conceptual relation node having n arcs

Example of Conceptual Graph person: john agent eat object soup instrument hand part

Conceptual Graphs Each conceptual graph represents a single proposition Knowledge base contains a set of conceptual graphs Graphs may be arbitrary complex but must be finite

Frame Based Systems The term frame is coined by Minsky in Frame is a static data structure used to represent stereotypical information about some concept. Frame is like a template that contains generic information about some concept that you could refer to for describing a given instance of the concept.

Frame Based Systems A frame has: –Name – frame identification information. –Slots with labels describing the attributes and possible values for each attribute.

Frame Based Systems The slots contain: –Frame identification information –Relationship of this frame to other frames –Frame default information (slot values that are taken to be true when no evidence to the contrary has been found) –New instance information (slots may be left unspecified) –Procedural information (procedural code may be attached to the slot) –Descriptors of requirements for frame match

Frame Based Systems ATTRIBUTES –STATIC –DYNAMIC VALUES –NUMERIC –SYMBOLIC –BOOLEAN

Frame Based Systems Sources of attribute values : –Initialize –Database –Procedure –Expert System –User –Inheritance –Other Frame (object)

Frame Based Systems CLASS Definition: Class is a collection of objects that share some common properties (attributes). A class frame contains –A descriptive name of the concept –A set of attributes that are characteristic of all its associated objects –Attribute values that considered common to these objects It may contain: 1) An explicit reference to all of its associated subclasses; 2) Information describing the behaviour of the concept.

Frame Based Systems SUBCLASSES Definition: Subclasses are classes that represent subsets of higher level classes. Three kinds of class relationships: –Generalization – “Kind of” relationship –Aggregation – “Part of” relationship –Association – “Semantic” relationship

Frame Based Systems INSTANCE Definition: Instance is a specific object from a class of objects. AN INSTANCE FRAME –Describes A specific object from its related class. –Contains All of the characteristics of the class frame as well as a specific information (specific features and property values).

Example of Frame Based System superclass: vehicle reg. number producer model owner truck class: vehicle reg. number producer model owner tonnage part ofbasket car class: vehicle reg. number producer model owner number of doors4 horse-power John’s car class: car reg. numberLV97 producerBMW model520 ownerJohn number of doors2 horse-power150 basket dimensions2*3*1.5 materialtin John age22 length of driving2

Frame Based Systems INHERITANCE Definition: Inheritance is the process by which the characteristics of a parent frame are assumed by its child frame. Note: In general, a child frame will inherit information from its parents, grandparents, great- grandparents, etc. MULTIPLE INHERITANCE An object could inherit information from more than one parent (in this case objects relate to different worlds)

Frame Based Systems INHERITANCE Practical value: –Easier coding of the system –Easier modification of information in a frame (adding new property to the class frame it will be inherited automatically by all of its instances) Potential problem: exception handling –Any frame that is an exception from the norm, that is, the frame has some property value unique to itself, this value must be explicitly encoded in the frame.

Frame Based Systems FACETS Definition: A facet is an extended knowledge about a frame’s property. Facets provide additional control over property value and the operation of the system.

Frame Based Systems A facet extends the information in the following ways: –TYPE – Defines a type of value that can be associated with the property –DEFAULT – Defines a default value, i.e., an initial value for the property –CONSTRAINT – Defines the allowable value

Frame Based Systems –MINIMUM CARDINALITY – Establishes min number of values –MAXIMUM CARDINALITY – Establishes max number of values –IF-NEEDED – Specifies action to be taken if the property’s value is needed –IF-CHANGED – Specifies action to be taken if the property’s value is changed

Frame Based Systems METHODS (1) Definition: Method is a procedure attached to an object, that will be executed whenever requested. An IF-NEEDED method is written in order to obtain the property’s value only when it is needed. IF-NEEDED facet executes some method only when it is needed (method acts like a demon).

Frame Based Systems METHODS (2) An IF-CHANGED method is written in order to change an object’s property value, access database, etc. IF-CHANGED facet executes some method that performs some function in the event the property's value changes. Methods designed to perform both operations can be inherited.

Example of Methods Temperature sensors Name UnknownCritical Value Unknown ValueUnknown StatusUnknown Get-Value (Self.Name) IF Self.Status = Alert THEN Sound-Alert Value If-Needed Method Status If-Changed Method IF Self.Value > Self.Critical Value THEN Self.Status = Alert Value If-Changed Method

Frame Based Systems COMMUNICATION BETWEEN OBJECTS (1) Interobject communication using facets –Using facet methods objects can communicate with one another. A change to only one property value may cause a series of changes in a number of objects (frame system is dynamic!). Objects can influence the property values in other objects, or even itself.

Frame Based Systems COMMUNICATION BETWEEN OBJECTS (2) Message passing Definition: Message passing is a signal to an object to which the object responds by executing a method. –Sending messages involves using a function: (SEND message-name, object-name, arguments)