Objects and Inheritance

Using objects lets us define static facts, represent generalities, and write more powerful and expressive rules.

1) Uses of and motivations behind objects and inheritance

• 1) They mimic the way we think (in categories; not each case separately).
• 2) They allow much more elegant, expressive, understandable knowledge bases.
• 3) KB maintenance is far easier.

2) Fundamental distinctions

• We will focus on ISA hierarchies. (AKA: abstraction hierarchies).
• Compare to: HAS hierarchies (AKA: aggregation hierachies).
• Heuristic: for each link (bottom to top), you should be able to say "child ISA parent" and have that make sense.

3) Building blocks

• Abstraction hierachies are composed of Classes.
• Classes can be subdivided into additonal Classes (sub-classes) until, finally, we arrive at:
• Instances (which are at the bottom level of any hierachy).
• Sorry: I can't create a suitable graphic to put in here. It would show:
• animal: top-level class
• bird and elephant: as second level classes
• robin and crow: as sub-classes of bird
• and these instances as "leaves": Tweety, Trixie (robins); Blackie (crow); Clyde and Jumbo (elephants)
• Attributes
• Basic idea (example): All animals have Heights, Weights, and Ages. Thus (if these are useful attributes for our problem), define them ONCE (and for all) for Animals.
• Then, by inheritance: the classes bird, elephant, robin, and crow have these attributes.
• And so do all instances.
• Different classes can have different attributes.
• Example: wing-span is a good attribute for birds, not for animals. Define it for birds.
• Values
• Basic idea: Establish "default" (commonly expected values) at as general a level as possible
• (This means: as "high up" in the hierarchy as possible)
• Example: if you want to set a value for the attribute "breathes Oxygen," set it to True for Animals, instead of setting it true separately for birds and elephants.
• Use: "exceptions" to over-ride defaults.
• Example: set Num Tusks = 2 for the Class elephants. Set Jumbo | Num Tusk = 1 if one of his fell off!
• Exceptions to exceptions are possible, too.

4) Building Hierachies

• Establish that your prooblem has exploitable generality
• Identify classes involved and arrange them in hierachy.
• can have two hierachies. E.g.: one for customers; another for products
• Establish (i.e, define) attributes at the appropriate, highest possible (most general) level.
• Set values for attributes at the highest possible level, too.
• If you know values even before you ask a user questions, set them for appropriate object | attributes.
• Never re-define attributes at lower levels
• eg: if you defined Num legs as an attribute for Animals, don't redine it for elephants
• let inheritance do it this instead
• Note: understand the difference between defining attributes and setting values
• the value for an attribute may be set and re-set at different levels of the hierachy (this corresponds to having defaults and exceptions)

5) Class Rules

• Definition: Rules that mention Classes (rather than instances)
• Use: short-hand for rules that mention instances. Ie., write one rule; have it do the work of many. This gives your ES power!
• Examples (all valid):
• IF VCR | cost < 300 THEN recommendation is buy from our store /* VCR is a Class */
• Short hand for:
• IF VCX-1 | cost < 300 THEN recommendation is buy from our store /* VCX-1 is instance */
• IF VCX-2 | cost < 300 THEN recommendation is buy from our store /* another instance */
• etc.
• IF customer | max price will spend > <VCR | price> THEN recommendation is consider /* VCR = class */
• IF customer | purchase category is electronics THEN VCR | acceptability is a possibility /* still a class */
• IF VCR | price < <Customer | max price will pay> AND ... THEN recom is objectname(<VCR>)
• Note: "bindings" are consistent.
• E.g.: If VCR | price < 300 THEN purchase | pricewith tax = <VCR | price> * 1.06 refers to
• VCX-1 twice: If VCX-1 | price < 300 THEN purchase | pricewith tax = <VCX-1 | price> * 1.06
• VCX-2 twice; and
• Beta-Junior twice;
• but is NOT shorthand for If VCX-1 | price < 300 THEN purchase | pricewith tax = <VCX-2 | price> * 1.06