[xsl] sorting the result of a calculation on xml that can have missing/blank tags

[<srguard2000-triallicense@xxxxxxxxxxx>]

XSL Processor: Xalan C++ 1.8.0
on MSVC6.0 and MS Windows XP Professional

Hello, I am new to this list and have a question about XSL.

I need to sort the results of an xsl calculation, while allowing for tags that are missing or
empty.
I tried using format-number() to replace blanks with '0' and even 'number(0)', but when the output
of format-number is used in a calculation, the result is NaN.

The result should have "CALC div: 5" as the first item.  The result is just text at the moment.

I'm under time pressure to get this working... tempting to just do it in C++ but it would be nicer
to have it in xsl...

Thanks!

Sean



Sample XML:
===========
<?xml version="1.0" encoding="UTF-8"?>
<Message>
	<Classes>
		<Class>
			<Teachers>2</Teachers>
			<HomePupils></HomePupils>
			<GuestPupils>10</GuestPupils>
		</Class>
	</Classes>
</Message>

Sample XSL:
===========
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform";
xmlns:fo="http://www.w3.org/1999/XSL/Format";>

	<xsl:decimal-format name="CCMDecimalFormat" NaN="0" />

	<xsl:template match="*">
		<xsl:for-each select="//Classes/Class[(											
Teachers &gt; 0 )]">
			<xsl:sort select="(HomePupils + 
 GuestPupils)	div Teachers" data-type="number" order="descending"/>
			<!--xsl:if test="position()=1"-->
												HP:<xsl:value-of select="HomePupils"/> ; 
												GP:<xsl:value-of select="GuestPupils"/> ; 
T:<xsl:value-of select="Teachers"/> ; 

												CALC number: <xsl:value-of select="HomePupils"/>

												CALC sum: <xsl:value-of select="HomePupils + 
GuestPupils"/>

												CALC div: <xsl:value-of select="
(HomePupils + 
GuestPupils div Teachers)		"/>
			<!--/xsl:if-->
			
			==============
		</xsl:for-each>
	</xsl:template>
</xsl:stylesheet>



___________
Sean Ryan