Re: [xsl] XSL, when elements don't exist

[Wendell Piez <wapiez@xxxxxxxxxxxxxxxx>]

Tanya,

At 02:37 PM 10/28/2003, you wrote:
> I'm trying to process an XML document for which certain elements may or may
> not exist.
> How can I handle this?
> E.g.
> <xsl:if test= "element exists"></xsl:if>
> <xsl:if test= "element does not exist"></xsl:if>
> ...

The trick to this is knowing some of the details of how XPath works.

XPath has four data types. (XSLT 1.0 adds a fifth, the 
result-tree-fragment, which needn't concern us here.) They are:

node sets (sets of nodes in a source document)
numbers
strings
Booleans

In addition, XPath has rules about how to convert from any of these to the 
others. One of these rules, for converting a node set into a Boolean, is 
particularly handy: if the node set contains any nodes, it's true; if it 
doesn't, it's false.

Since a test attribute in a conditional expects a Boolean true or false, 
the trick for seeing whether a node exists is simply to retrieve it. If the 
set of nodes you get back contains any members, it passes the truth test; 
if it doesn't, it fails.

As I understand it, you need to do something special when nodes like these 
don't exist:

> <property name="d" displayname="Defer Node Expansion" value="true" />
> <property name="tr" displayname="Traverse DOM" value="true" />

...this would be (assuming your context node is the PropertyGroup):

<xsl:if test="not(child::property[@displayname='Defer Node Expansion']
               or child::property[@displayname='Traverse DOM'])">

using the Boolean 'or' operator (and the not() function), or

<xsl:if test="not(child::property[@displayname='Defer Node Expansion'] |
                  child::property[@displayname='Traverse DOM'])">

using the union operator '|', which combines two node sets.

If you apply the rule for converting node sets to Booleans in each case, 
you can see why they do the same thing here -- even though the 'or' 
operator is *not* the same as the '|' operator.

Incidentally, the casting rules referred to above make it generally 
superfluous in XSLT to write tests such as 
test="string-length(normalize-space($node) = 0", since you can always do 
things like test="not(normalize-space($node))" and get the same answer.

I hope this helps,
Wendell



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Wendell Piez                            mailto:wapiez@xxxxxxxxxxxxxxxx
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