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Lesson 16
For loop can be used in the code of Registration form to check that users do not type invalid characters in
the text box. For example, in case a user types a "," in the text box for user login, an alert box can be made
to display informing him that it is an invalid user login. See Fig. 1 below.
Fig. 1
Look at the code to understand For statement/ loop. The initialization statement is executed only at the
beginning of the For loop's execution. The condition is then tested, and if it is true the statements enclosed
within the curly brackets are executed. If the condition is false, the loop is terminated and the statement
following the For statement is executed.
Another check can also be applied to see that the passwords entered in two different text boxes by the user
are the same. In case the two passwords do not match an alert box can inform the user about it (see Fig. 2
below). In case no condition applied in the code is violated then the function checkValues returns true at
`onsubmit'. Consequently, the information provided by the user in the form is forwarded to the server side.
Fig. 2
Extensible markup language
World Wide Web Consortium (W3C), a non-profit organization that maintains standards for the web
presented the draft of XML in late 1990's. It is also used for web page creation and includes data
management capabilities that HTML cannot provide. Consider the example of a list of planets. Suppose that
same HTML heading tags are decided to be used each planet. Also, suppose that it is decided to display
different pieces of information about a planet in different heading sizes. Then, there is a shortcoming in
respect of HTML that it can only supply upto 6 different levels of headings. In case there are more than six
different pieces of information to display, then HTML loses its efficacy. That is why web professionals have
found XML as a list formatting alternative of HTML. Following is a simple HTML code for preparing the
list of planets:
<HTML> <HEAD><TITLE>Planets</TITLE> </HEAD> <BODY> <h1>Planets</h1>
<h3>36 million miles</h3>
<h5>176 Days</h5>
<h3>67 million miles</h3>
<h5>117 days</h5>
<h3>93 million miles</h3>
<h5>24 Hours</h5>
Result is shown in Fig. 3 below.
Fig. 3
XML differs from HTML in two important respects. Firstly, XML is not a markup language with defined
tags; rather, one can create one's own set of tags in XML. Secondly, XML tags do not provide information
how text would appear on a web page. Instead of that XML tags convey meaning of information included
within them. To understand these distinctions consider the example of planets' list again. Following is the
XML code for that:
<?xml version="1.0"?>
<Planet Name="Mercury"> <Distance>36 million miles</Distance> <Moons>None</Moons>
<DayLength>176 days</DayLength> </Planet>
<Planet Name="Venus"> <Distance>67 million miles</Distance> <Moons>None</Moons>
<DayLength>117 days</DayLength> </Planet>
<Planet Name="Earth"> <Distance>93 million miles</Distance> <Moons>One</Moons>
<DayLength>24 Hours</DayLength> </Planet>
First line of the code is a declaration that it is an XML document (version 1). Second and last lines of the
code are called root element tags. We enclose other elements within the root element tags. We assign a
name to the root element that best describes the purpose of our file. Other elements are called child
elements. Thus, planet is a child element of planetlist. Further, each property of a planet is the child element
of the planet element. So, distance, moons and daylength are the child elements of planet element. Name is
the attribute of the planet element. Names of child elements can be different between two organizations,
which can make the sharing of information difficult. For instance, some may describe the property of a
planet as Day and others may use the word Daylength for that purpose. This has led to the necessity of
having uniform standards for writing different types of XML documents. Many companies have agreed to
follow common standards for XML tags. A file that uses XML tags is called data type definition (DTD) or
XML schema. Different DTDs are available for different industries. We now have accounting information,
legal information standards etc.
Rules for writing an XML code
-All elements must be properly nested <outer><inner>content</inner></outer>
-All attribute values must be quoted <FRIES SIZE="LARGE">
-All elements with empty content must be identified by ending in />
<BR/>, <img src="image2.gif" />
- All elements must be cased consistently <PART> must not be closed as </part>
- Certain characters having reserved meanings cannot be used e.g, & , < etc. Embedding XML into
HTML documents
<XML> element can be used anywhere within HTML document to enclose XML content.
See the following example in this regard:
<h1>HTML text here</h1>
Also, <script> element can be used for such purpose e.g, <script language="xml"
Extensible Style sheet Language (XSL) XML files are translated using another file which contains
formatting instructions. Formatting instructions are often written in Extensible Style sheet Language (XSL).
These formatting instructions are read over by special programs ­ usually these programs are written in Java
programming language- called XML Parsers. Following diagram (Fig. 4) explains how web server might
process http request for an XML page.
Fig. 4
We write XSL rules that match various xml elements. For that consider the following example:
XML code <?xml version="1.0"?> <?xml-stylesheet href="catalog.xsl" type="text/xsl" ?>
<PART> <NAME>Switch</NAME> <DESCRIPTION>A very efficient device</DESCRIPTION>
<PRICE>Rs. 1000</PRICE>
.... </CATALOG> </xml>
XSL code
We can provide the formatting instructions for the above XML code by writing an XSL
code as given below:
<?xml version="1.0"?>
<root /> <html> <body bgcolor=yellow> <children />
</html> </rule> <rule>
<target-element type="PART" /> <DIV style="margin-bottom:20px"> <children /> </DIV>
<rule> <element type="PART"> <target-element type="NAME" /> </element>
<B> <children />
<B> <BR /> </rule> ......... </xsl>
Note that we use <rule> elements and <target-element> in our XSL code to provide formatting
instructions for the corresponding XML elements.
HTML and XML editors
General purpose text-editors for HTML are Notepad, Wordpad etc. However, there are certain HTML
editors that help create web pages more easily, e.g, Macromedia Dreamweaver and Microsoft FrontPage.
XML code can also be written in any general purpose text editor. However, there are special programs such
as Epic Editor, TurboXML which can facilitate the editing job considerably.