A brief introduction to xml  (Page 3/3)

No small task

That is no small task. Over the years, dozens of different types of computers have been built, operating under several different operating systems, andrunning thousands of different programs. As a result, insofar as the exchange of structured documents is concerned, the computer world is a modern manifestationof the "Tower of Babel" where everyone spoke a different language. XML attempts to rectify this situation by providing a common language for structureddocuments.

What Does XML Contribute?

Without getting into the technical details at this point, XML provides a definition of a simple scheme by which the structure and the content of adocument can be established. The resulting physical document is so simple that any computer (or any human) can read it with only a modest amount of preparation. You will sometimes see XML referred to as a "meta" language.

What Does Meta Mean?

In computer jargon, the term meta is often used to identify something that provides information about something else. (If you want to impress someone at your next cocktail party, mention that meta information is information aboutinformation.)

For example, consider the listings of stock prices, bond prices, and mutual fund prices that commonly appear in most daily newspapers. The various tables onthe page provide information about the bid and ask prices for the various stock, bond, and mutual fund instruments.

Usually somewhere on the page, you will find an explanation as to how to interpret the information presented throughout the remainder of the page. Youcould probably think of the information contained in the explanation as metainformation. It provides information about other information.

So, why might people refer to XML as a meta language?

If you write a book, XML doesn't tell you how to structure the document that represents your book. Rather, it provides you with a set of rules that you canuse to establish structure and content when you create the document that represents your book. It is up to you to decide how you will use those rules toestablish the structure and content of your book.

Information about new languages

You might say that XML is a language that provides information about a new language that you are free to invent. For example, Flex is a specializedprogramming language that is based on XML. XML doesn't specify the language. Instead, XML provides the tools used by the inventors of the Flex programminglanguage to specify the structure of the language.

Different flavors of XML

Similarly, XML doesn't specify CNXML. Instead, XML provides the tools used by the inventors of CNXML to specify the format of documents suitable forpublication on the Connexions website. In the past, I have also published documents on a particular IBMwebsite. That website uses a different flavor of XML to specify the format of documents suitable for publication on the website.

Transportable

If you follow the rules for creating an XML document, the document that you create can be easily transported among various computers and rendered in avariety of different ways.

Multiple renderings

For example, you might want to have two different renderings of your book. One rendering might be in conventional printed format and the other renderingmight be in an online format. The use of XML makes it practical to render your book in two or more different ways without any requirement to modify theoriginal document that you produce.

Applying XML

At this point, I am going to provide two different examples of actual XML code, either of which might reasonably represent the simple book examplepresented earlier in Figure 1. The first example is shown in Figure 2.

If you compare this example with the book example given earlier , you should be able to see a one-to-one correspondence between the "elements" in this XML code and the description ofthe book presented earlier.

Introducing attributes

The example in Figure 3 provides an improvement over the example in Figure 2. Figure 3 provides an "attribute" in each of the chapter elements. Each attributespecifies the chapter number.

That's a wrap

That's enough for this module. In the next module, I will begin discussing the syntax shown in Figure 3 and begin the explanation of tags , elements , content , and attributes .

Miscellaneous

This section contains a variety of miscellaneous materials.

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