XPath predicates¶

A predicate is a [...] filter attached to a location step. The step first selects a set of nodes; the predicate then keeps only those for which the bracketed expression is true. You can read cd[price > 10] as "the cd elements that have a price child greater than 10".

For these examples the catalog gains a genre attribute, and the first CD keeps its optional <year> child while the others omit it:

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<catalog>
  <cd genre="rock"><title>Empire Burlesque</title><artist>Bob Dylan</artist><price>10.90</price><year>1985</year></cd>
  <cd genre="pop"><title>Hide your heart</title><artist>Bonnie Tyler</artist><price>9.90</price></cd>
  <cd genre="country"><title>Greatest Hits</title><artist>Dolly Parton</artist><price>9.90</price></cd>
</catalog>

Boolean and comparison predicates¶

The expression inside [...] is evaluated relative to each candidate node and coerced to a boolean. Comparisons are the common case:

<xsl:for-each select="catalog/cd[price &gt; 10]">     <!-- (1)! -->
  <p><xsl:value-of select="title"/></p>
</xsl:for-each>

1. Only CDs whose <price> child is numerically greater than 10.

String equality works the same way. catalog/cd[artist = 'Bob Dylan'] keeps the CDs whose <artist> child equals that string:

Existence predicates¶

A bare child or attribute name is true when that node exists, because XPath treats a non-empty node-set as true. This is the standard way to filter on optional content.

<xsl:for-each select="catalog/cd[year]">     <!-- (1)! -->
  <p><xsl:value-of select="title"/></p>
</xsl:for-each>

1. Keep only CDs that have a <year> child.

Wrap the name in not(...) to invert it. catalog/cd[not(year)] selects the CDs that have no <year> child:

Attribute predicates¶

Attributes are addressed with the @ prefix, and behave exactly like child elements in a predicate — test their value, or just their existence.

<xsl:value-of select="catalog/cd[@genre = 'pop']/title"/>   <!-- (1)! -->

1. The title of the CD whose genre attribute is pop.

catalog/cd[@genre] would match every CD here, since all three carry a genre attribute; catalog/cd[@id] would match none, since no CD has an id.

Positional predicates¶

A numeric predicate selects by position within the current node-set. Positions are 1-based, and [1] is simply shorthand for [position() = 1].

<xsl:value-of select="catalog/cd[1]/title"/>
<xsl:value-of select="catalog/cd[last()]/title"/>

position() and last() are also useful arithmetically. A common trick is row striping — shading every even row — with the mod operator:

<xsl:for-each select="catalog/cd">
  <tr>
    <xsl:if test="position() mod 2 = 0">          <!-- (1)! -->
      <xsl:attribute name="bgcolor">#eeeeee</xsl:attribute>
    </xsl:if>
    <td><xsl:value-of select="title"/></td>
  </tr>
</xsl:for-each>

1. True for positions 2, 4, 6, … — i.e. every other row.

The selector catalog/cd[position() mod 2 = 0] likewise keeps just the even-positioned CDs.

Chaining predicates¶

Multiple predicates apply left to right, and each one re-numbers the node-set it receives. Order therefore matters:

<xsl:value-of select="catalog/cd[price &gt; 9][1]/title"/>    <!-- (1)! -->

1. All three CDs exceed 9, so this is the first of them.

Here both expressions happen to yield Empire Burlesque, but they would differ if the first CD were the cheap one: cd[1][price > 9] could then select nothing, while cd[price > 9][1] would still return the first qualifying CD.

The //cd[1] gotcha¶

(//cd)[1]      <!-- the one first cd in the whole document -->
//cd[1]        <!-- each cd that is its parent's first cd -->

In this flat catalog the two expressions coincide, but as soon as cd elements appear under more than one parent they diverge. When you mean "the first match anywhere", reach for (//cd)[1].

Predicates in match patterns¶

Everything above applies wherever an XPath expression is used — the select of xsl:for-each, xsl:apply-templates, and xsl:value-of, and also the match pattern of a template. A predicate in a match pattern restricts which nodes the template fires for:

<xsl:template match="cd[price &gt; 10]">      <!-- (1)! -->
  <p class="premium"><xsl:value-of select="title"/></p>
</xsl:template>

<xsl:template match="cd">                    <!-- (2)! -->
  <p><xsl:value-of select="title"/></p>
</xsl:template>

1. Fires only for CDs over 10; the more specific pattern wins.
2. The fallback for every other cd.

Summary¶

Next¶

Predicates often test text, and real filters need to trim, match, and compare strings precisely — see String functions.