Some F-Logic engines such as FLORA and FLORID support aggregation operators. These operators compute values pertained to the set of variable instantiations that occur in certain query. It is good to illustrate that by an example. The first rule computes the yearly average of the daily temperature in Burlington:
burlington[ avg_yearly_temperature--> TAvg] :-
TAvg= avg{ T | burlington[ temperature( Month)--> T]}.
Here TAvg is the average value of all T which would occur in query ?-burlington[ temperature( Month)--> T].
Here is another rule. It computes the minimum daily temperature in Burlington during the year.:
burlington[ min__yearly_temperature --> TMin] :-
TMin= min{ T | burlington[ temperature( Month)--> T]}.
TMin is the minimum value of all T which would occur in query ?-burlington[ temperature( Month)--> T].
Besides avg and min other aggregation operators are supported by FLORA and FLORID. Here is the full list of those supported by FLORA : min, max, count, sum, avg, collectset, collectbag. All these aggregation operators have similar graphic representation. Here is the graphic representation of the first rule mentioned above .
C 2003 S. Krivov Send your comments to skrivov@zoo.uvm.edu