Examples

We have more to tell before we present "Location Steps in Summary," a later section that concisely explains what location steps do. For now, let's look at some more examples.

• As mentioned, /Insured/CarPolicy/Vehicle makes available all Vehicle nodes that are children of any CarPolicy child within Insured. The following expression resolves to the node whose string value is Buick.

   /Insured/CarPolicy/Vehicle[@Category='Coupe']/Make 

• In contrast, /Insured/CarPolicy[1]/Vehicle is more restrictive, making available all Vehicle nodes that are children of the first CarPolicy child within Insured. The following expression resolves to an empty set.

   /Insured/CarPolicy[1]/Vehicle[@Category='Coupe']/Make 

• /Insured/CarPolicy[last()] selects (within Insured) the last CarPolicy node, which is the node whose PolicyType attribute has the string value Antique.

• /Insured/CarPolicy[last()]/Vehicle[last()] selects the third Vehicle node within the second CarPolicy node. That Vehicle node has details on the Porsche Speedster.

Let's talk about the returned nodes whose string values are not available to you. The following expression resolves to a set of two Make nodes (one for Ford, one for Buick).

 /Insured/CarPolicy/Vehicle[2]/Make 

Only the first Make node seems to be available. The reason is that most XPath processors (such as the ones used with BPEL) provide the string value only of the first returned node.

The following expression returns an empty set.

 /Insured/CarPolicy/Vehicle[2]/Make[2] 

The set is empty because each of the two Vehicle nodes selected by /Insured/CarPolicy/Vehicle[2] has only one Make child, and the predicate [2] refers not to the whole expression but to the Make node.

How do you access only the Make node that refers to Buick? The solution is to use parentheses, which selects a node set that can be filtered by a predicate. In the next example, the XPath processor makes available only the second node in the node set.

 (/Insured/CarPolicy/Vehicle[2]/Make)[2] 

Incidentally, a parenthetical expression is valid only at the beginning of a location path in XPath 1.0. The next expression is valid only if you're working with XPath 2.0.

 <!- not a valid expression with XPath 1.0 -> /Insured/CarPolicy/(Vehicle) 

Now, let's look again at the effect of the last() function. The following expression returns, for each CarPolicy node, the Make child of the second Vehicle element:

 /Insured/CarPolicy/Vehicle[2]/Make[last()] 

The value of last() is 1, and although the XPath processor returns two nodes (one for Ford, one for Buick), the only returned string value is Ford.

The following expression returns the last Make node of the nodes returned from the parenthesized expression:

 (/Insured/CarPolicy/Vehicle[2]/Make)[last()] 

The value of last() is 2, and the related string value is Buick.

A predicate operates in the context of the previous location steps and the syntax that precedes the predicate in the same location step. Therefore … what is the result of the next expression?

 /Insured/CarPolicy/Vehicle[@Category='Sport'][2]/Make 

This expression evaluates to a single node whose string value is Porsche. The reason is that the predicate [2] is operating on each node set that is created by the syntax Vehicle[@Category='Sport']:

• Subordinate to the first CarPolicy node is a node set that contains all Vehicle nodes whose Category attribute equals Sport. One node is present, and its Make child has the string value Ford. When applied to this set, the predicate [2] yields an empty set.

• Subordinate to the second CarPolicy node is a node set that also contains all Vehicle nodes whose Category attribute equals Sport. Two nodes are present, and their Make children have the respective string values Triumph and Porsche. When applied to this set, the predicate [2] yields the second node.

Here's a variation:

 /Insured/CarPolicy/Vehicle[@Category='Sport'][last()]/Make 

Each of the branches mentioned earlier now contributes one node that is returned by last(). The overall expression yields a node set whose string values are Ford and Porsche, though only the string Ford is immediately available.

Order counts, even within a location step. The next example returns the Make node for the second Vehicle child within each CarPolicy node, with the further restriction that the value of attribute Category is Sport:

 /Insured/CarPolicy/Vehicle[2][@Category='Sport']/Make 

The string value of the single returned node is Ford.