XSLT, 2nd Edition

Predicates

There’s one more aspect of XPath expressions that we haven’t discussed: predicates. These are filters that restrict the nodes selected by an XPath expression. Each predicate is evaluated and converted to a Boolean value (either true or false). If the predicate is true for a given node, that node will be selected; otherwise, it isn’t. Predicates always appear inside square brackets ([]). Here’s an example:

<xsl:apply-templates select="line[position() = 7]"/>

This expression selects the seventh <line> element in the current context. If there are six or fewer <line> elements in the current context, this XPath expression returns an empty node-set. Several things can be part of a predicate; we’ll go through them here.

Numbers in predicates

Instead of using the position() function, we can use a number. For example, the XPath expression line[7] selects the seventh <line> element in the context node; this means exactly the same thing as line[position() = 7]. XPath also provides the boolean and and or operators as well as the union operator (|) to combine predicates. The expression line[position()=3 and @style] matches all <line> elements that occur third and that have a style attribute, while line[position()=3 or @style] matches all <line> elements that either occur third or have a style attribute.

You can use more than one predicate if you like; line[3][@style] or line[@style][3] are both legal. They aren’t equivalent, however. Predicates are evaluated from left to right. The XSLT ...