courtxml.org

XML (Extensible Markup Language) is a popular and widely-implemented standard: you can use XML to create documents and data records that are fully portable and platform-independent.Learn the introductory basics of how to use XML for data exchange; including DTDs (Document Type Definitions), XSL (the Extensible Style sheet language) and how to construct an XML document.

INTRODUCTION:

XML is a markup language much like HTML.
XML was designed to describe data.
XML tags are not predefined in XML. You must define your own tags.
XML is self describing.
XML uses a DTD (Document Type Definition) to formally describe the data.

Why XML?

Structure is important to electronic documents.

When a program reads a file it tries to structure it into something it can work with. If the structure of the file is not the one the program was expecting, it will generate an error. In the old method of data file processing, files were structured by specifying a record size and specifying the location of each element in the record as a character position in that record. All it takes is being off by one character and everything is messed up. For instance, this situation is going to leave many people short-changed, and probably very unhappy.

Markup languages, although slightly more verbose in terms of file size, offer a benefit over data files whose structure is determined by character position. A markup language uses commands embedded right in the data file that explain the structure of the document to the program reading it. Markup based data files are longer than simple data files, but they are also much more flexible. U could add another element to the above records, for instance, employee name, and would probably not have to change a single program that reads the file. Programs that are not looking for an employee name field would just ignore it. In data files structured by position, every time you modify the structure of the file, you have to rewrite (or at least recompile) every program that reads or writes to that file.

This makes markup a very powerful tool. With advances in computing power and storage capabilities, its only major drawback, which is that it is verbose, is becoming less of an issue.

One such markup language is HTML. Another is XML. Both work by allowing structural information about a file to be encoded directly in that file. This means that the file, when it is opened, is capable of explaining itself to anything that can read the embedded commands.

Document structure is different than data file structure. The way markup languages work is based on the idea of document structure. Therefore, before we begin to look at XML, we should take a look at how documents are structured.

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