[rules-users] how to do things like "distinct"?

[Wolfgang Laun]

On 1 June 2011 16:55, Simon Chen <simonchennj at gmail.com> wrote:

> Hi all,
>
> I have the following problem.
>
> Let's say, I have a class Number3(), with three fields a, b, and c.
>
> I have the following rule:
>
> rule "test3"
>    when
>        Number3($a: a, $b: b)
>        $minvalue: Number()
>            from accumulate(Number3(a == $a, b == $b, $c: c), min($c))
>    then
>        System.out.println($a + " " + $b + " " + $minvalue);
> end
>
>
> Say, I have four objects (1,2,3), (1,2,4), (1,3,4), (1,3,5).
>
> The result shows four lines:
> 1 3 4.0
> 1 3 4.0
> 1 2 3.0
> 1 2 3.0
>
> But I want the output to be:
> 1 3 4.0
> 1 2 3.0
>
> meaning that for each unique pair of a and b, I want the rule to fire only
> once.
>
>
In this particular case you might try

rule "test3"
   when
      Number3($a: a, $b: b, $c)
      not Number3( a == $a, b == $b, c < $c )
   then
       System.out.println($a + " " + $b + " " + $c);
end

Event this will fail, if there are two identical (w.r.t. a, b and c) Number3
objects.

For the general case, you must have some way of picking a single Number3
with some a/b-combination.

Alternatively, insert possibly multiple triplets <a,b,min(c)> and harvest
one from each group, using additional rules.

Or use an additional fact containing <a,b(,c)> to indicate that you have
computed the minimum. But then you must get rid of these auxiliaries.

-W


> Is this doable?
>
> Thanks.
> -Simon
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