Learning XML, 2nd Edition

Finding Nodes

There are still a few cultures on earth who can name their ancestors back ten generations or further. “Here is Sam, the son of Ben, the son of Andrew, the son of...” This chain of generations helps establish the identity of a person, showing that he or she is a member of such and such a clan or related to another person through some shared great-great-uncle.

XPath, too, uses chains of steps, except that they are steps in an XML tree rather than an actual family tree. The terms “child” and “parent” are still applicable. A location path is a chain of location steps that get you from one point in a document to another. If the path begins with an absolute position (say, the root node), then we call it an absolute path . Otherwise, it is called a relative path because it starts from a place not yet determined.

A location step has three parts: an axis that describes the direction to travel, a node test that specifies what kinds of nodes are applicable, and a set of optional predicates that use Boolean (true/false) tests to winnow down the candidates even further.

The axis is a keyword that specifies a direction you can travel from any node. You can go up through ancestors, down through descendants, or linearly through siblings. Table 6-1 lists all the types of node axes.

Table 6-1. Node axes

Axis type	Matches
Ancestor	All nodes above the context node, including the parent, grandparent, and so on up to the root node.
Ancestor-or-self	The ancestor node plus the context node. ...