XSLT Cookbook

Processing Nodes by Position

Problem

You want to process nodes in a sequence that is a function of their position in a document or node set.

Solution

Use xsl:sort with the select set to the position( ) or last( ) functions. The most trivial application of this example processes nodes in reverse document order:

<xsl:apply-templates>
     <xsl:sort select="position(  )" order="descending" data-type="number"/>
</xsl:apply-templates>

or:

<xsl:for-each select="*">
     <xsl:sort select="position(  )" order="descending" data-type="number"/>
     <!-- ... -->
</xsl:for-each>

Another common version of this example traverses a node set as if it were a matrix of a specified number of columns. Here, you process all nodes in the first column, then the second, and then the third:

<xsl:for-each select="*">
     <xsl:sort select="(position(  ) - 1)  mod 3" />
     <!-- ... -->
</xsl:for-each>

Or, perhaps more cleanly with:

<xsl:for-each select="*[position(  ) mod 3 = 1]">
    <xsl:apply-templates 
         select=". | following-sibling::*[position(  ) &lt; 3]" />
</xsl:for-each>

Sometimes you need to use position( ) to separate the first node in a node set from the remaining nodes. Doing so lets you perform complex aggregation operations on a document using recursion. I call this example recursive-aggregation. The abstract form of this example follows:

<xsl:template name="aggregation"> <xsl:param name="node-set"/> <xsl:choose> <xsl:when test="$node-set"> <!--We compute some function of the first element that produces a value that we want to aggregate. ...