为什么减法和除法会在XSLT中给我NaN？

Question

So, I'm trying to make a symbolic derivative calculator and it works with a single hitch. It gives me NaN with this XML

<?xml version="1.0"?>
<sum>
    <mono>  
        <arg>3</arg>
        <var>x</var>
        <exp>2</exp>
    </mono>
</sum>

And this stylesheet that gives the numbers the behaviour they should have in a derivative.

<xsl:stylesheet version="1.0"        xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="xml" encoding="UTF-8" indent="yes"/>
<xsl:template match = "/">
    <mono>
        <arg>
            <xsl:value-of select="arg div exp"></xsl:value-of>
        </arg>
    <var>
        <xsl:if test="exp > 0"/>
        X
    </var>
    <exp>
        <xsl:value-of select="exp - 1"></xsl:value-of>
    </exp>
</mono>
</xsl:template>
</xsl:stylesheet>

Answer 1

Your main template is matching the root element:

<xsl:template match = "/">

Change it to match mono elements instead:

<xsl:template match = "mono">

Then your NaN issue will go away because the arg and exp elements will really be children of the current mono element now, whereas they were not children of the / element before.

Here's your XSLT with the above a few other improvements:

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:output method="xml" encoding="UTF-8" indent="yes"/>
  <xsl:strip-space elements="*"/>

  <xsl:template match="mono">
    <mono>
      <arg>
        <xsl:value-of select="arg div exp"></xsl:value-of>
      </arg>
      <var>
        <xsl:if test="exp > 0">X</xsl:if>
      </var>
      <exp>
        <xsl:value-of select="exp - 1"/>
      </exp>
    </mono>
  </xsl:template>
</xsl:stylesheet>

Here's the XML output, without the NaNs, as requested:

<?xml version="1.0" encoding="UTF-8"?>
<mono>
   <arg>1.5</arg>
   <var>X</var>
   <exp>1</exp>
</mono>