Knowledge Representation and Reasoning - Dublin Institute of 都柏林学院知识表示与推理.pptVIP

*/46 Problems with semantic networks Searching possible combinatorial explosion, especially if the response to a query is negative. Semantic networks, as we know, were originally devised as models of human associative memory in which one node has links to others and information retrieval occurs due to a spreading activation of the nodes. However, other forms of reasoning must be available to the brain, as it does not take a long time to answer the question “Is there a football team on Pluto?” Unable to represent Negation Quantification Disjunction */46 Semantic networks Semantic networks nowadays Conceptual graphs (John Sowa) SNePS (Stuart Shapiro) */46 Frames Frames are a variant of semantic networks A frame is “a data structure for representing a stereotypical situation like …going to a child’s birthday party” (Minksy 1981). All the information relevant to a concept is stored in a single complex entity called a frame. Superficially, frames look like record data structures or class, however, at the very least frames also support inheritance. class Book { Person author; String title; int price; } */46 Frames In frame-based systems we refer to objects – Mammal, Elephant, and Nellie; slots – properties such as colour and size; slot-values – values stored in the slots, e.g. grey and large. Slots and the corresponding slot-values are inherited through the class hierarchy */46 Frames - example The previous example can be also represented as frames: Mammal: subclass: Animal has-part: head Elephant: subclass: Mammal colour: grey size: large Nellie: instance: Elephant likes: apples Clyde: instance: Elephant animal mammal isa elephant isa head has_part Clyde Nellie large size instance_of instance_of apples likes grey colour */46 Inheritance and default values In general children classes inherit the properties of the parent class Default values - properties that are typical for a class Instances or subclasses whose properties differ from th