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Sams Teach Yourself XML in 24 Hours
You may want to keep a bookmark around for this page, as several of the XPath examples throughout the next section rely on the training log sample code.
Referencing Nodes
The most basic of all XPath patterns is the pattern that references the current node, which consists of a simple period:
.
If you're traversing a document tree, a period will obtain the current node. The current node pattern is therefore a relative pattern because it makes sense only in the context of a tree of data. As a contrast to the current pattern, which is relative, consider the pattern that is used to select the root node of a document. This pattern is known as the root pattern and consists of a single forward slash:
/
If you were to use a single forward slash in an expression for the training log sample
document, it would refer to the trainlog element (line 4) because this element is the
root element of the document. Because the root pattern directly references a specific
location in a document (the root node), it is considered an absolute pattern. The root
pattern is extremely important to XPath because it represents the starting point of
any document's node tree.
As you know, XPath relies on the hierarchical nature of XML documents to reference
nodes. The relationship between nodes in this type of hierarchy is best described
as a familial relationship, which means that nodes can be described as parent,
child, or sibling nodes, depending upon the context of the tree. For example,
the root node is the parent of all nodes. Nodes might be parents of some nodes
and siblings of others. To reference child nodes using XPath, you use the name
of the child node as the pattern. So, in the training log example, you can reference
a session element (line 6, for example) as a child of the root
node by simply specifying the name of the element: session. Of
course, this assumes that the root node (line 4) is the current context for
the pattern, in which case a relative child path is okay. If the root node isn't
the current context, you should fully specify the child path as /session.
Notice in this case that the root pattern is combined with a child pattern to
create an absolute path.
If there are child nodes there must also be parent nodes. To access a parent node, you must use two periods:
..
As an example, if the current context is one of the distance elements (line 15, for
example) in the training log document, the .. parent pattern will reference the parent
of the node, which is a session element (line 13). You can put patterns together to get
more interesting results. For example, to address a sibling node, you must first go to
the parent and then reference the sibling as a child. In other words, you use the parent
pattern (..) followed by a forward slash (/) followed by the sibling node name, like this:
../duration
This pattern assumes that the context is one of the child elements of the session
element (other than duration). Assuming this context, the ../duration pattern will
reference the duration element (line 14) as a sibling node.
Thus far I've focused on referencing individual nodes. However, it's also possible to select multiple nodes. For example, you can select all of the child nodes (descendants) of a given node using the double slash pattern:
//
As an example, if the context is one of the session elements in the training log
document (line 20, for example), you can select all of its child nodes by using
double slashes. This results in the duration (line 21), distance (line 22), location
(line 23), and comments (line 24) elements being selected.
Another way to select multiple nodes is to use the wildcard pattern, which is an asterisk:
*
The wildcard pattern selects all of the nodes in a given context. So, if the context
was a session element and you used the pattern */distance, all of the distance
elements in the document would be selected. This occurs because the wildcard
pattern first results in all of the sibling session elements being selected, after
which the selection is limited to the child distance elements.
To summarize, following are the primary building blocks used to reference nodes in XPath:
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These pattern building blocks form the core of XPath, but they don't tell the whole story. The next section explores attributes and subsets and how they are referenced.
Created: March 27, 2003
Revised: December 12, 2005
URL: http://webreference.com/programming/xml_24/1
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