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Can't get node with libxml2 and xpath

 原文  2014-01-30 15:50:02  0   2    c++/ xpath/ libxml2

Question

I have the following piece of code to find all elements from an HTML page: 

string AParser::cleanHTMLDocument(const string& aDoc) {
   vector<xmlNodePtr> nodesToRemove;
   xmlDocPtr doc = xmlParseDoc((xmlChar *)aDoc.c_str());
   xmlXPathContextPtr context = xmlXPathNewContext(doc);
   xmlXPathObjectPtr result = xmlXPathEvalExpression(
                                 (const xmlChar *)string("//link").c_str(), context);
   if (xmlXPathNodeSetIsEmpty(result->nodesetval)) {
       xmlXPathFreeObject(result);
       xmlXPathFreeContext(context);
       xmlFreeDoc(doc);
       LOG(WARNING)<< "XPath is invalid, bailing out.";
       return string();
   }
   const int size = result->nodesetval->nodeNr;
   for(int i = size - 1; i >= 0; i--) {
      LOG(DEBUG)<< result->nodesetval->nodeTab[i]->name;
   }
}

But for some reason xmlXPathNodeSetIsEmpty is always true. Am I missing something here?

Update: Input Document 

<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE html PUBLIC '-//W3C//DTD XHTML 1.1//EN' 'http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd'>
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
<head>
<meta name="generator" content="HTML Tidy for Linux (vers 7 December 2008), see www.w3.org"/>
<title>The Republic, by Plato</title>
<link href="0.css" type="text/css" rel="stylesheet"/>
<link href="1.css" type="text/css" rel="stylesheet"/>
<link href="pgepub.css" type="text/css" rel="stylesheet"/>
<meta content="EpubMaker 0.3.20a6 by Marcello Perathoner &lt;webmaster@gutenberg.org&gt;" name="generator"/>
</head>
<body>
<div xml:space="preserve" class="pgmonospaced pgheader"><br/>The Project Gutenberg EBook of The Republic, by Plato<br/><br/>This eBook is for the use of anyone anywhere at no cost and with<br/>almost no restrictions whatsoever.  You may copy it, give it away or<br/>re-use it under the terms of the Project Gutenberg License included<br/>with this eBook or online at www.gutenberg.org<br/><br/><br/>Title: The Republic<br/><br/>Author: Plato<br/><br/>Translator: B. Jowett<br/><br/>Release Date: August 27, 2008 [EBook #1497]<br/>Last Updated: November 5, 2012<br/><br/>Language: English<br/><br/><br/>*** START OF THIS PROJECT GUTENBERG EBOOK THE REPUBLIC ***<br/><br/><br/><br/><br/>Produced by Sue Asscher, and David Widger<br/><br/><br/><br/><br/><br/></div>
<p><br/>
<br/></p>
<h1 id="pgepubid00000">THE REPUBLIC</h1>
<p><br/></p>
<h2>By Plato</h2>
<p><br/></p>
<h3 id="pgepubid00001">Translated by Benjamin Jowett</h3>
<p><br/>
<br/>
<br/>
Note: See also "The Republic" by Plato, Jowett, etext #150<br/>
<br/></p>
<hr/>
<p><br/>
<br/></p>
<h2 id="pgepubid00002">Contents</h2>
<table summary="">
<tbody><tr>
<td>
<p class="toc"><a href="@public@vhost@g@gutenberg@html@files@1497@1497-h@1497-h-0.htm.html#link2H_INTR" class="pginternal">INTRODUCTION AND ANALYSIS.</a></p>
<br/>
<p class="toc"><a class="c1 pginternal" href="@public@vhost@g@gutenberg@html@files@1497@1497-h@1497-h-7.htm.html#link2H_4_0002">THE REPUBLIC.</a></p>
<p class="toc"><a href="@public@vhost@g@gutenberg@html@files@1497@1497-h@1497-h-7.htm.html#link2H_4_0003" class="pginternal">PERSONS OF THE DIALOGUE.</a></p>
<p class="toc"><a href="@public@vhost@g@gutenberg@html@files@1497@1497-h@1497-h-7.htm.html#link2H_4_0004" class="pginternal">BOOK I.</a></p>
<p class="toc"><a href="@public@vhost@g@gutenberg@html@files@1497@1497-h@1497-h-8.htm.html#link2H_4_0005" class="pginternal">BOOK II.</a></p>
<p class="toc"><a href="@public@vhost@g@gutenberg@html@files@1497@1497-h@1497-h-9.htm.html#link2H_4_0006" class="pginternal">BOOK III.</a></p>
<p class="toc"><a href="@public@vhost@g@gutenberg@html@files@1497@1497-h@1497-h-10.htm.html#link2H_4_0007" class="pginternal">BOOK IV.</a></p>
<p class="toc"><a href="@public@vhost@g@gutenberg@html@files@1497@1497-h@1497-h-11.htm.html#link2H_4_0008" class="pginternal">BOOK V.</a></p>
<p class="toc"><a href="@public@vhost@g@gutenberg@html@files@1497@1497-h@1497-h-12.htm.html#link2H_4_0009" class="pginternal">BOOK VI.</a></p>
<p class="toc"><a href="@public@vhost@g@gutenberg@html@files@1497@1497-h@1497-h-14.htm.html#link2H_4_0010" class="pginternal">BOOK VII.</a></p>
