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Re: dtd attributes

• To: xml-dev@l...
• Subject: Re: dtd attributes
• From: "G. Ken Holman" <gkholman@C...>
• Date: Mon, 26 Jul 2004 07:32:22 -0400
• In-reply-to: <BAY17-F183qrB0Xn2Pi000829a2@h...>

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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