Ask Ben: Selecting XML Attributes Given Other XML Attributes : Kinky Solutions : A Student's Perspective by Ben Nadel

Ask Ben: Selecting XML Attributes Given Other XML Attributes

Posted September 19, 2007 at 6:39 PM

Earlier today, someone asked me about searching XML documents in such a way that he only wanted to select a tag attribute if the parent tag had another attribute set to a given value. This is actually a simple task with the use of a Predicate. As a review from my ColdFusion XPath and XmlSearch() presentation, predicates are XPath constructs contained in square brackets that filter the results of the returned nodes. Predicates need to result in a boolean-true in order for the selected node to be returned in the final node set.

In our case, there are two ways we can look at the problem that have slightly different XPath values. We want to:

Select an attribute node that is part of a element node that has an attribute of a given value.
Select an attribute node that has a sibling attribute node of a given value.

While these might sound like the same thing, and do, in fact, result in the same node set, they require different XPath values. Both of these situations are demonstrated in this example:

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<cfxml variable="xmlGirls">
<girls>
<girl
name="Samantha"
age="27"
hair="Blonde"
/>
<girl
name="Kim"
age="32"
hair="Brunette"
/>
<girl
name="Cindi"
age="25"
hair="Black"
/>
</girls>
</cfxml>
<!---
Get the Name attribute nodes of all the girls
who are brunetted. We are going to be doing this
by only looking in girl nodes that have a hair
attribute that is brunette.
--->
<cfset arrNodes1 = XmlSearch(
xmlGirls,
"//girl[ @hair = 'Brunette' ]/@name"
) />
<!---
Get the Name attribute nodes of all the girls
who are brunetted. We are going to be doing this
by getting all name nodes who have a sibling
attribute node, hair, that is Brunette.
--->
<cfset arrNodes2 = XmlSearch(
xmlGirls,
"//girl/@name[ ../@hair = 'Brunette' ]"
) />
<cfdump
var="#arrNodes1#"
label="Names of Burnette Girls - Method ##1"
/>
<cfdump
var="#arrNodes2#"
label="Names of Burnette Girls - Method ##2"
/>

Notice that in our first ColdFusion XmlSearch() call, our XPath value is first limiting on the Girl node, using a predicate that requires the Hair attribute to be Brunette. Then in our second ColdFusion XmlSearch() call, our predicate is requiring a sibling Hair attribute with no explicit mention of the parent tag (other than by relative relationship).

Running the above tag, we get the two CFDump outputs:

As you can see, both return the proper Name attribute, Kim. Now, is there a difference between the two different XPath values used? From a readability standpoint, I think the first one is better. From a performance standpoint, I am going to assume that the first one is also a better choice. Just as with a SQL WHERE clause, I think in an XPath statement, you are gonna get better performance by putting the most limiting statements first; filtering on the Girl node will result in less Name attribute node evaluations and therefore might perform better.

Hope that helps a bit.

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