[rules-users] finding matching entries in parallel arrays

[Mark Proctor]

We don't really recommend you assert ArrayLists etc as facts, as they 
have no contextual meaning

$first : ArrayList( )
$second : ArrayList( this != $first )
$fi Object() from $first
$si Object from $second
eval( $fi == $si )

'from' allows iteration of lists, so you could potentially do it this 
way, although it doesn't provide an index number for you.... You could 
possibly have a global that as part of a function in eval get 
incremented and that global is available in the consequence. But you'll 
have to be very careful with concurrency.... Might be easier to have a 
hashmap of indexes where the key is made up of $first and $second. Btw 
== checks for same instances, not an equality check, is that what you 
wanted?

Mark

Aaron Dixon wrote:
> My problem boils down to finding matching entries in parallel arrays.
>
> Here is a rule that succesfully does just that:
>
> rule "Find matching entry in parallel arrays"
>     when
>         $first : ArrayList( )
>         $second : ArrayList( this != $first )
>         $i : Integer( this < $first.size )
>         eval( $first.get($i) == $second.get($i) )
>     then
>         System.out.println ( "Found match at index " + $i + "!" );
> end
>
>
> To execute this rule, I must insert (assert) the two ArrayList facts 
> as well as at least as many Integer facts as there are items in the 
> ArrayLists.
>
> What I don't like about this rule is that
>
>     (1) I have to assert the Integers
>     (2) I can't support arbitrarily-sized lists in my rules (without 
> asserting that many Integer facts)
>
> So -- Is there a better way?
>
> What if a future version of Drools supported implicit Number facts 
> that allowed for these kinds of indexing rules?
>
> Aaron
>
>
>
>
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