xsl-list
[Top] [All Lists]

Re: [xsl] following-sibling is evil

2014-07-06 05:33:38
Use a key, and count how many entries there are in the key with that value
combo.

On 6 Jul 2014 10:33, "Costello, Roger L. costello(_at_)mitre(_dot_)org" <
xsl-list-service(_at_)lists(_dot_)mulberrytech(_dot_)com> 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

--~----------------------------------------------------------------
XSL-List info and archive: http://www.mulberrytech.com/xsl/xsl-list
EasyUnsubscribe: http://lists.mulberrytech.com/unsub/xsl-list/1167547
or by email: xsl-list-unsub(_at_)lists(_dot_)mulberrytech(_dot_)com
--~--
<Prev in Thread] Current Thread [Next in Thread>