On Nov 5, 4:29 pm, Joseph Kesselman <keshlam-nos...@comcast.net>
wrote:
> mit...@juno.com wrote:
> > I am a newbie to XML. I have the following question: how do I express
> > something in XML when the subsequent tag structures depends on the
> > value of a preceeding tag?
>
> Neither DTDs nor XML Schema allow you to use the value of an element or
> attribute to constrain the presence or another -- with the exception of
> the xsi:type attribute; see a good XML Schema tutorial (or a great deal
> of recent discussion in this very newsgroup) for details of how to use
> that sub-typing feature.
>
> You *can* structure things so the presence or absence of your <TAG2>
> element gates whether TAG3 or TAG4 was expected after it, by writing the
> appropriate content models in your DTD or Schema.
>
> Or you can enforce the cross-constraint in your application rather than
> expecting the document formalism to do it all. Remember, neither DTDs
> nor schemas are intended to capture all the semantics of a document, and
> some checking will always have to be done by the application --
> vaidation only provides an initial layer and some "machine-readable
> documentation" of what's expected.
>
> --
> Joe Kesselman / Beware the fury of a patient man. -- John Dryden

Thanks. I will look into XML Schema tutorial.

However, from your respnse it seems I might not have communicated the
objective behind doing this. While I can enforce the check of the
subsequent tag structure (TAG3 or TAG4) in the application domain, the
primary objective is to define *one* XML structure that (based on
TAG2) defines the *entire* structure. This will be akin to saying, in
this example,

if (TAG2 value is 1) then
   <TAG3>some_value</TAG3>
else
   <TAG4>some_value2</TAG4>

Perhaps, XML Schema does it?