Knowledge Representation
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Propositional Logic
Predicate Logic
Rules
Semantic Nets
Frame
Object
Propositional Logic
• It is raining
– RAINING
• It is sunny
– SUNNY
• We can deduce whether a certain
proposition is true or false
Proposition Logic
• Socrates is a man
– SOCRATESMAN
• Plato is a man
– PLATOMAN
• we can not draw any conclusions about
similarities between Socrates and Plato
Predicate Logic
• Socrates is a man
– MAN (SOCRATES)
• Plato is a man
– MAN (PLATO)
• Now the structure of representation reflects
the structure of knowledge itself
Predicate Logic
• Marcus is a man
– MAN(Marcus)
• Marcus is a Pompeian
– POMPEIAN (Marcus)
• All Pompeians were Romans
– Vx POMPEIAN(x) -> ROMAN(x)
Predicate Logic
• All Romans were either loyal to Caesar or
hated him
• Vx ROMAN(x) -> loyalto (x, Caesar) v hate (x, Caesar)
• It is difficult to represent knowledge in predicate logic
Rules
If
(conditions)
Then
(actions)
Else
(actions)
Semantic Nets
• Semantic net is a knowledge presentation
method based on a network structure
• It consists of
– points called nodes connected by
– links called arcs
• Nodes - object, concepts, events
• Arcs - relationships between nodes
Semantic Nets
• Common arcs used for representing
hierarchies include isa and has-part.
Example:
The queen mary is an ocean liner.
Every ocean liner is a ship
Ship
isa
Ocean Liner
isa
Queen Mary
isa
Ocean Liner
Has-part
SHIP
Oil Tanker
Engine
Hull
isa
Swimming
Pool
Queen Mary
Liver Pool
Boiler
Bill gives Judy a gift
Bill told Laura that he gave Judy a gift
Frame 1
• a data structure for representing a
stereotyped situation
• a network of nodes and relations organized
in a hierarchy
• the topmost nodes - general concepts
• the lower nodes - more specific instances
Frame 2
• The concepts at each node is described by a
set of attributes and values of those
attributes
• Attributes are called slots
• Each slot can have procedures (codes)
• Typical procedures
– if added procedure
– if deleted procedure
– if needed procedure
Report
isa
Progress Report
isa
DSS Project Process Report
isa
Technical Report
A node in a frame system
Slot 1
Value 1
Procedure 1
Slot 2
Value 2
Procedure 2
Slot 3
Value 3
Procedure 3
Comparisons of KR Methods
• Rules
– Adv.
• simple syntax, easy to understand, simple
interpreter, high modular, flexible
– Disadv.
• Hard to follow hierarchies, inefficient for large
systems, not all knowledge can be expressed as rules
Comparisons of KR Methods
• Semantic Nets
– Adv.
• Easy to follow hierarchy, easy to trace association,
flexible
– Disadv.
• Meaning attached to nodes might be ambiguous
• exception handling is difficult
• difficult to program
Comparisons of KR Methods
• Frames
– Adv.
• Expressive power, easy to set up slots for new
properties and relations
• easy to create specialized procedures
• easy to include default information and detect
missing values
– Disadv.
• Difficult to program
• difficult for inference