[rules-users] Calculate the sum of "price" attribute from many facts.

[Edson Tirelli]

    Oana,

    Your reasoning is correct. The only thing I would change in your 
rule is that you don't need the predicate. Just do the comparison directly:

        Drink($speciality : speciality == $drinkS, $price : price)        

     The only better way I can think of is using the "accumulate" 
conditional element, but this is only available in trunk:

when
        Likes($nameS : name, $drinkS : drinkSpeciality)
        $total : Double() from accumulate ( Drink($speciality : 
speciality == $drinkS, $price : price)
                                                              init( 
double total = 0 )
                                                              action( 
total += $price )
                                                              result( 
new Double( total ) ) )      
 then
      // do something with the $total
end
  
     You can use a global variable instead of an Accumulator class, but 
this would force you to set the global variable in your java code 
(before you call fireAllRules()), what may not ideal for such 
situations. Using an accumulator instance allows you to define a setup 
rule whose consequence asserts the accumulator instance, keeping your 
launching code not dependent from your rules.

    []s
   Edson


nicolae oana wrote:

> Dear all,
>
> In my problem, I have to calculate the sum of "price" field from many 
> Drink facts. Here is the code I have:
>
> when
>     Likes($nameS : name, $drinkS : drinkSpeciality)
>     Drink(speciality == $drinkS, $price : price)    
> then
>     //calculate something like:   totalPrice+=$price
>
> Inspired from what Edson said, I've defined an Accumulator class and I 
> solve the problem:
>
> when
>         $a:Accumulator()
>         Likes($nameS : name, $drinkS : drinkSpeciality)
>         Drink($speciality : speciality ->($speciality == $drinkS), 
> $price : price)        
>     then
>         $a.incrementDrinksPrice($price.intValue());
>
> In this context, my questions are:
>
> 1) Is there another appropriate solution to calculate the sum of prices?
>
> 2) What is the use of global variables? (because I think I 
> missunderstood their meanings). Can I solve my problem using a global 
> variable like an accumulator, knowing that a global variable is acting 
> like a static variable?
>
> Best regards,
>                    Oana.
>
>
> */Edson Tirelli <tirelli at post.com>/* wrote:
>
>
>     Jeevan, sorry, I missed your e-mail in the limbo.
>
>     Just create a class or an attribute to be your accumulator...
>     For instance, if you create a "branchCounter" attribute in the Tree
>     object, you can do that:
>
>     rule "Count branches" salience 10
>     when
>     $t : Tree()
>     Branch( tree == $t )
>     then
>     $t.incrementBranchCounter();
>     modify( $t );
>     end
>
>     rule "Check branch count" salience 0
>     when
>     $t : Tree( branchCounter > 100 )
>     then
>     // do something
>     end
>
>     The only trick is the salience. The first rule must have a higher
>     salience in order to only fire the second rule after all branches are
>     counted.
>
>     Yes, in trunk you could do better with "accumulate" or "collect". For
>     your case, "collect" would be simpler:
>
>     rule "Check branches"
>     when
>     $t : Tree()
>     $branches : ArrayList( size > 100 ) from collect( Branch( tree ==
>     $t ) )
>     then
>     // do something
>     end
>
>     []s
>     Edson
>
>     Jeevan Tambuluri wrote:
>
>     > Hello all,
>     >
>     > I have been told that you could implement a helper class to
>     simulate
>     > "accumulate" command in drools. Could you please ealborate on
>     how to
>     > do this? Some code snippet to illustrate this would be very much
>     > appreciated.
>     >
>     > My problem is as follows.I have two types of facts - Tree and
>     Branch.
>     > Each Branch object has a reference to the Tree object it belongs
>     to. I
>     > want to write a rule: The number of branches in a tree must not
>     > exceed 100. I guess this would be easy to write with "accumulate"
>     > command, which is not yet available in 3.0.x.
>     >
>     > Thanks
>     >
>     > Jeevan
>     >
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>
>
>     -- 
>     Edson Tirelli
>     Software Engineer - JBoss Rules Core Developer
>     Office: +55 11 3124-6000
>     Mobile: +55 11 9218-4151
>     JBoss, a division of Red Hat @ www.jboss.com
>
>
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-- 
 Edson Tirelli
 Software Engineer - JBoss Rules Core Developer
 Office: +55 11 3124-6000
 Mobile: +55 11 9218-4151
 JBoss, a division of Red Hat @ www.jboss.com