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Frames, Semantic Networks, Objects (Development of Frame Based System (1.…
Frames, Semantic Networks, Objects
Frame
What is a Frame?
A frame is a data structure with typical knowledge about a particular object or concept.
Frame was proposed by Marvin Minsky used to capture and represent knowledge in frame based expert system.
Each frame has its own name and a set of attributes also known as slots associated with it.
Model, processor, memory and price are slots in the frame Computer.
Each slot have a value attached to it.
Sometimes instead of a particular value a slot may have a procedure that determines the value.
However in expert system frames are often used in conjunction with production rules
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Frame Relations
Relation between frames are in a hierarchy which constitute a process of specialisation.
The class-frame on the top of the hierarchy represent some generic concept, class-frames further down stand for a more restricted concept and instances are basically exemplification
Is there any differences between the main steps in building a rule-based expert system and a frame based one?
In frame based system the overall hierarchy of the knowledge is decided first. Classes and their attributes are identified and the relationship between frames are established.
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Slot Information
Slot may include information such as:
- Frame name
- Relation of the frame to other frames
- Slot value-Symbolic, Numeric or Boolean
- Default slot value
- Range of the slot value
- Procedural information:- a slot can have a procedure attached to it which is executed if the slot value is changed or needed. Two types of procedures often attached to slots is 1) WHEN CHANGED 2) WHEN NEEDED
What is facet?
A facet is means of providing extended knowledge about an attribute of a frame.
Facets are used to establish attribute value, control end user queries and tell the inference engine how to process the attribute.
Frame based expert system allows us to attach value, prompt and inference facets to attributes.
Types of facet
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Prompt facets enable the end user to enter the attribute value online during a session with the expert system.
Inference facets allows us to stop the inference process when the value of specified attribute changes.
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Method and Demons
Method and demons is basically used to manipulate and validate knowledge
Basically it is used to add actions to frames.
Method is a procedure associated with a frame attribute that is executed whenever requested.
Method is written for a specific attribute to determine the attribute value or execute a series of actions when the attribute value change
A demon has and IF-THEN structure.
It is executed whenever an attribute in the demon’s IF statement changes its values.
Methods are more appropriate if we need to write complex procedure.
Demons are usually limited to if then statements.
Types
WHEN CHANGED
An expert system is required to assist a loan officer in evaluating credit request from small bussiness venture. A credit request is to be classified into one of three categories, ‘Give credit’,’Deny credit’ or ‘Consult a superior’ based on the collateral and financial rating of the bussiness and the banks expected yield from the loan.
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Method Workings
Any method begins with the reserved words WHEN CHANGED or WHEN NEEDED, which are followed by the reserved word BEGIN and a series of commands executed. The reserved word END completes a method. To refer to a particular attribute in a method, we must specify the class name as well as the attribute name. The syntax is: <attributename> OF <classname> For example, the statement Goto Next OF Action Data referes to the attribute Goto Next of the class Action Data. The Next pushbutton on the input display is attached to the attribute Goto Next of the class Action Data.
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Semantic Network
What is?
A semantic network is a classical AI representation technique used for propositional information
A semantic net is sometimes called a propositional net.
In mathematical term a semantic net is a label directed graph.
The structure of semantic net is shown graphically in terms of nodes and the arcs that connects them.
Node are referred as objects
Arcs are referred as links or edges
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IS-A, KIND-OF, HAS A
Rather then defining new relationships for different problems it is customary to use the these three types of relationship
IS-A
IS-A relationship means “an instance of” and refers to a specific member in a class.
A class refers to a set of objects
A-KIND-OF
A-KIND-of relationship basically relates one class to another.
The AKO class relates an individual class to a parent class of classes of which the individual class is a child class.
The more general class that an AKO arrow points to is a superclass.
HAS A
A HAS-A relationship basically relates a class to a subclass.
The HAS-A relationship points opposite to the AKO and is often used to relate an object to a part of the object.
Eg. car HAS-A engine car HAS-A tyres car IS-A ford
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Disadvantage
No standard definition for link names.
Combinatorial explosions of searching nodes.
Cannot define knowledge in a way that a logic can.
Objects
An object is defined as a collection of information representing an entity in the real world (the data).
A description of how information is manipulated is the code.
Property
The object in a class have one or more attributes in common. Each attribute has a value.
The combination of value and attribute is called a property.
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