Context Changes

David Carlisle

> Does the xpath expression  "A/C[position()=2]"   select the 3rd child
> of A, or the second one.

 A/C[position()=2 applies to the current step so it selects the second child of  C here.

If you want to select  C children that are the second child of A then

"A/*[position()=2 and self::C]
[Ednote: the self referred to here is the new context which changes 
  within the predicate to become the child node of A]
  
> (I mean, it requires an xsl:if test="name(*[position()=2])='C']"      )

name() isn't so safe, as it will be wrong if unexpected namespace
prefixes are used.

Wendell Piez

What you're missing is the concept of the "context node" for evaluating the expression. The expression "Conn" is short for "child::Conn". The expression "./Conn" is short for "self::node()/child::Conn".

In the example provided, the context node for the expression "@id=Conn" or "@id=./Conn" is the same -- as Mike said, the connection. (The context node for a predicate is each of the nodes being predicated.) Both of these expressions return the same thing when the context is the same. When you use a variable, you have already bound your Conn child to the variable when the expression is evaluated, so the context node is not a concern. Note that where you bind your variable does matter. If you did extra work to do the same thing, and said

<xsl:for-each select="document('connections.xml')/connections/connection">
   <xsl:variable name="conn" select="Conn"/>
   <xsl:if test="@id = $conn">
     <xsl:value-of select="."/>
   </xsl:if>
</xsl:for-each>

it wouldn't work. (Nor would you, an XSLT expert, expect it to.) But this would:

<xsl:variable name="conn" select="Conn"/> <xsl:for-each select="document('connections.xml')/connections/connection">
   <xsl:if test="@id = $conn">
     <xsl:value-of select="."/>
   </xsl:if>
</xsl:for-each>

Right? (And this is the better analogue to what you're doing when your predicate has "@id=$conn")

The short answer: you probably want your predicate to read "@id=current()/Conn", though without seeing the rest of the logic it's impossible to be certain. Or just use the variable: we do this all the time.

Wendell Piez

What you're missing is the concept of the "context node" for evaluating the expression.

The expression "Conn" is short for "child::Conn".

The expression "./Conn" is short for "self::node()/child::Conn".

In the example provided, the context node for the expression "@id=Conn" or "@id=./Conn" is the same, the connection. (The context node for a predicate is each of the nodes being predicated.) Both of these expressions return the same thing when the context is the same.

When you use a variable, you have already bound your Conn child to the variable when the expression is evaluated, so the context node is not a concern. Note that where you bind your variable does matter.

If you did extra work to do the same thing, and said

<xsl:for-each select="document('connections.xml')/connections/connection">
   <xsl:variable name="conn" select="Conn"/>
   <xsl:if test="@id = $conn">
     <xsl:value-of select="."/>
   </xsl:if>
</xsl:for-each>

it wouldn't work. (Nor would you, an XSLT expert, expect it to.) But this would:

<xsl:variable name="conn" select="Conn"/> <xsl:for-each select="document('connections.xml')/connections/connection">
   <xsl:if test="@id = $conn">
     <xsl:value-of select="."/>
   </xsl:if>
</xsl:for-each>

Right? (And this is the better analogue to what you're doing when your predicate has "@id=$conn")

The short answer: you probably want your predicate to read "@id=current()/Conn", though without seeing the rest of the logic it's impossible to be certain. Or just use the variable: we do this all the time.