XSLT Cookbook

Performing a Preorder Traversal

Problem

You want to recursively process an element first and then process its child elements.

Solution

Solutions to this recipe have the following general form:

<xsl:template match="node(  )">
     <!-- Do something with current node -->
   
     <!--Process children -->
     <xsl:apply-templates/>
</xsl:template>

Discussion

The term preorder is computer-science jargon for traversing a tree so you visit the root and recursively visit the children of the root in preorder. This process is arguably the most common means of processing XML. Of this idiom’s many applications, this chapter will consider two. As you review recipes in later sections, you will see this mode of traversal arise frequently.

Consider an organization chart (Example 4-2) encoded in a simplistic fashion so that an employee element B is a child of another employee element A if B reports to A. Example 4-16 employs a preorder traversal to explain who manages whom. Example 4-17 shows the output.

Example 4-16. Stylesheet

<?xml version="1.0" encoding="UTF-8"?> <xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"> <xsl:output method="text"/> <xsl:strip-space elements="*"/> <xsl:template match="/employee" priority="10"> <xsl:value-of select="@name"/><xsl:text> is the head of the company. </xsl:text> <xsl:call-template name="HeShe"/><xsl:text> manages </xsl:text> <xsl:call-template name="manages"/> <xsl:apply-templates/> </xsl:template> <xsl:template match="employee[employee]"> <xsl:value-of ...