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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