Re: [xsl] How to specify a path to a node in a nodeset using a variable

[Jeni Tennison <mail@xxxxxxxxxxxxxxxx>]

Hi Shakeel,

>> If you want to learn about XPath expressions, you should have a look
>> at the XPath Recommendation at http://www.w3.org/TR/xpath.  Or one of
>> the online tutorials about them.
>
> The XPath TR contains just 5 references to '$' and another 5
> references to 'VariableReference'. There arent any examples or
> explanation of VariableReference.

Yeah, it has to be said that specs are usually clear as mud when it
comes to learning anything.  Online tutorials are probably better at
explaining the basics.  But anyway...

VariableReference is defined in production 36 in XPath as:

  VariableReference ::= '$' QName

In other words, it's a '$' followed by a qualified name.  Qualified
names sometimes look like 'foo' or sometimes look like 'foo:foo' - the
':' is used to separate the prefix from the local name.

So '$container' is indeed a valid variable reference.

> Can VariableReference contain VariableReferences?

No (and I'm not sure what it would mean if it did?). The '$' that
starts a variable reference isn't a valid QName character.

> Can an expression contain more than one VariableReference?

Sure, for example:

  $foo = $bar

Variable references are primary expressions, so they can be used with
operators or at the beginning of a path expression (which is what
$container//account/person/@name is).  You can't use a variable
reference in the middle of a path expression (except within a
predicate [i.e. square brackets in the path] - predicates can hold any
expressions).  So:

  $foo/$bar

is illegal, although:

  $foo[$bar]

is fine.

The description of VariableReference is in Section 3.1
(http://www.w3.org/TR/xpath#section-Basics):

  "A VariableReference evaluates to the value to which the variable
  name is bound in the set of variable bindings in the context. It is
  an error if the variable name is not bound to any value in the set
  of variable bindings in the expression context."

In other words, a variable reference resolves to the value of the
named variable.  The QName in the VariableReference production is that
name.

I hope that helps,

Jeni

---
Jeni Tennison
http://www.jenitennison.com/



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