| MST 2004 John Fleck | d14bccc | 2004-02-15 01:57:42 +0000 | [diff] [blame] | 1 | <html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>Using XPath to Retrieve Element Content</title><meta name="generator" content="DocBook XSL Stylesheets V1.61.2"><link rel="home" href="index.html" title="Libxml Tutorial"><link rel="up" href="index.html" title="Libxml Tutorial"><link rel="previous" href="ar01s04.html" title="Retrieving Element Content"><link rel="next" href="ar01s06.html" title="Writing element content"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Using XPath to Retrieve Element Content</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ar01s04.html">Prev</a> </td><th width="60%" align="center"> </th><td width="20%" align="right"> <a accesskey="n" href="ar01s06.html">Next</a></td></tr></table><hr></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="xmltutorialxpath"></a>Using XPath to Retrieve Element Content</h2></div></div><div></div></div><p>In addition to walking the document tree to find an element, |
| MDT 2003 John Fleck | 63f3a47 | 2003-07-24 21:48:30 +0000 | [diff] [blame] | 2 | <span class="application">Libxml2</span> includes support for |
| 3 | use of <span class="application">XPath</span> expressions to retrieve sets of |
| 4 | nodes that match a specified criteria. Full documentation of the |
| 5 | <span class="application">XPath</span> <span class="acronym">API</span> is <a href="http://xmlsoft.org/html/libxml-xpath.html" target="_top">here</a>. |
| 6 | </p><p><span class="application">XPath</span> allows searching through a document |
| 7 | for nodes that match specified criteria. In the example below we search |
| 8 | through a document for the contents of all <tt class="varname">keyword</tt> |
| 9 | elements. |
| MST 2004 John Fleck | d14bccc | 2004-02-15 01:57:42 +0000 | [diff] [blame] | 10 | </p><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><table border="0" summary="Note"><tr><td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="images/note.png"></td><th align="left">Note</th></tr><tr><td colspan="2" align="left" valign="top"><p>A full discussion of <span class="application">XPath</span> is beyond |
| 11 | the scope of this document. For details on its use, see the <a href="http://www.w3.org/TR/xpath" target="_top">XPath specification</a>.</p></td></tr></table></div><p> |
| MDT 2003 John Fleck | 63f3a47 | 2003-07-24 21:48:30 +0000 | [diff] [blame] | 12 | Full code for this example is at <a href="apd.html" title="D. Code for XPath Example">Appendix D, <i>Code for XPath Example</i></a>. |
| 13 | </p><p>Using <span class="application">XPath</span> requires setting up an |
| 14 | xmlXPathContext and then supplying the <span class="application">XPath</span> |
| 15 | expression and the context to the |
| 16 | <tt class="function">xmlXPathEvalExpression</tt> function. The function returns |
| 17 | an xmlXPathObjectPtr, which includes the set of nodes satisfying the |
| 18 | <span class="application">XPath</span> expression.</p><p> |
| MST 2003 John Fleck | 731967e | 2003-01-27 00:39:50 +0000 | [diff] [blame] | 19 | </p><pre class="programlisting"> |
| MDT 2003 John Fleck | 63f3a47 | 2003-07-24 21:48:30 +0000 | [diff] [blame] | 20 | xmlXPathObjectPtr |
| 21 | getnodeset (xmlDocPtr doc, xmlChar *xpath){ |
| 22 | |
| 23 | <a name="cocontext"></a><img src="images/callouts/1.png" alt="1" border="0">xmlXPathContextPtr context; |
| 24 | xmlXPathObjectPtr result; |
| MDT 2002 John Fleck | 598f6eb | 2002-06-04 15:10:36 +0000 | [diff] [blame] | 25 | |
| MDT 2003 John Fleck | 63f3a47 | 2003-07-24 21:48:30 +0000 | [diff] [blame] | 26 | <a name="cocreatecontext"></a><img src="images/callouts/2.png" alt="2" border="0">context = xmlXPathNewContext(doc); |
| 27 | <a name="corunxpath"></a><img src="images/callouts/3.png" alt="3" border="0">result = xmlXPathEvalExpression(xpath, context); |
| 28 | <a name="cocheckxpathresult"></a><img src="images/callouts/4.png" alt="4" border="0">if(xmlXPathNodeSetIsEmpty(result->nodesetval)){ |
| MDT 2004 John Fleck | 4c3bb7d | 2004-08-25 02:51:27 +0000 | [diff] [blame^] | 29 | xmlXPathFreeObject(result); |
| MDT 2003 John Fleck | 63f3a47 | 2003-07-24 21:48:30 +0000 | [diff] [blame] | 30 | printf("No result\n"); |
| MDT 2004 John Fleck | 4c3bb7d | 2004-08-25 02:51:27 +0000 | [diff] [blame^] | 31 | return NULL; |
| MST 2003 John Fleck | 731967e | 2003-01-27 00:39:50 +0000 | [diff] [blame] | 32 | </pre><p> |
