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RE: [xsl] Pattern Matching in XSl - find groups defined in one Xml in another Xml.

Subject: RE: [xsl] Pattern Matching in XSl - find groups defined in one Xml in another Xml.
From: "G. Ken Holman" <gkholman@xxxxxxxxxxxxxxxxxxxx>
Date: Thu, 23 Aug 2012 07:59:59 -0400

At 2012-08-23 10:07 +0000, Kerry, Richard wrote:
Thinking about it overnight I think I could get the search done by using two passes. Firstly the RE (or key) based search we've already seem some examples of. And then a second pass to check whether the groups have been found in thier entireties.

I'm about to try this latest example of yours. At a very quick glance I think I can see two passes.

Not quite ... the "second" pass is only going over those groups that were found, not all groups. So it isn't really a second pass over all of the data.

Note that my first focus is from the group level, because only there can you tell if for a given area all of the members of the group are found.

Then I go over the area alarms, finding which of the found groups (if any) each alarm was in. There is no need to go over all groups, only over the found groups. And if there are no found groups, there is no loop for the area alarms.

In the classroom I see students take traditional programming approaches to issues that are best declared. Why best? Because then the processor can do the optimization and not be hobbled by the stylesheet writer's implementation of the algorithm.

For an example, take the declarative approach in finding out which groups are fully populated by the area alarms:

    <xsl:variable name="foundGroups" select="$groupedAlarms
        (:there can be no mis-matches:)
        [every $groupAlarm in alarm satisfies
             some $areaAlarm in $thisAlarmArea/alarm
                satisfies matches( $areaAlarm/@equipment,

This takes advantage of the two standalone quantified expressions that return true()/false() values:

   every tuple-list in set satisfies boolean
   (all tests must be true() in order to return true())


   some tuple-list in set satisfies boolean
   (any test must be true() in order to be true()

In my case, I've nested the "some" expression to return the boolean evaluation for the "every" expression. Having the variables available in scope made the "some" evaluation straightforward and self-documenting.

Note the processor can stop evaluation of the "every" expression as soon as it hits a false() value, and it can stop evaluation of the "some" expression as soon as it hits a true() value. If the stylesheet writer takes a programming approach and replaces the above with an algorithmic walking of the tree, even with the use of a function call, it could end up doing a lot more work than the processor would.

I hope this is helpful.

. . . . . . . . . . . . Ken

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