<p class="toc"><a href="@public@vhost@g@gutenberg@html@files@1497@1497-h@1497-h-14.htm.html#link2H_4_0011" class="pginternal">BOOK VIII.</a></p>
<p class="toc"><a href="@public@vhost@g@gutenberg@html@files@1497@1497-h@1497-h-16.htm.html#link2H_4_0012" class="pginternal">BOOK IX.</a></p>
<p class="toc"><a href="@public@vhost@g@gutenberg@html@files@1497@1497-h@1497-h-16.htm.html#link2H_4_0013" class="pginternal">BOOK X.</a></p>
</td>
</tr>
</tbody></table>
<p><br/>
<br/></p>
<hr/>
<p><br/>
<br/>
<a id="link2H_INTR"><!--  H2 anchor --></a></p>
<h2 id="pgepubid00003">INTRODUCTION AND ANALYSIS.</h2>
<p>The Republic of Plato is the longest of his works with the exception of the Laws, and is certainly the greatest of them. There are nearer approaches to modern metaphysics in the Philebus and in the Sophist; the Politicus or Statesman is more ideal; the form and institutions of the State are more clearly drawn out in the Laws; as works of art, the Symposium and the Protagoras are of higher excellence. But no other Dialogue of Plato has the same largeness of view and the same perfection of style; no other shows an equal knowledge of the world, or contains more of those thoughts which are new as well as old, and not of one age only but of all. Nowhere in Plato is there a deeper irony or a greater wealth of humour or imagery, or more dramatic power. Nor in any other of his writings is the attempt made to interweave life and speculation, or to connect politics with philosophy. The Republic is the centre around which the other Dialogues may be grouped; here philosophy reaches the highest point (cp, especially in Books V, VI, VII) to which ancient thinkers ever attained. Plato among the Greeks, like Bacon among the moderns, was the first who conceived a method of knowledge, although neither of them always distinguished the bare outline or form from the substance of truth; and both of them had to be content with an abstraction of science which was not yet realized. He was the greatest metaphysical genius whom the world has seen; and in him, more than in any other ancient thinker, the germs of future knowledge are contained. The sciences of logic and psychology, which have supplied so many instruments of thought to after-ages, are based upon the analyses of Socrates and Plato. The principles of definition, the law of contradiction, the fallacy of arguing in a circle, the distinction between the essence and accidents of a thing or notion, between means and ends, between causes and conditions; also the division of the mind into the rational, concupiscent, and irascible elements, or of pleasures and desires into necessary and unnecessary—these and other great forms of thought are all of them to be found in the Republic, and were probably first invented by Plato. The greatest of all logical truths, and the one of which writers on philosophy are most apt to lose sight, the difference between words and things, has been most strenuously insisted on by him (cp. Rep.; Polit.; Cratyl), although he has not always avoided the confusion of them in his own writings (e.g. Rep.). But he does not bind up truth in logical formulae,—logic is still veiled in metaphysics; and the science which he imagines to 'contemplate all truth and all existence' is very unlike the doctrine of the syllogism which Aristotle claims to have discovered (Soph. Elenchi).</p>
</body></html>

2 answers

solution1
 4 ACCPTED  2014-01-30 22:14:16

The document you're querying uses an XML namespace. You have to either ignore the namespace or register and use it.

To ignore namespaces , query for all nodes and compare the local name (without namespaces) in a predicate, like in //*[local-name(.) = 'link'] .

To register a namespace , call xmlXPathRegisterNs and afterwards prefix all nodes having that namespace with [ns-prefix]: . For example: 

xmlXPathContextPtr context = xmlXPathNewContext(doc);
xmlXPathRegisterNs(context, 'xhtml', 'http://www.w3.org/1999/xhtml');
xmlXPathObjectPtr result = xmlXPathEvalExpression(
                       (const xmlChar *)string("//xhtml:link").c_str(), context);

solution2
 1  2014-01-30 22:19:41

As an alternative solution to the one posted by Jens, you could parse the HTML document using libxml2's HTML parser . All you have to do is to replace xmlParseDoc with htmlParseDoc .

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