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Re: [xsl] position() ne last()

2009-12-03 08:40:08
That's remarkable. Somehow it is similar to the prepend optimization I
wrote about in my blog last December:

  http://dnovatchev.spaces.live.com/Blog/cns!44B0A32C2CCF7488!981.entry

Does a catalog of such optimizations exist?


-- 
Cheers,
Dimitre Novatchev
---------------------------------------
Truly great madness cannot be achieved without significant intelligence.
---------------------------------------
To invent, you need a good imagination and a pile of junk
-------------------------------------
Never fight an inanimate object
-------------------------------------
You've achieved success in your field when you don't know whether what
you're doing is work or play


On Thu, Dec 3, 2009 at 4:43 AM, Michael Kay <mike(_at_)saxonica(_dot_)com> 
wrote:

A little coding tip. I've no idea why this hasn't come up before. It's very
common to write things like

<xsl:for-each select="para">
 <xsl:apply-templates/>
 <xsl:if test="position() ne last()">
    <br/>
 </xsl:if>
</xsl:for-each>

But a much better way of coding this is:

<xsl:for-each select="para">
 <xsl:if test="position() ne 1">
    <br/>
 </xsl:if>
 <xsl:apply-templates/>
</xsl:for-each>

Why? Because however hard the optimizer works, the last() function is hard
work: it involves some kind of lookahead. With "position() ne last()" the
lookahead might be limited to one element, but it's still a lot more
complicated than testing whether the position is 1.

With streaming coming along, the latter formulation is also more likely to
be streamable (because lookahead is impossible with streaming).

I might think about doing the above rewrite automatically...


Regards,

Michael Kay
http://www.saxonica.com/
http://twitter.com/michaelhkay


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