Modify attribute of certain XML elements

Question

I would like to modify an attribute of a very long xml like this:

<element index="0">
    <subelement bla="asdf" />
    <subelement bla="asdf" />
</element>
<element index="1">
    <subelement bla="asdf" />
    <subelement bla="asdf" />
</element>
...

I need to add N the value of each index attribute. Say N=5. The result would be:

<element index="5">
    <subelement bla="asdf" />
    <subelement bla="asdf" />
</element>
<element index="6">
    <subelement bla="asdf" />
    <subelement bla="asdf" />
</element>
...

What's the easiest way to do this? I presume it would be with XSLT, but I don't know how to do it.

Thanks!

Answer 1

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

  <!-- copy everything verbatim -->
  <xsl:template match="@*|node()">
    <xsl:copy>
      <xsl:apply-templates select="@*|node()" />
    </xsl:copy>
  </xsl:template>

  <!-- except "element" nodes -->
  <xsl:template match="element">
    <xsl:copy>
      <xsl:attribute name="index">
        <xsl:value-of select="@index + 5"/>
      </xsl:attribute>
      <xsl:apply-templates />
    </xsl:copy>
  </xsl:template>

</xsl:stylesheet>

Answer 2

svick answer it's a good solution. Just in case you have to keep exact structure, try:

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">

<xsl:param name="increment" select="5" />

<xsl:template match="@*|node()">
 <xsl:copy>
  <xsl:apply-templates select="@*|node()" />
 </xsl:copy>
</xsl:template>

<xsl:template match="node()[not(node())]">
 <xsl:element name="{name()}" >
  <xsl:apply-templates select="@*" />
 </xsl:element>
</xsl:template>

<xsl:template match="element/@index">
 <xsl:attribute name="index">
  <xsl:value-of select="$increment + ." />
 </xsl:attribute>
</xsl:template>

</xsl:stylesheet>