Turning Up The Heat - XML

Just as you can declare elements, a DTD also allows you to define the attributes attached to each element. An attribute declaration typically looks like this:

<!ATTLIST elementName attributeName contentType modifier>

In order to demonstrate, consider the following XML document, which adds a couple of attributes to the previously declared XML elements.

<?xml version="1.0"?>
<weather>
<city state="NY">New York</city>
<temperature>
<high 
units="celsius">23</high>
<low units="celsius">10</low>
</temperature>
<forecast>sun</forecast>
</weather>

Here's the corresponding DTD:

<!-- element declarations -->
<!ELEMENT weather (city, temperature, forecast)>
<!ELEMENT 
city (#PCDATA)>
<!ELEMENT forecast (#PCDATA)>
<!ELEMENT high (#PCDATA)>
<!ELEMENT 
low (#PCDATA)>
<!ELEMENT temperature (high, low)>
<!-- attribute declarations 
-->
<!ATTLIST city state CDATA #REQUIRED>
<!ATTLIST high units CDATA #REQUIRED>
<!ATTLIST 
low units CDATA #REQUIRED>

Let's examine the first one a little more closely:

<!ATTLIST city state CDATA #REQUIRED>

In English, this declares that the element "city" has an attribute named "state" containing character data. The additional #REQUIRED modifier indicates that this is a required attribute - failure to include it will render the XML document invalid.

A number of different content types are available for attributes. You're already familiar with character data - here's a quick list of the others:

type  description
------------------------------------------
CDATA  character 
data
ID  unique identifier
IDREF  reference to unique identifier of another element
IDREFS 
list 
of identifiers
NMTOKEN string literal or token
NMTOKENS list of tokens
ENTITY 
entity
ENTITIES list 
of entities

You can also specify a list of allowed attribute values by enclosing them in parentheses and separating them with the | operator. The following attribute declaration does just that, limiting the list of allowed values for the "units" attribute to either "celsius" or "fahrenheit".

<!ATTLIST high units (celsius | fahrenheit) #REQUIRED>

A number of modifiers are available, each applying a special characteristic to the attribute. For example, you can specify a default value for the attribute by enclosing it in quotes; this default value is used if the attribute is absent.

<!ATTLIST high units (celsius | fahrenheit) "fahrenheit">

The #IMPLIED modifier is used to declare a particular attribute as optional.

<!ATTLIST high units #IMPLIED>

And finally, the #FIXED keyword is used to fix an attribute value to something specific, allowing the XML document author no choice in the matter.

<!ATTLIST high units (celsius | fahrenheit) #FIXED "fahrenheit">

If an element has more than one attribute, you can declare them all within the same attribute declaration. Consider the following XML document,

<?xml version="1.0"?>
<movie id="42" genre="sci-fi">
<title>Star Wars</title>
<cast>Mark 
Hamill, Carrie Fisher, Harrison Ford</cast>
<director>George Lucas</director>
</movie>

and its associated DTD, which demonstrates how this works.

<!-- element declarations -->
<!ELEMENT movie (title, cast, director)>
<!ELEMENT 
title (#PCDATA)>
<!ELEMENT cast (#PCDATA)>
<!ELEMENT director (#PCDATA)>
<!-- 
attribute declarations -->
<!ATTLIST movie
id CDATA #REQUIRED
genre CDATA 
#REQUIRED

Let's move on to entities.

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

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