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Re: valid maxOccurs values

From: Henry S. Thompson <ht@cogsci.ed.ac.uk>
Date: 01 Feb 2001 09:56:42 +0000
To: "Weber, Heiko" <Heiko.Weber@softwareag.com>
Cc: "'xmlschema-dev@w3.org'" <xmlschema-dev@w3.org>
Message-ID: <f5b7l3aenfp.fsf@cogsci.ed.ac.uk>
"Weber, Heiko" <Heiko.Weber@softwareag.com> writes:

> Hello,
> 
> as I was reading through the XML Schema 1 Specification I found
> a section which said:
> 
> "If maxOccurs is not unbounded, that is, it has a numeric value, then
> maxOccurs must [be] greater than or equal to 1"
> 
> on the other hand, I found "minOccurs=maxOccurs=0", for example:
> 
> "otherwise if the <element> element information item has <complexType> 
> or <group> as an ancestor and the ref [attribute] is absent, the
> corresponding 
> schema components are as follows (unless minOccurs=maxOccurs=0, in 
> which case the item corresponds to no component at all):"
> 
> So, can maxOccurs take the value 0 or not?  I'm a little confused here.
> Am I misunderstanding something?

Both are true, but you must keep track of the difference between a
component and its XML representation, to understand why.

Particle components have a {maxOccurs} property, which must be >=1 or
_unbounded_.  The first quote you give above is from the formal
constraint on particles [1] which enforces this.

Element information items in the XML representation of schemas may
have a 'maxOccurs' attribute, which may be 0 to effectively remove an
optional element during derivation of a type definition by
restriction.  The second quote you give above is from the description
of the mapping from EII to component [2], which actually fits
perfectly with the component constraint: if the attribute is 0, there
_is_ no corresponding component, so the constraint is not violated.

Hope this helps.

ht

[1] http://www.w3.org/TR/xmlschema-1/#p-props-correct
[2] http://www.w3.org/TR/xmlschema-1/#declare-element
-- 
  Henry S. Thompson, HCRC Language Technology Group, University of Edinburgh
          W3C Fellow 1999--2001, part-time member of W3C Team
     2 Buccleuch Place, Edinburgh EH8 9LW, SCOTLAND -- (44) 131 650-4440
	    Fax: (44) 131 650-4587, e-mail: ht@cogsci.ed.ac.uk
		     URL: http://www.ltg.ed.ac.uk/~ht/
Received on Thursday, 1 February 2001 04:56:45 UTC

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