XSLT Cookbook

Computing Sums and Products

Problem

You need to sum or multiply functions of numbers contained in a node set.

Solution

The abstract form of sum for processors that support tail-recursive optimization is as follows:

<xsl:template name="math:sum">
  <!-- Initialize nodes to empty node set -->
  <xsl:param name="nodes" select="/.."/>
  <xsl:param name="result" select="0"/>
  <xsl:choose>
    <xsl:when test="not($nodes)">
      <xsl:value-of select="$result"/>
    </xsl:when>
    <xsl:otherwise>
        <!-- call or apply template that will determine value of node 
          unless the node is literally the value to be summed -->
      <xsl:variable name="value">
        <xsl:call-template name="some-function-of-a-node">
          <xsl:with-param name="node" select="$nodes[1]"/>
        </xsl:call-template>
      </xsl:variable>
       <!-- recurse to sum rest -->
      <xsl:call-template name="math:sum">
        <xsl:with-param name="nodes" select="$nodes[position(  ) != 1]"/>
        <xsl:with-param name="result" select="$result + $value"/>
      </xsl:call-template>
    </xsl:otherwise>
  </xsl:choose>
</xsl:template>

Two techniques can handle a large number of nodes in the absence of tail-recursive optimization. The first is commonly called divide and conquer. The idea behind this technique is to reduce the amount of work by at least a factor of two on each recursive step:

<xsl:template name="math:sum-dvc"> <xsl:param name="nodes" select="/.."/> <xsl:param name="result" select="0"/> <xsl:param name="dvc-threshold" select="100"/> <xsl:choose> <xsl:when test="count($nodes) &lt;= $dvc-threshold"> <xsl:call-template ...