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@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@sbb.ch> wrote:
>
>> From: CSchr <christian.schrinner@sbb.ch>
>> Subject: [rules-users] Newbie question on accessing predecessor and
>> successor in a list
>> To: rules-users@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@lists.jboss.org
>> https://lists.jboss.org/mailman/listinfo/rules-users
>
>
>
> _______________________________________________
> rules-users mailing list
> rules-users@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.

_______________________________________________
rules-users mailing list
rules-users@lists.jboss.org
https://lists.jboss.org/mailman/listinfo/rules-users



--
 Edson Tirelli
 JBoss Drools Core Development
 JBoss, a division of Red Hat @ www.jboss.com