Question

Is it possible for me to have a conditional in XSLT such that I find and replace only the FIRST tag of a particular tag name?

For example, I have an XML file with many <title> tags. I would like to replace the first of these tags with <PageTitle>. The rest should be left alone. How would I do this in my transform? What I currently have is this:

<xsl:template match="title">
     <PageTitle>
       <xsl:apply-templates />
     </PageTitle>
</xsl:template>

which finds all <title> tags and replaces them with <PageTitle>. Any help would be greatly appreciated!

Answer 1

The first title element in the document is selected by:

(//title)[1]

Many people mistakenly think that //title[1] selects the first title in the document and this is a frequently committed error. //title[1] selects every title element that is the first title child of its parent — not what is wanted here.

Using this, the following transformation produces the required output:

<xsl:stylesheet version="1.0"
 xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
 <xsl:output omit-xml-declaration="yes" indent="yes"/>

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

 <xsl:template match=
  "title[count(.|((//title)[1])) = 1]">

     <PageTitle>
       <xsl:apply-templates />
     </PageTitle>
 </xsl:template>
</xsl:stylesheet>

When applied on this XML document:

<t>
 <a>
  <b>
    <title>Page Title</title>
  </b>
 </a>
 <b>
  <title/>
 </b>
 <c>
  <title/>
 </c>
</t>

the wanted result is produced:

<t>
 <a>
  <b>
    <PageTitle>Page Title</PageTitle>
  </b>
 </a>
 <b>
  <title />
 </b>
 <c>
  <title />
 </c>
</t>

Do note how we use the well-known Kaysian method of set intersection in XPath 1.0:

If there are two nodesets $ns1 and $ns2, the following expression selects every node which belongs to both $ns1 and $ns2:

$ns1[count(.|$ns2) = count($ns2)]

In the specific case when both node-sets contain only one node, and one of them is the current node, the following expression evaluates to true() exactly when the two nodes are identical:

count(.|$ns2) = 1

A variation of this is used in the match pattern of the template that overrides the identity rule:

title[count(.|((//title)[1])) = 1]

matches only the first title element in the document.