XML Attributes
XML elements can have attributes in the start tag, just like HTML.
Attributes provide additional information about elements.
XML Attributes
From HTML you will remember this: <img src="computer.gif">. The
"src" attribute provides additional information about the <img>
element.
In HTML (and in XML) attributes provide additional information about
elements:
<img src="computer.gif">
<a href="demo.asp">
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Attributes often provide information that is not
a part of the data. In the example
below, the file type is irrelevant to the data, but
important to the software that wants to manipulate the element:
<file type="gif">computer.gif</file>
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XML Attributes Must be Quoted
Attribute values must always be enclosed in quotes, but
either single or double quotes can be used. For a person's
sex, the person tag can be written like this:
or like this:
If
the attribute value itself contains double quotes you can use
single quotes, like in this example:
<gangster name='George "Shotgun" Ziegler'>
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or you can use character entities:
<gangster name="George "Shotgun" Ziegler">
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XML Elements vs. Attributes
Take a look at these examples:
<person sex="female">
<firstname>Anna</firstname>
<lastname>Smith</lastname>
</person>
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<person>
<sex>female</sex>
<firstname>Anna</firstname>
<lastname>Smith</lastname>
</person>
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In the first example sex is an attribute. In the last, sex is an
element. Both examples provide the same information.
There are no rules about when to use attributes and when to
use elements. Attributes are handy in HTML. In XML my advice is to avoid
them. Use elements instead.
My Favorite Way
The following three XML documents contain exactly the same information:
A date attribute is used in the first example:
<note date="10/01/2008">
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>
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A date element is used in the second example:
<note>
<date>10/01/2008</date>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>
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An expanded date element is used in the third: (THIS IS MY FAVORITE):
<note>
<date>
<day>10</day>
<month>01</month>
<year>2008</year>
</date>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>
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Avoid XML Attributes?
Some of the problems with using attributes are:
- attributes cannot contain multiple values (elements can)
- attributes cannot contain tree structures (elements can)
- attributes are not easily expandable (for future changes)
Attributes are
difficult to read and maintain. Use elements for data. Use attributes for
information that is not relevant to the data.
Don't end up like this:
<note day="10" month="01" year="2008"
to="Tove" from="Jani" heading="Reminder"
body="Don't forget me this weekend!">
</note>
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XML Attributes for Metadata
Sometimes ID references are assigned to elements. These IDs can be used to
identify XML elements in much the same way as the
ID attribute in HTML. This example demonstrates this:
<messages>
<note id="501">
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>
<note id="502">
<to>Jani</to>
<from>Tove</from>
<heading>Re: Reminder</heading>
<body>I will not</body>
</note>
</messages>
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The ID above is just an identifier, to identify
the different notes. It is not a part of the note itself.
What I'm trying to say here is that metadata (data about data) should be stored
as attributes, and that data itself should be stored as elements.
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