XPath Axis

You'll remember from the lesson on location paths that each location path is made up of location steps. Location steps can also include an axis.

In this lesson, we'll look at XPath axes (plural for axis).

What is an Axis?

An axis stores certain information about the context node or other nodes within the document. The information it stores depends on the axis being used. For example, an axis called "child" stores information about the children of the context node. Therefore, we could use this axis to select a child from the context node.

Syntax

You use an axis by using it's name, along with a node test, and optionally, one or more predicates. The axis and node test are separated by ::, and the predicate/s are enclosed in [].

Axis without predicates - Here's the syntax you use if you don't have any predicates:

axis::node_test

Axis with predicates - Here's the syntax you use if you have one or more predicates:

axis::node_test[predicate/s]

Axis Usage

Up until now, our location steps have specified the exact nodes in the path, or they have used a wildcard to specify unknown nodes. For example, they have been constructed something like this:

node1/node2/*

We have other options when building our XPath expressions - we could use an axis.

Consider the following expression:

child::title

This expression looks a little different to the ones we've looked at so far. The expression is selecting a node called "title", but only when it's a child of the context node (remember that context node refers to the current node).

You might be thinking, "Yeah that's fine, but how do I know what the context node is?"

Example

Here's an example of how we could use the axis mentioned above:

<xsl:template match="book">
  <xsl:value-of select="child::title"/>
</xsl:template>

You'll remember from our XSLT lessons that this statement is selecting the value from a node within the "book" context. We know we're in the "book" context because of the match="book" (which is selecting a node called "book"). Therefore, we can explicitly state that we want to select the "title" node which is a child of the context node.

List of Axes

There are many other axes you can use within your XPath expressions. Here's a list of the axes you can use with XPath:

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