I am confused between supporting page numbers in our application. The choices I have are as an element or as a processing instruction. Which is better, and why? I see the advantage of being able to link to a page number if implemented as an element, since you can't do that in a processing instruction.
what I would like to know is what can I do with a processing instruction that I can't do in an element, and visa versa.

Thanks,
Bill

The only two differences that seem important are:

1. Elements can have attributes and children, processing instructions can't.

2. The existence and content of an element can be constrained by a DTD or schema. Processing instructions are not subject to such constraints. They can appear anywhere and they can contain arbitrary text.

and perhaps 3: Everyone uses elements and understands them. Hardly anyone uses processing instructions.

Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference

Thanks Mike. This helps a lot.

2. The existence and content of an element can be constrained by a DTD or schema. Processing instructions are not subject to such constraints. They can appear anywhere and they can contain arbitrary text.
This is interesting. That means we can have some weird results if the XML file is not checked carefully. But can you please give me an example of arbitary text?

The only advantage I see for using processing instruction is there is no need to send out a new version of your software! do you agree?

Bill

>But can you please give me an example of arbitary text?

E'WIORVB

Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference

>The only advantage I see for using processing instruction is there is no need to send out a new version of your software! do you agree?

I've been known to use them to trigger processing by an XSLT stylesheet. For example

<?current-date?>

causes the stylesheet to insert the current date. The advantage of using a PI rather than an element <current-date/> is, I suppose, that you don't have to change the DTD/Schema (or negotiate with the owner of the DTD/Schema). The disadvantage is that what you've done isn't documented in the DTD/Schema, which is where people will tend to look when they want to know what game you're playing at.

Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference

You mean if I have a page number as a processing instruction, this will be valid and display without a problem!
<?page E'WIORVB?>

Thanks Mike for making all this more clear to me.

Bill