Overview. COMP 4200: Expert Systems. Knowledge Representation 2. Knowledge Representation 1

Transcription

1 Overview COMP 4200: Expert Systems Dr. Christel Kemke Department of Computer Science University of Manitoba Knowledge Representation for XPS Associative Nets and Frame Systems (Jackson, Chapter 6) Object-oriented Programming (Jackson, Chapter 7) C. Kemke Knowledge Representation 1 C. Kemke Knowledge Representation 2 Knowledge Representation 1 Representation of declarative knowledge (what, objects, structure) procedural knowledge (how, actions, performance) Representation Formalisms for declarative knowledge Frames, Semantic Nets, Inheritance Hierarchies, Schemata,... procedural knowledge Algorithms, Procedures, Plans, Rules,... Knowledge Representation 2 Representation of declarative knowledge: descriptions of objects, concepts: relations between them e.g. has-parts, father features e.g. number-of-legs, age mechanisms to work with this knowledge: store, retrieve information (accessing KB) e.g. TELL (KB1, IS-A (square,polygon)) KB1 is a knowledge base; in KB1 a square is a sub-concept or subclass of the polygon-concept/class; TELL asserts this. reason about objects (inferencing) if square(x) then polygon(x) if square(x) then number-of-sides (X)=4 If something is a square, it is also a polygon. A square thing has 4 sides. C. Kemke Knowledge Representation 3 C. Kemke Knowledge Representation 4 1

2 Graphs as Representations I: Nodes, Links, and what they represent Semantic Networks (Quilian 1968) nodes represent concepts, objects, events e.g. person, John, restaurant links (arcs) represent any kind of relation or association between concepts, objects, events e.g. IS-A, owns, lives-at, name This allows the representation of all kinds of semantic expressions BUT the semantics is not clearly defined (Woods 1975) -> epistemological confusion. C. Kemke Knowledge Representation 5 Causal, Temporal, and Inheritance Networks Causal Networks (Jackson, Fig. 6.2, see next slide) nodes represent concepts, objects, events links represent causal relationship between these concepts, objects, events Temporal Networks nodes represent events links represent temporal relationship between events, like before, after,... Inheritance Networks (Terminologies, Taxonomies) nodes represent concepts links represent class-/subclass-relationship IS-A: superclass subclass or super-concept / sub-concept C. Kemke Knowledge Representation 6 Causal Network Example (Jackson, Fig. 6.2) Classification Hierarchy (Jackson, Fig. 6.3) C. Kemke Knowledge Representation 7 C. Kemke Knowledge Representation 8 2

3 Graphs as Representations II: Nodes, Links, and what they represent Semantics of Knowledge Representation Languages: formal semantics e.g. Predicate Logic Interpretation, derive meaning of complex expressions based on meaning of atomic expressions plus construction mechanism use / reasoning e.g. Spreading Activation positive or negative association between concepts; see Neural Networks Inheritance Networks Inheritance Networks (Terminologies, Taxonomies) nodes represent concepts (events, objects, actions,...) links represent super-concept / sub-concept-relationships IS-A: specialization / subsumption of concepts concept-instance-relationships instance-of relationships between concepts role (slot) attributes/features/properties of concept constraints attached to roles, e.g. number of fillers Closely related to First-Order Predicate Logic. C. Kemke Knowledge Representation 9 C. Kemke Knowledge Representation 10 Terminological Network Example Example: Concepts: bird, robin, flying-animal, Speedy Feature: color IS-A (robin,bird), IS-A (bird,flying-animal) instance-of ( Speedy, robin) color (robin)= grey Superclass Instance Feature Task: Express that a typical elephant has legs, usually 4 of them, has a certain color, and there is a specific elephant named Clyde. elephant, color, legs, has (usually) 4 legs, Clyde Solution: Concepts: Roles: Feature: Terminological Network - Solution elephant, legs, Clyde (instance) or Clyde (individual concept), (color and grey) has-legs, (has-color) color Specific Representation of Clyde: has-legs (elephant, legs, 4) has-color (elephant, grey) or color (elephant) = grey instance-of ( Clyde, elephant) specific object Clyde IS-A (Clyde, elephant) individual concept Clyde C. Kemke Knowledge Representation 11 C. Kemke Knowledge Representation 12 3

4 Frames Frames, Schemas, Prototypes concepts as record-like structures slots relationships to other concepts, attributes fillers values for slots (other concept or value) Schema-Theory / Prototypes some objects are more typical for a certain class of objects precise definition for concepts sometimes not possible, then reference to prototypes Frames, Schemas, Prototypes Typical bird is robin take all robin-features as description for class bird. This forms a prototype. Take typical chair as prototype. Other chairs are more or less similar to this prototypical chair. The class of all chairs is fuzzy since there are no precise or exact boundaries for the class 'chair', i.e. to decide when something is a chair or not. C. Kemke Knowledge Representation 13 C. Kemke Knowledge Representation 14 Frames, Defaults, and Demons Frames represent concepts as record-like structures slots relationships to other concepts, attributes fillers values for those slots attached procedures to determine slot-fillers Defaults represent standard values (fillers) for some attributes (slots) of a concept (frame) Demons are activated by a certain action or pattern if-added demon - activated when value is added or updated, e.g. re-calculate area-value if side-info changes if-needed demon - activated when value is accessed, e.g. calculate area-value based on default assumptions if this slot-filler is required Defaults Defaults represent standard-values for some attributes of a concept, e.g. the standard number of legs of an elephant is 4. (Inherited) defaults may be overwritten at lower-level concepts, or for individual concepts. Clyde - the famous 3-legged AI-elephant. Problem: If roles, attributes etc. in a concept description can be changed or cancelled, what is the definition of a concept. How can we classify? And reason? C. Kemke Knowledge Representation 15 C. Kemke Knowledge Representation 16 4

