《人工智能（智能系统指南，第二版）》读书笔记——6、第五章

1. introduction to knowledge-based intelligent systems(summary / questions for review / references)

2. rule-based expert systems

3. uncertainty management in rule-based expert systems

4. fuzzy expert systems

5. frame-based expert systems

6. artificial neural networks

7. evolutionary computation

8. hybrid intelligent systems

9. knowledge engineering and data mining

5. frame-based expert systems

A frame is a data structure with typical knowledge about a particular object or concept.

A rule-based expert system may search through the knowledge that is not relevant to a given problem, and the search may take a great deal of time. In this situation, we need frames to collect the relevant facts within a single structure.

Frame are used to represent knowledge in a frame-based expert system. A frame contains knowledge of a given object, including its name and a set of attributes also called slots(槽). Name, weight, height and age are attributes of the frame Person.

Attributes or slots are used to store values. An attribute may contain a default value or a pointer to another frame, set of rules or procedure by which the attribute value is obtained.

Frame-based systems can also extend the attribute-value structure through the application of facets(侧面). Facets are used to establish the attribute value, control end-user queries, and tell the inference engine how to process the attribute.

Basically, frames are an application of object-oriented programming(面向对象编程) for expert system.

A frame may refer to a group of similar objects, or a particular object. A class-frame describes a group of objects with common attributes. Animal, person, car and computer are all class-frames. An instance-frame describes a particular object.

Class inheritance(类继承) is an essential feature of the frame-based systems. Class inheritance i.e.(即) the process by which all characteristics of a class-frame are assumed by the instance-frame. The fundamental idea of inheritance is that attributes of the class-frame represent things that are typically true for all objects in  the class, but slots in the instance-frames are filled with actual data that is unique for each instance.

In general, there are three types of relationships between objects: generalisation, aggregation and association(泛化、聚合、关联). Generalisation denotes "a-kind-of" or "is-a" relationship between a superclass and its subclasses.  For example, a car is a vehicle. Aggregation is "a-part-of" or "part-whole" relationship in which several subclasses representing components are associated with a superclass representing a whole. For example, an engine is a part of a car. Association describes some semantic(语义学的) relationship between different classes which are unrelated otherwise. Associations are inherently bi-directional(双向继承), for example, we can say a computer own Mr Black.

A frame can inherit attributes from more than one parent through multiple-parent inheritance. The only requirement for multiple parent inheritance is that attributes of all parents must be uniquely specified.

Frames communicate with each other by methods and demons(方法和守护程序). A method is a procedure associated with a frame attribute; it is executed whenever requested. Most frame-based expert systems use two types of methods: WHEN CHANGED and WHEN NEEDED. The WHEN CHANGED method is executed when new information is placed in the slot, and the WHEN NEEDED method is executed when information is needed for solving the problem but the slot value is unspecified.

Demons are similar to methods, and the terms are often used as synonyms. However, methods are more appropriate if we need to write complex procedures. Demons, on the other hand, are usually limited to IF-THEN statements.

In expert systems, frames are often used in conjunction(连接;结合) with production rules. We use rules to evaluate information contained in frames. In frame-based expert systems, rules often use pattern matching clauses(模式匹配子句). These clauses contain variables that are used for finding matching conditions(查找匹配条件) among all instance-frames. We use methods and demons cause a corespond rule to fire.

Although frames provide a powerful tool for combining declarative and procedural knowledge(陈述性知识和过程性知识), they leave the knowledge engineer with difficult decisions about the hierarchical structure(分层结构) of the system and its inheritance paths.