How to apply successively two xpath expressions with libxml?

Question

Hi,

To sum up, i'm a totally beginner in libxml and I have to use an existing source code. The main idea is to apply a first xpath expression to extract a set of nodes from an xml file. Then, for each node, the second xpath expression shall be applied to extract some values.

Existing source code is:

int xt_parseXmlResult(xmlDocPtr doc, const char *xpath, assoc_arrayc_t expr, arrayc_t *result) { xmlXPathContextPtr xpathCtx = xmlXPathNewContext(doc);

// Register namespaces ...

/*
 * Evaluate main xpath expression
 */
xmlXPathObjectPtr xpathNodes = xmlXPathEvalExpression((xmlChar *)xpath, xpathCtx);

/*
 * Now we apply the xpath expressions on each node returned by the first xpath request
 */
// First loop is on the XML document as we have to create a new context each
// time we change the document
int nbDocs = xpathNodes->nodesetval->nodeNr;

for (row = 0; row < nbDocs; row++)
{
    xmlXPathContextPtr  subCtx = xmlXPathNewContext(doc);

 // Register namespaces ...

    // Update context to use the nodeset related to this row
    subCtx->node = xpathNodes->nodesetval->nodeTab[row];

    for (col = 0; col < expr.nbItems; col++)
    {
        // Evaluate expression
        xpathRows = xmlXPathEvalExpression((xmlChar *)expr.itemList[col].val, subCtx);

        result->data[(row + 1) * result->nbCols + col] = strdup((char *)xmlXPathCastToString(xpathRows)); 
        xmlXPathFreeObject(xpathRows);
    }
    xmlXPathFreeContext(subCtx);
    subCtx = NULL;
}

xmlFreeDoc(doc); 
xmlXPathFreeContext(xpathCtx);
xmlXPathFreeObject(xpathNodes);
return 0;

}

I think that the problem comes from this line

    // Update context to use the nodeset related to this row
subCtx->node = xpathNodes->nodesetval->nodeTab[row];

Because the second xpath expression is applied from the root of the xml file, not the root of each node.

Any idea on how to do such thing?

Best regards, Quentin

Answer 1

you could concatinate your xpath expressions.

edit

//FORECAST/DAY/descendant::content/meteo/desc should work

Answer 2

Some sample code. Modify to suit your needs and language. This is C#, but it should be largely the same. Notice the second xpath is not starting with a "/" and is using an instance of the node returned from the first one. Neither of the xpaths end in a "/".

XmlDocument doc = new XmlDocument();
doc.Load(docfile);
XmlNodeList items = doc.SelectNodes("/part1/part2");
foreach (item in items)
{
    XMLNode x = item.SelectNodes("part3");
    //Dostuff
}