Microsoft Word Applied xml a toolkit for Programmers Wiley doc

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Ceponkus, Hoodbhoy - Applied XML - Toolkit for Programmers

DTD Basics—The How Tos
We now enter the realm of creating XML DTDs. We’ve already mentioned what DTDs
are, but we haven’t stressed enough yet why they are important.
DTDs are important because they provide a way for explicitly declaring the structure of
your XML documents. From a programming perspective, dealing with unstructured XML
is pretty difficult because you can’t really predict what information is going to be coming
next. Even intuitive structures can take on different forms; for example, in an address
data set, you may have name, address, and telephone as your fields, but who’s to say
that address can’t be preceded by telephone? Thus you end up having to create
programming instances to accommodate as many scenarios as you can to further
process information. But that solution doesn’t scale well; after all, the permutations of
markup structures increases exponentially as data sets increase in size and complexity.
Imagine what a nightmare that would be.
To make things easier, DTDs are a method for explicitly and completely defining the
structure of a given XML document. DTDs are used by validating parsers that read a
DTD, interpret the rules dictated within, and rigorously check to see if the XML data
contained within the document is in conformance with the DTD.

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Now we’re ready to take a closer look at what XML DTDs are made of and how to make
your own custom DTD.
A DTD can consist of the following parts:
•
Element type declarations
•
Attribute list declarations
•
Entity declarations
•
Notations declarations
Let’s take a more detailed look at each of these declarations and their subsections, which
we’ve illustrated with many examples for your convenience. If some sound silly, bear with
us.

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