XPath - The Wizzart

In the following example, I demonstrate how to modify an XML attribute value in a particular XML node using XPath in MATLAB. XPath stands for XML Path Language and is a query language for selecting nodes in an XML document. The XPath language is based on a tree representation of the XML document.

Let’s consider the following sample XML file

<?xml version="1.0" encoding="utf-8"?>
<xml>
    <busStop endPos="30" id="BusStop0" lane="1to2_0" startPos="20"/>
    <busStop endPos="80" id="BusStop1" lane="1to2_0" startPos="70"/>
    <chargingStation chrgPower="220" endPos="30" id="ChrgStn1" lane="1to2_0" startPos="45"/>
</xml>

I am going to select the chargingStation node which id has the value of ChrgStn1. The XPath in this case is xml/chargingStation[@id="ChrgStn1"]. The full code in MATLAB would be:

import javax.xml.xpath.*
fXML=xmlread('SampleXML.xml');

factory = XPathFactory.newInstance;
xpath = factory.newXPath;

expression = xpath.compile('xml/chargingStation[@id="ChrgStn1"]');
chargingStation = expression.evaluate(fXML, XPathConstants.NODE); 

if ~isempty(chargingStation)
     chargingStation.setAttribute('startPos', '45')        
end

xmlwrite('SampleXMLOut.xml', fXML);

If you need to select multiple nodes, you shall change the XPathConstants.NODE to XPathConstants.NODESET.

In the following example, I demonstrate how to get all the node values from an XML based on the XPath in MATLAB and Java. XPath stands for XML Path Language and is a query language for selecting nodes in an XML document. The XPath language is based on a tree representation of the XML document. As I am going to show in this post, XPath provides a straightforward way to navigate the tree and select nodes according to various criteria.

Let’s consider the following example XML file (book.xml):

<?xml version="1.0"?>
<catalog>
   <book id="bk101">
      <author>Gambardella, Matthew</author>
      <author>Ralls, Kim Inside</author>
      <title>XML Developer's Guide</title>
      <genre>Computer</genre>
      <price>44.95</price>
      <publish_date>2000-10-01</publish_date>
      <description>An in-depth look at creating applications 
      with XML.</description>
   </book>
   <book id="bk101">
      <author>Ralls, Kim bk101</author>
      <title>Midnight Rain</title>
      <genre>Fantasy</genre>
      <price>5.95</price>
      <publish_date>2000-12-16</publish_date>
      <description>A former architect battles corporate zombies, 
      an evil sorceress, and her own childhood to become queen 
      of the world.</description>
   </book>   
   <book id="bk102">
      <author>Ralls, Kim</author>
      <title>Midnight Rain</title>
      <genre>Fantasy</genre>
      <price>5.95</price>
      <publish_date>2000-12-16</publish_date>
      <description>A former architect battles corporate zombies, 
      an evil sorceress, and her own childhood to become queen 
      of the world.</description>
   </book> 
</catalog>

What I would like to achieve is to get all the authors names for the books which have id of bk101. This can be easily achieved by using xpath:

mport javax.xml.xpath.*

factory = XPathFactory.newInstance;
xpath = factory.newXPath;

fXML=xmlread('books.xml');
authors = xpath.evaluate('catalog/book[@id="bk101"]/author', fXML, XPathConstants.NODESET);

for i=0:authors.getLength-1
    disp(authors.item(i).getFirstChild().getNodeValue())
end

If you are using the expression in a, for example, for or while loop, I recommend compiling it first to prevent performance drawbacks => The evaluate function will call the compile function each time. After updating the code, by including the compile function, the whole example would be:

import javax.xml.xpath.*

factory = XPathFactory.newInstance;
xpath = factory.newXPath;

fXML=xmlread('books.xml');
expression = xpath.compile('catalog/book[@id="bk101"]/author');
authors = expression.evaluate(fXML, XPathConstants.NODESET);

for i=0:authors.getLength-1
    disp(authors.item(i).getFirstChild().getNodeValue())
end

The Wizzart

Tag: XPath

How to modify an XML attribute value using Matlab?

How to get all the node values from an XML in MATLAB?