Re: Complex Path Checking

[Jeni Tennison <jeni at friday dot u-net dot com>]

Heiner,

>This needs to stay as is. My problem is, I want to check the content 
>of the first <node2> (with "some content") before I decide if I 
>display the content of the other <node2> (with "other content"). I
>need to do this through the whole large document.
>
>Now, I'm not exactly experienced when it comes to XPath. I tried the 
>following, but as I already expected ;) , it didn't work.
>
> <xsl:template match="//node2">

I'm just going to go over your XPaths to make sure that you understand what
they're doing before helping you with your actual problem.

The syntax '//' within an XPath is short for
'/descendant-or-self::node()/', so this expands to:

  /descendant-or-self::node()/node2

If you were using this expression to *select* nodes (e.g. in
xsl:apply-templates or xsl:for-each), then it would select:

all the 'node2' elements that are a child of
  any node that is a descendent of (or is itself)
    the root node

When you use an XPath to *match* nodes, as you are here (or if you were
matching nodes to key into), it will match:

any 'node2' element that is a child of
  any node that is a descendent of (or is itself)
    the root node

In other words, this matches any node2 element in the document.  Another
XPath that matches any node2 element in the document is simply 'node2', so
you may as well say:

<xsl:template match="node2">
  ...
</xsl:template>

It's just a small thing, but it took me ages to get my head around the fact
that the 'match' attributes don't *select* nodes, they test a node you've
already select, which means they can usually be fairly simple.

> <xsl:if 
>test="contains(ancestor::following-sibling::child::node(),'X')">

You're going wrong here because you're forgetting to separate your steps
properly.  Each step is made up of an axis (like ancestor,
following-sibling or child) and a node test.  The node test usually gives
the name of the node.  Each step is separated from the next step using a
'/'.  I think you were trying for something like:

  ancestor::*/following-sibling::*/child::node()

This will select:

any node (of any type) that is a child of
  any element that is a sibling that follows
    any element that is an ancestor of
      the current node

I doubt that you are really after any node of any type - are you really
interested if an attribute or a comment contains an 'X'?  If you are just
after elements, then it would help to say so.  You can also lose the
child:: axis if you want - it's assumed by default.  So try:

  <xsl:if test="contains(ancestor::*/following-sibling::*/*, 'X')">
    ...
  </xsl:if>

Anyway, your actual problem was that you wanted to check the content of the
first node2 element to see whether you should process the second node2
element.

Within XSLT, processing flows from the top of the *tree* to the bottom of
the tree rather than from the top of the *document* to the bottom of the
document.  So, the children of the root node are processed, and their
processing involves the processing of their children, which involves the
processing of their children and so on.

This means that the right place to decide whether to process a particular
node is either higher up the *tree* rather than higher up the *document*.
Another approach is is to process the node, but not produce any output
unless certain conditions are met.

Here's an example that always processes the first node2, but only processes
the second node2 if there's an ancestor of the first node2 which has a
following sibling which has a child that contains an 'X':

<xsl:template match="root">
  <xsl:apply-templates select="node1/node2[1]" />
  <xsl:if
      test="contains(node1/node2[1]/ancestor::*/following-sibling::*/*, 'X')">
    <xsl:apply-templates select="node1/node2[2]" /> 
  </xsl:if>
</xsl:template>

Here's another example which decides whether to produce any output within
the node2-matching template:

<xsl:template match="root">
  <xsl:apply-templates select="node1/node2" />
</xsl:template>

<xsl:template match="node2">
  <xsl:if
    test="position() = 1 or
          contains(node1/node2[1]/ancestor::*/following-sibling::*/*, 'X')">
    ...produce some output...
  </xsl:if>
</xsl:template>

I hope that this helps get your project finished,

Jeni

Dr Jeni Tennison
Epistemics Ltd * Strelley Hall * Nottingham * NG8 6PE
tel: 0115 906 1301 * fax: 0115 906 1304 * email: jeni.tennison@epistemics.co.uk


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