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At 2004-07-26 10:46 +0100, james walker wrote:
>Does anybody know how to force an attribute to be used when another
>attribute is used?
This is called a co-occurrence constraint.
>I have a set of tags below:
><portlet imageurl="http://....." alt="this is an image" >
></portlet>
>sometimes the portlet may have an image and sometimes it may not. How do i
>construct the dtd attributes to force the person writing the xml to always
>have an alt attribute for every imageurl attribute.
DTD syntax is not expressive enough to express such a co-occurrence
constraint. Neither is W3C Schema.
You can express this in RELAX-NG. An example is below. Note how the
parentheses makes the pair of attributes an item that, as a pair, is
optional. This means that neither may be allowed, or both may be allowed,
but not only just one.
I hope this helps.
T:\ftemp>type walker.rng
start = element portlet
{
(
attribute imageurl { text },
attribute alt { text }
)?
}
T:\ftemp>type walker1.xml
<portlet imageurl="http://....." alt="this is an image" >
</portlet>
T:\ftemp>jing -c walker.rng walker1.xml
T:\ftemp>type walker2.xml
<portlet>
</portlet>
T:\ftemp>jing -c walker.rng walker2.xml
T:\ftemp>type walker3.xml
<portlet imageurl="http://.....">
</portlet>
T:\ftemp>jing -c walker.rng walker3.xml
T:\ftemp\walker3.xml:1: error: required attributes missing
T:\ftemp>
--
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