5 Multiple Inheritance and Views Multiple Inheritance - Views (Jackson, Fig. 6.7) Multiple inheritance Sub-Concept inherits descriptions from several superconcepts. Possibly conflicting information ( = ambiguity) skeptical reasoners: don t know (no conclusion) credulous reasoners: whatever (several conclusions) Views Description of concept from different viewpoints. Inheritance of multiple, complementing descriptions e.g. view computer as machine or as equipment C. Kemke Knowledge Representation 17 C. Kemke Knowledge Representation 18 Multiple Inheritance - Ambiguity (Jackson, Fig. 6.8) Object-Oriented Programming Objects represent concepts similar to frames structured declarative representation (like record) procedural methods define the (external) behavior of object Objects communicate via sending messages to other objects to invoke their methods. C. Kemke Knowledge Representation 19 C. Kemke Knowledge Representation 20 5

6 Object-Oriented Programming Example Class Radio Class Robot slots slots power:{on,off}... volume:{1,...,10} methods methods v-control(v) - shut-down(...) set volume = V send (radio-1, switch(off)) switch(o) - set power on/off for O=on/off radio-1 of-class Radio robot-1 of-class Robot C. Kemke Knowledge Representation 21 Object-Oriented Programming (OOP) - Languages - SIMULA67, SmallTalk... KRL Knowledge Representation Language LOOPS Lisp Object-Oriented Programming System Flavors CLOS Common Lisp Object System COOL CLIPS Object-Oriented Language... C ++, Java,... C. Kemke Knowledge Representation 22 objects: OOP Terminology and Concepts data structures + procedures Objects are often called classes. Procedures are often called methods. Encapsulation: object-information can only be accessed by specifying the object and using the methods defined for this object ( message passing). Message Passing: objects can send messages to other objects by addressing the object and one of it s methods. Distinction: private and public variables / procedures Computation in OOP involves mainly communication between objects, and little or no global control. Basic Functions in OOP CREATING Define a class, including data structures/slots and methods. Define inheritance hierarchy, i.e. IS-A relationships. Create / Delete instances of a class. WORKING Message passing. (COOL) In CLIPS: Application of rules. C. Kemke Knowledge Representation 23 C. Kemke Knowledge Representation 24 6

7 OOP Inheritance Objects/classes arranged in inheritance hierarchy class ship class motorship IS-A ship Classes are also called generic objects / concepts Their methods are called generic methods / procedures Specific objects are instances of classes. class ship instance-of ship Titanic Inheritance of data structures (slots, fillers) and procedures (methods). OOP Inheritance of Data Structures Inheritance of data structures define data structures (slots) for super-class define defaults and common values (for slots) for super-class inherit to sub-classes and instances class ship x-velocity INTEGER y-velocity INTEGER... class motorship IS-A ship instance-of ship Titanic Class motorship, as well as instance Titanic inherit all slotspecifications, defaults, etc. from ship. C. Kemke Knowledge Representation 25 C. Kemke Knowledge Representation 26 OOP Inheritance of Methods Inheritance of procedures (methods) define generic method for super-class inherit to sub-classes (and instances) class ship method calc-speed... (self:x-velocity, self:y-velocity)... self is object itself; self:x-velocity refers to slot x-velocity of self. instance-of ship Titanic send Titanic calc-speed Method calc-speed known for all ships; uses concrete values for x-velocity, y-velocity for instance of ship (e.g. Titanic) in actual calculation. Multiple Inheritance and Method Combination Inheritance of methods / procedures in multiple inheritance hierarchies (heterarchies). Problem of method combination. Use before- and after-methods ( e.g. Flavors) take main method (inherited from super-class); add special before and after methods (from class or superclasses) which are executed before / after the main method. before-method preparation for main method after-method clean-up and adjustments around-methods or wrappers and whoppers additional surrounding code C. Kemke Knowledge Representation 27 C. Kemke Knowledge Representation 28 7

8 Method Combination and Multiple Inheritance Inherit main methods (e.g. refresh) from Window, and add special methods as before- and after-methods from subclasses. Window with Border Window Window with Border and Label Window with Label Method Combination and Multiple Inheritance in CLIPS/COOL Problem of method combination for multiple inheritance 1) Class Precedence List (CLOS, CLIPS/COOL) take methods as listed in their super-classes : class republican-quaker IS-A quaker and republican consult quaker first for inheritance; then republican. 2) before- and after-methods add special before and after methods C. Kemke Knowledge Representation 29 C. Kemke Knowledge Representation 30 Meta-Classes Classes and Instances in COOL - Instance-Relation - Meta-Classes are used to describe classes, e.g. create- and delete-functions, access-functions etc. The members of Meta-Classes are classes. Meta-Classes are classes themselves. system classes Object MetaClass Class domain classes Ship Liner domain instances Titanic C. Kemke Knowledge Representation 31 C. Kemke Knowledge Representation 32 8

9 Classes and Meta-Classes in COOL - IS-A Relation - system classes Class Object Ship domain classes MetaClass Liner Titanic domain instances C. Kemke Knowledge Representation 33 9