How to select a node based on attribute values?

Fist off, I'm a total newbie when it comes to libxml.  I've
been able to 
work my way through some examples, and produce working code,
but I'm not 
entirely happy with the result, and I would like some
advice.

I have an XML tree that looks like this:

<root>
    <plc id="1">
       <some stuff/>
    </plc>
    <plc id="2">
       <some other stuff/>
    </plc>
</root>

I have a relatively small number (< 50) of the plc nodes,
but the whole 
tree is pretty big.

What is the best (or simplest way) to retrieve the
information for plc 2?

Should I pull all of the plc elements using
xmlXPathEvalExpression, and 
then iterate over them with xmlGetProp looking for an id
attribute with 
a value of "2"?

Or is there some other way to directly specify, I want this
element with 
these attributes?

Thanks for any guidance....  The docs are really thorough,
but libxml is 
so rich that the learning curve is a killer.....

--Yan

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On Tue, Nov 28, 2006 at 09:35:38PM -0500, John Dennis wrote:
> no need to iterate, just create an xpath expression
which specifies id=2

  Well libxml2 XPath implementation will iterate for you
more precisely.

> off the top of my head the expression would be
"/plc[id="2"]"
> 
> just google xpath syntax or xpath examples, you'll turn
up a ton
> 
> libxml2 xpath example C code here:
> http://x
mlsoft.org/examples/index.html#XPath
> 
> The xpath spec has examples too: http://www.w3.org/TR/xpath

> 
> If you're comfortable with python or another scripting
language you can
> construct a simple test program to exercise your xpath
syntax in about 5
> minutes.

  You can test that even faster using xmllint --shell
test.xml
and using the xpath command directly there

Daniel

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John Dennis wrote:

> On Tue, 2006-11-28 at 16:57 -0800, Yan Seiner wrote:
>
>>Fist off, I'm a total newbie when it comes to
libxml.  I've been able to 
>>work my way through some examples, and produce
working code, but I'm not 
>>entirely happy with the result, and I would like
some advice.
>>
>>I have an XML tree that looks like this:
>>
>><root>
>>    <plc id="1">
>>       <some stuff/>
>>    </plc>
>>    <plc id="2">
>>       <some other stuff/>
>>    </plc>
>></root>
>>
>>I have a relatively small number (< 50) of the
plc nodes, but the whole 
>>tree is pretty big.
>>
>>What is the best (or simplest way) to retrieve the
information for plc 2?
>>    
>>
>  
>
> xpath is the simplest and most powerful
>
>>Should I pull all of the plc elements using
xmlXPathEvalExpression, and 
>>then iterate over them with xmlGetProp looking for
an id attribute with 
>>a value of "2"?
>>    
>>
>
> no need to iterate, just create an xpath expression
which specifies id=2
>
> off the top of my head the expression would be
"/plc[id="2"]"
>
> just google xpath syntax or xpath examples, you'll turn
up a ton

OK, thanks a lot for the above!

I'm working with an embedded platform, so it took me a bit
to get all of 
the required libxml stuff together and rebuild PHP with
libxml support....

Now I have xpath working with PHP.   Yaay!

Ok, now for the PHP + xml question:

I am trying to get the base node of a schedule; I've
attached a small 
portion of my XML file below.  What I would like to do is to
get the 
root of a schedule, and then iterate with foreach through
all of the 
children.  This is the function I came up with and of course
it doesn't 
work.  I don't understand how I am supposed to work with the
object that 
the xpath_eval[_expression] returns.  I've also tried the
child_nodes 
method and that fails as well.

Hopefully there is a PHP expert on the list who can shed
some light on this.

Thanks,

--Yan

    function loadSchedule($pivotID, $schedName, $schedNum) {
    $dom = domxml_open_file("/tmp/test.conf");
    $xpath = xpath_new_context($dom);
    // find base node for our schedule
    $pat = '//root/pivot[id="' . $pivotID .
'"]/'
        . $schedName . '[id="' . $schedNum .
'"]';
    print $pat;
    $baseNode = xpath_eval($xpath,$pat);
    var_dump($baseNode);
    $lines =
xpath_eval_expression($xpath,'/line',$baseNode);
    var_dump($lines);
    print "nOKn";
    foreach($lines->nodeset as $line) {
        var_dump($line);
        }
    }

<root>
    <pivot id="1">
        <water id="1">
            <line id="1">
                <bearingTo>120.0</bearingTo>
                <WetDry>1</WetDry>
                <FwdRev>0</FwdRev>
                <event>0</event>
                <speed>10</speed>
            </line>
            <line id="2">
                <bearingTo>180.0</bearingTo>
                <WetDry>1</WetDry>
                <FwdRev>1</FwdRev>
                <event>0</event>
                <speed>45</speed>
            </line>
        </water>
    <pivot>
</root>
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Yan,

I answered your question off list as it doesn't pertain to
libxml2 but 
rather coding with domxml from PHP 4.

Seeing that there happens to be a good number of PHP based
questions 
coming up, I'd like to make sure people are aware of the
general PHP 
mailing list (php-generallists.php.net). The people
that follow that 
list can probably better handled a lot of these questions.


Rob

Yan Seiner wrote:
>
> I'm working with an embedded platform, so it took me a
bit to get all of 
> the required libxml stuff together and rebuild PHP with
libxml support....
>
> Now I have xpath working with PHP.   Yaay!
>
> Ok, now for the PHP + xml question:
>
> I am trying to get the base node of a schedule; I've
attached a small 
> portion of my XML file below.  What I would like to do
is to get the 
> root of a schedule, and then iterate with foreach
through all of the 
> children.  This is the function I came up with and of
course it doesn't 
> work.  I don't understand how I am supposed to work
with the object that 
> the xpath_eval[_expression] returns.  I've also tried
the child_nodes 
> method and that fails as well.
>
>   
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