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From: William Bagby <williamb@adone.com>
To: "XML Mailing List (E-mail)" <general@xml.apache.org>
Subject: Setting defaults in DTD
Date: Tue, 8 May 2001 13:17:22 -0400 
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I'm still sort of a novice when it comes to DTD's, so please bear with me...

I have an element called <animal-list> which contains one or more
<animal>'s, each with a unique type attribute.  Additionally, each <animal>
contains the same elements.  Here's an example:

<animal-list>
	<animal type="cat">
		<name>Felix</name>
		<color>black</color>
	</animal>  
	<animal type="dog">
		<name>Fido</name>
		<color>brown</color>
	</animal>  
	<animal type="bird">
		<name>Tweety</name>
		<color>yellow</color>
	</animal>
</animal-list>

which has the corresponding DTD:

<!ELEMENT animal-list (animal+)>
<!ELEMENT animal (name,color)>
<!ATTLIST animal type (cat,dog,bird,moose,cow,horse,rabbit,snake) #REQUIRED>

What I'd like to do is specify a default <name> based on what type of
<animal> it is, for example, the default name for a "cat" is Felix, and so
on.  Unless I am mistaken, there is no way to do this.  So, I restructured
the XML so that each child of <animal-list> is distinct:

<animal-list>
	<cat>
		<name>Felix</name>
		<color>black</color>
	</cat>  
	<dog>
		<name>Fido</name>
		<color>brown</color>
	</dog>  
	<bird>
		<name>Tweety</name>
		<color>yellow</color>
	</bird>
</animal-list>

This has it's own drawbacks, not the least of which is a rather unruly DTD,
and it still doesn't allow for defaults.  So I try again:

<animal-list>
	<cat name="Felix" color="black"/>
	<dog name="Fido" color="brown"/>  
	<bird name="Tweety" color="yellow"/>
</animal-list>

OK, now I'm getting somewhere.  Now I can specify defaults.  But my DTD is
still unruly, and it seems a terrible waste to have different elements when
they are really the same element with different characteristics.

Is there a simple and elegant solution to this problem?  I'm of course
making the assumption that one of you has run into this problem before.

Please help me...

William





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