Home arrow XML arrow Page 6 - The Fundamentals of DTD Design

Turning Up The Heat - XML

Ever tried to read a DTD, and failed miserably? Ever wondered what all those symbols and weird language constructs meant? Well, fear not - this crash course will get you up to speed with the basics of DTD design in a hurry.

TABLE OF CONTENTS:
  1. The Fundamentals of DTD Design
  2. DTD Who?
  3. Rainy Days
  4. Simply Elementary
  5. What's The Frequency, Bobby?
  6. Turning Up The Heat
  7. An Entity In The Attic
  8. The Old Popcorn Trick
By: Vikram Vaswani, (c) Melonfire
Rating: starstarstarstarstar / 6
September 27, 2001

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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.

 
 
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