| MDT 2004 John Fleck | 4c3bb7d | 2004-08-25 02:51:27 +0000 | [diff] [blame^] | 33 | </p><div class="calloutlist"><table border="0" summary="Callout list"><tr><td width="5%" valign="top" align="left"><a href="#cocontext"><img src="images/callouts/1.png" alt="1" border="0"></a> </td><td valign="top" align="left"><p>First we declare our variables.</p></td></tr><tr><td width="5%" valign="top" align="left"><a href="#cocreatecontext"><img src="images/callouts/2.png" alt="2" border="0"></a> </td><td valign="top" align="left"><p>Initialize the <tt class="varname">context</tt> variable.</p></td></tr><tr><td width="5%" valign="top" align="left"><a href="#corunxpath"><img src="images/callouts/3.png" alt="3" border="0"></a> </td><td valign="top" align="left"><p>Apply the <span class="application">XPath</span> expression.</p></td></tr><tr><td width="5%" valign="top" align="left"><a href="#cocheckxpathresult"><img src="images/callouts/4.png" alt="4" border="0"></a> </td><td valign="top" align="left"><p>Check the result and free the memory allocated to |
| 34 | <tt class="varname">result</tt> if no result is found.</p></td></tr></table></div><p> |
| MDT 2003 John Fleck | 63f3a47 | 2003-07-24 21:48:30 +0000 | [diff] [blame] | 35 | </p><p>The xmlPathObjectPtr returned by the function contains a set of nodes |
| 36 | and other information needed to iterate through the set and act on the |
| 37 | results. For this example, our functions returns the |
| 38 | <tt class="varname">xmlXPathObjectPtr</tt>. We use it to print the contents of |
| 39 | <tt class="varname">keyword</tt> nodes in our document. The node set object |
| 40 | includes the number of elements in the set (<tt class="varname">nodeNr</tt>) and |
| 41 | an array of nodes (<tt class="varname">nodeTab</tt>): |
| MST 2003 John Fleck | 731967e | 2003-01-27 00:39:50 +0000 | [diff] [blame] | 42 | </p><pre class="programlisting"> |
| MDT 2003 John Fleck | 63f3a47 | 2003-07-24 21:48:30 +0000 | [diff] [blame] | 43 | <a name="conodesetcounter"></a><img src="images/callouts/1.png" alt="1" border="0">for (i=0; i < nodeset->nodeNr; i++) { |
| 44 | <a name="coprintkeywords"></a><img src="images/callouts/2.png" alt="2" border="0">keyword = xmlNodeListGetString(doc, nodeset->nodeTab[i]->xmlChildrenNode, 1); |
| 45 | printf("keyword: %s\n", keyword); |
| MST 2004 John Fleck | d14bccc | 2004-02-15 01:57:42 +0000 | [diff] [blame] | 46 | xmlFree(keyword); |
| MDT 2003 John Fleck | 63f3a47 | 2003-07-24 21:48:30 +0000 | [diff] [blame] | 47 | } |
| MST 2003 John Fleck | 731967e | 2003-01-27 00:39:50 +0000 | [diff] [blame] | 48 | </pre><p> |
| MDT 2003 John Fleck | 63f3a47 | 2003-07-24 21:48:30 +0000 | [diff] [blame] | 49 | </p><div class="calloutlist"><table border="0" summary="Callout list"><tr><td width="5%" valign="top" align="left"><a href="#conodesetcounter"><img src="images/callouts/1.png" alt="1" border="0"></a> </td><td valign="top" align="left"><p>The value of <tt class="varname">nodeset->Nr</tt> holds the number of |
| 50 | elements in the node set. Here we use it to iterate through the array.</p></td></tr><tr><td width="5%" valign="top" align="left"><a href="#coprintkeywords"><img src="images/callouts/2.png" alt="2" border="0"></a> </td><td valign="top" align="left"><p>Here we print the contents of each of the nodes returned. |
| MST 2004 John Fleck | d14bccc | 2004-02-15 01:57:42 +0000 | [diff] [blame] | 51 | </p><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><table border="0" summary="Note"><tr><td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="images/note.png"></td><th align="left">Note</th></tr><tr><td colspan="2" align="left" valign="top"><p>Note that we are printing the child node of the node that is |
| MDT 2003 John Fleck | 63f3a47 | 2003-07-24 21:48:30 +0000 | [diff] [blame] | 52 | returned, because the contents of the <tt class="varname">keyword</tt> |
| MST 2004 John Fleck | d14bccc | 2004-02-15 01:57:42 +0000 | [diff] [blame] | 53 | element are a child text node.</p></td></tr></table></div><p> |
| MDT 2003 John Fleck | 63f3a47 | 2003-07-24 21:48:30 +0000 | [diff] [blame] | 54 | </p></td></tr></table></div><p> |
| 55 | </p></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ar01s04.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="index.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ar01s06.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Retrieving Element Content </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Writing element content</td></tr></table></div></body></html> |