Rainy Days - XML

Once you have an XML document and a DTD linked together, an XML parser can verify the document against the DTD and let you know if it finds errors. A number of tools are available online to perform this validation - my favourite is the XML Spy editor, available at http://www.xmlspy.com/ , although you can also try out expat, at http://sourceforge.net/projects/expat/, and rxp, at http://www.cogsci.ed.ac.uk/~richard/rxp.html

Here's what rxp has to say when I run it on the XML document above.

$ rxp -V weather.xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE weather 
SYSTEM "weather.dtd">
<weather>
<city>New York</city>
<high>26</high>
<low>18</low>
<forecast>rain</forecast>
</weather>

In other words - no error.

Let's suppose I altered the XML document instance a little, by modifying one of the element names and adding a new element.

<?xml version="1.0"?>
<!DOCTYPE weather SYSTEM "weather.dtd">
<weather>
<city>New 
York</city>
<high>26</high>
<low>18</low>
<weekly_mean>21</weekly_mean>
<daily_forecast>rain</daily_forecast>
</weather>

While this version of the document is still well-formed, it no longer follows the rules laid down in "weather.dtd" and hence cannot be considered valid - which is why rxp barfs and generates a list of errors.

$ rxp -V weather.xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE weather 
SYSTEM "weather.dtd">
<weather>
<city>New York</city>
<high>26</high>
<low>18</low>
Warning: Start tag 
for undeclared element weekly_mean
in unnamed entity at line 7 char 14 of weather.xml
Warning: 
Content model for weather does not allow element weekly_mean here
in unnamed 
entity at line 7 char 14 of weather.xml
<weekly_mean>21</weekly_mean>
Warning: Start tag for undeclared element daily_forecast
in unnamed entity 
at line 8 char 17 of weather.xml
<daily_forecast>rain</daily_forecast>
</weather>

Incidentally, it's also possible to place the DTD within the XML document itself. Although this is quite rare - the DTD is usually stored in a central place so that it can be referenced by different XML documents - you should know how to do it in case you're ever home on a Saturday evening and feel like experimenting.

<?xml version="1.0"?>
<!DOCTYPE weather [
<!ELEMENT weather (city, 
high, low, forecast)>
<!ELEMENT city (#PCDATA)>
<!ELEMENT high (#PCDATA)>
<!ELEMENT 
low (#PCDATA)>
<!ELEMENT forecast (#PCDATA)>
]>
<weather>
<city>New 
York</city>
<high>26</high>
<low>18</low>
<forecast>rain</forecast>
</weather>

>>> More XML Articles          >>> More By Vikram Vaswani, (c) Melonfire

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