I think Richard Kasperowski wrote:
> how would I find the two automobiles that have the same model but
> different years?
>
> This rule almost does what I want:
>
> (defrule find-different-years
> (automobile (model ?model-1) (year ?year-1))
> (automobile (model ?model-2) (year ?year-2))
> (test (eq ?model-1 ?model-2))
> (test (not (eq ?year-1 ?year-2)))
First, note the following two general rules:
1) Don't use "test" if you don't need to, and
2) direct matching is more efficient than function calls.
Therefore, I'd recomment rewriting your rule above as:
(defrule find-different-years
(automobile (model ?model) (year ?year-1))
(automobile (model ?model) (year ?year-2&~?year-1))
Note that using "?model" to match both facts means that both facts
must contain the same slot value. This rule says exactly the same
thing as the original, but does a lot less work during pattern
matching.
Now, as written, the rule matches any time the two years are
different, which means that any pair will match twice: 1,2 and 2,1 are
both valid matches. To remove the duplicates, just pick one specific
ordering. Since "year" is numeric, this is easy here:
(defrule find-different-years
(automobile (model ?model) (year ?year-1))
(automobile (model ?model) (year ?year-2&:(> ?year-1 ?year-2)))
i.e., the rule matches only one of the two possible orderings, when
?year-1 comes after ?year-2.
---------------------------------------------------------
Ernest Friedman-Hill
Distributed Systems Research Phone: (925) 294-2154
Sandia National Labs FAX: (925) 294-2234
PO Box 969, MS 9012 [EMAIL PROTECTED]
Livermore, CA 94550 http://herzberg.ca.sandia.gov
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