I would like to determine whether a element C with a selected attribute 'car' is the first element within <A>,but one point is very import , something like this Structure can appear in font of this.In other way there is no element which satisfies ancestor::A and preceding::C.Thanks a lot.
<A>
  <B>
    <C name="car"> </C>
   </B>
     <C name="car"> </C>
    <D> </D>
</A>

Sorry, but your question isn't simple at all!

>I would like to determine whether a element C with a selected attribute 'car' is the first element within <A>

Do you mean

(1) "is the first child element of <A>"

(2) "is the first child element of <A> whose name is <C>"

(3) "is the first child element of <A> that has a selected attribute whose name is 'name' and whose value is 'car'

(4) "is the first descendant element of <A>"

(5) "is the first descendant element of <A> whose name is <C>"

(6) something else....?

>something like this Structure can appear in font of this

What does "this Structure" refer to?

(1) The whole of the XML fragment you have shown?

(2) The <B> element?

(3) Something else?

>In other way there is no element which satisfies ancestor::A and preceding::C

In your example, that condition is satisfied by the first C, and is not satisfied by the second C. But until now, I thought your rules would match the second C but not the first. Please be more clear!



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

@Volder: Two points about your example above - firstly, the "//C" will mean the the 'C' element wouldn't have to be the child of 'A'. Secondly I believe that ".//C[1][@name='car']" will have the same effect as the above code, and may be faster.

/- Sam Judson : Wrox Technical Editor -/

Looking at Volder's reply:

  <xsl:template match="A">
      <xsl:choose>
         <xsl:when test="generate-id(//C[1])=generate-id(//C[@name='car'][1])">the first C has name "car"</xsl:when>
         <xsl:otherwise>the first C's name is not "car"</xsl:otherwise>
      </xsl:choose>
   </xsl:template>

(1) I suspect that "//C" (both occurrences) was supposed to be ".//C"; I also suspect that ".//C[1]" should be "(.//C)[1]" or "descendant::C[1]" - the first descendant C, as distict from every C descendant that is the first child of its parent.

(2) If the OP really meant to ask the question "Is the name attribute of the first C descendant equal to 'car'?" (and it's not at all clear to me that this was the intent), then the question can be answered with the rather simpler expression

test="descendant::C[1]/@name = 'car'"

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

>test="descendant::C[1]/@name = 'car'"

yep, this is what I meant
and accept my mistakes.

dear All,
my purpose is making xslt intelligent as human being has ,i give quantification 'a' in front of the first appearance of 'Car',and give 'the' to the following 'car' .i simplify this code in order to be be understood easily,but this code is more complex and this is just a piece of whole code which is comprised of many units having <A> element. one thing we should care about fore unit is that there is possibility that a element with attribute whose value 'car' .i want to make sure in this unit the 'car' is the first 'car'.so xslt parses <C> of every unit,when it find that it is ,then add 'a' ,otherwise 'the' .i hope that i make the thing clear.what i have to mention is that our template is on every <C>

it works using function generate-id().thanks to everyone.

if I were a developer and someone wrote requirments sepcification in that a manner - I'd better crash my head over the wall )))

petergoodman, please read it yourself, just to see whether you can understand it yourself?

if you were attentive reading the thread you would realize, that
1) it is not totally correct
2) there is much simpler version to do the same