[rules-users] Newbie question on accessing predecessor and successor in a list

[Edson Tirelli]

   Christian,

   My guess is that there are better approaches to do what you want (i.e.,
write your business rules) without using specific indexes in a list, since
that is usually a sign of trying to use a imperative algorithm in a
declarative engine.
   If you can share a mock of what your business rule is, maybe we can help
more.

   Having said that, about your questions:

* You can use eval inside patterns. Just remember that eval() uses a code
block in the same dialect you are using to write your rules (default is
java). So, $journey.segmentList is probably not valid in java (since
segmentList is probably private attribute), but is valid in MVEL that
transparently call getSegmentList(). So, either change your expression to a
correct java expression or change the rule dialect to MVEL.

* DISCLAIMER: THIS IS REALLY BAD PRACTICE: if you really want to emulate
"imperative programming" for this rule, you need a control fact to keep
track of the index you are testing:

rule "Children: do not do this at home, unless your parents says it is ok"
when
    $cf : ControlFact( $pos : position < $journey.segmentList.size )
    $s : Segment( countries contains Country.Switzerland, ... ) from
$journey.segmentList[$pos]
    $ps : Segment( countries not contains Country.Switzerland, ... ) from
$journey.segmentList[$pos-1]
then
    // do something
    // and also update control fact
    modify( $cf ) { setPosition( $pos + 1 ) }
end

Initialize control fact with position number 1 and it should be fine, but I
did not tested the above rule.

   []s
   Edson


2008/11/4 CSchr <christian.schrinner at sbb.ch>

>
> Thanks Greg for your fast reply. Unfortunately I do not get this to work:
>
> $preSegment : Segment (
>                       $journey.segmentList.indexOf(this) ==
> ($journey.segmentList.indexOf($segment) - 1),
>                        countries not contains Country.Switzerland && ...
>                     ) from $journey.segmentList
>
> Compiler throws:
>
> [36,48]: unknown:36:48 Unexpected token 'this'[36,98]: unknown:36:98
> Unexpected token '$segment'
>
> Is it possible to use eval() inside Segment ()?
>
> With eval the compiler throws something like this:
>
> $journey.segmentList cannot be resolved to a type
> Cannot use this in a static context
>
> I don't know how matching works in drools, but wouldn't this also be very
> expensive? As for every match of Segment ( ) the segmentList has to be
> iterated two times. These lists can get very long. A cheap approach would
> be
> if the from statement would "know" the index of the Segment () matched. I
> was hoping this would be possible.
>
>
> Regards,
>
> Chris
>
>
> Greg Barton wrote:
> >
> > You can't do $journey.segmentList.indexOf(this) and
> > $journey.segmentList.indexOf($segment), even in an eval?
> >
> > --- On Mon, 11/3/08, CSchr <christian.schrinner at sbb.ch> wrote:
> >
> >> From: CSchr <christian.schrinner at sbb.ch>
> >> Subject: [rules-users] Newbie question on accessing predecessor and
> >> successor in a list
> >> To: rules-users at lists.jboss.org
> >> Date: Monday, November 3, 2008, 9:56 AM
> >> Hi everyone!
> >>
> >> I'm relatively new to Drools and rule languages.
> >>
> >> I'm wondering if it is possible to access the
> >> predecessor and successor in a
> >> match using from operator on an (Array)List. I have
> >> something like this
> >> (simplified):
> >>
> >> ...
> >> $segment : Segment( countries contains Country.Switzerland
> >> && ... ) from
> >> $journey.segmentList
> >>
> >> $preSegment : Segment ( indexOf(this) == indexOf($segment)
> >> - 1,
> >>                                 countries not contains
> Country.Switzerland
> >> && ...
> >>                  ) from $journey.segmentList
> >>
> >> I need something like indexOf() for the exact predecessor
> >> of the matched
> >> segment.
> >>
> >> If the matched business object was an element of a double
> >> linked list it
> >> would be easy. Unfortunately I don't have the luxury to
> >> change the business
> >> object into knowing it's predecessor or successor. I
> >> have to prove that we
> >> could write our java coded business rules in Drools rule
> >> language without
> >> changing the business object model.
> >>
> >> Thanks in advance,
> >>
> >> Chris
> >>
> >>
> >> --
> >> View this message in context:
> >>
> http://www.nabble.com/Newbie-question-on-accessing-predecessor-and-successor-in-a-list-tp20302694p20302694.html
> >> Sent from the drools - user mailing list archive at
> >> Nabble.com.
> >>
> >> _______________________________________________
> >> rules-users mailing list
> >> rules-users at lists.jboss.org
> >> https://lists.jboss.org/mailman/listinfo/rules-users
> >
> >
> >
> > _______________________________________________
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> > rules-users at lists.jboss.org
> > https://lists.jboss.org/mailman/listinfo/rules-users
> >
> >
>
> --
> View this message in context:
> http://www.nabble.com/Newbie-question-on-accessing-predecessor-and-successor-in-a-list-tp20302694p20317549.html
> Sent from the drools - user mailing list archive at Nabble.com.
>
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-- 
 Edson Tirelli
 JBoss Drools Core Development
 JBoss, a division of Red Hat @ www.jboss.com
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