XML Elements vs. Attributes [duplicate]

Question

Possible Duplicate:
Should I use Elements or Attributes in XML?

I have never been able to figure out when to use xml attributes. I always use elements. I just read this w3schools article. The article states that it is bad practice to use attributes because:

• attributes cannot contain multiple values (child elements can)
• attributes are not easily expandable (for future changes)
• attributes cannot describe structures (child elements can)
• attributes are more difficult to manipulate by program code
• attribute values are not easy to test against a DTD

The only exception that it states is when you are assigning an id to a tag.

Is this correct? Why do attributes even exist then? