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Re: DTD Entity
- From: Danny Vint <firstname.lastname@example.org>
- To: email@example.com
- Date: Sat, 10 Mar 2001 12:11:13 -0800
This was one of those simplifications from SGML that XML has taken. In SGML
there was the '&' and that allowed you to say A and B and C in any order
but only once.
XML Schemas have added this feature back into their content models.
At 02:50 PM 3/10/2001 -0500, Charles Reitzel wrote:
>I don't think you're missing anything. It appears to be a limitation of the
>DTD content model rules.
>You might elaborate all the combinations:
><!ELEMENT Base ( First,
> ( Second? | Third? | Fourth?
> | (Second, Third) | (Third, Second)
> | (Third, Fourth) | (Fourth, Third)
> | (Second, Third, Fourth)
> | (Fourth, Second, Third)
> | (Third, Fourth, Second)
> | (Third, Second, Fourth)
> | ... you get the idea ...
>The problems w/ this are a) it runs the risk of becoming non-deterministic
>(I can't tell by looking) and b) the number of combinations will explode
>quickly with more child elements.
>As a practical matter, you'll have to choose. If lack of order is most
>important, go with ( First, (Second|Third|Fourth)* ). If ordinality is most
>important (0 or 1 vs. 0 or more vs. 1 or more), go with your original.
>take it easy,
>At 09:29 AM 3/10/01 -0500, you wrote:
>>Since it isn't quite dead yet (just mortally wouned), I have a question DTD
>>Note: Beginner's question is being posed. Please excuse any cases of gross
>lack of knowledge.
>>We are working on DTD with the following requirement:
>>- Base element consists of Child elements First, Second, Third, and Fourth.
>>- First must appear under Base.
>>- Second, Third, and Fourth may appear zero or one time.
>>- Order is not important for elements Second, Third, and Fourth.
>>The original statement in my DTD was
>><!ELEMENT Base (First, Second?, Third?, Fourth?)>
>>Which imposes an order constraint on the elements. It was suggested that
>><!ELEMENT Base (First (Second|Third|Fourth)?)>
>>was more correct. However, this appears it will allow only Second, Third,
>_or_ Fourth element, not all three. Is
>><!ELEMENT Base (First (Second|Third|Fourth)*)>
>>seems to allow enough elements (zero or one of each) but also allows more
>>Is there a good way to address this situation or is this a limit of DTDs?
>Do Schemas handle this any better?
>take it easy,