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

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(a)sbb.ch&gt; > > 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(a)sbb.ch&gt; wrote: > > > >> From: CSchr <christian.schrinner(a)sbb.ch&gt; > >> Subject: [rules-users] Newbie question on accessing predecessor and > >> successor in a list > >> To: rules-users(a)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-succes... > >> Sent from the drools - user mailing list archive at > >> Nabble.com. > >> > >> _______________________________________________ > >> rules-users mailing list > >> rules-users(a)lists.jboss.org > >> https://lists.jboss.org/mailman/listinfo/rules-users > > > > > > > > _______________________________________________ > > rules-users mailing list > > rules-users(a)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-succes... > Sent from the drools - user mailing list archive at Nabble.com. > > _______________________________________________ > rules-users mailing list > rules-users(a)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 _______________________________________________ rules-users mailing list rules-users(a)lists.jboss.org https://lists.jboss.org/mailman/listinfo/rules-users