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Re: [xsl] following-sibling is evil


Subject: Re: [xsl] following-sibling is evil
From: "Wolfgang Laun wolfgang.laun@xxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>
Date: Sun, 6 Jul 2014 15:31:59 -0000

The key of the "evll" is already in the proposition that determining
"a following Book with the same Author and Genre" should be done
for each Book. I can't imagine a user who'd be pleased with learning
(for instance) 46 times that Rex Stout has written yet another
Nero Wolfe mystery.

If the question is put as "Who has written more than one Book
of the same genre?", it is (probably) what the user really wants
to know and, above all, it should not push you towards an approach
that's oriented towards the presentaton of the data.

-W

On 06/07/2014, Costello, Roger L. costello@xxxxxxxxx
<xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx> wrote:
> Hi Folks,
>
> I have a list of <Book> elements. Each Book contains Title, Author, and
> Genre.
>
> For each Book I wish to determine if there is a following Book with the same
> Author and Genre.
>
> I am using Schematron. This XSLT is generated from my Schematron rule:
>
>     <xsl:template match="Book">
>         <xsl:variable name="currentAuthor" select="Author" />
>         <xsl:variable name="currentGenre" select="Genre" />
>         <xsl:if test="following-sibling::Book[Title eq $currentAuthor][Genre
> eq $currentGenre][1]">
>             <xsl:text>There is another Book with the same Author and
> Genre</xsl:text>
>         </xsl:if>
>     </xsl:template>
>
> Notice the XPath expression:
>
> following-sibling::Book[Title eq $currentAuthor][Genre eq $currentGenre][1]
>
> As I understand it, an XPath processor will evaluate that XPath expression
> like so:
>
> Step 1. Gather up all the following sibling Book elements. Let's denote the
> resulting set by S1.
>
> Step 2. Filter S1 by eliminating those Books that don't satisfy this
> predicate: [Title eq $currentAuthor]. Let's denote the resulting set by S2.
>
> Step 3. Filter S2 by eliminating those Books that don't satisfy this
> predicate: [Genre eq $currentGenre]. Let's denote the resulting set by S3.
>
> Step 4. Filter S3 by eliminating all Books except the first.
>
> Do I correctly understand how an XPath processor will evaluate the XPath
> expression?
>
> Suppose the list contains 100,000 Books.
>
> Consider evaluating the XPath expression for the first Book in the list. The
> XPath processor must collect 99,999 Books and apply the steps to them.
>
> For the second Book the XPath processor must collect 99,998 Books and apply
> the steps to them.
>
> For the third Book the XPath processor must collect 99,997 Books and apply
> the steps to them.
>
> And so forth.
>
> Yikes!
>
> That XSLT program will take a very long time to execute!
>
> Is there a way to modify my XPath so that the XPath processor stops as soon
> as it gets to a Book that matches all the predicates?
>
> /Roger


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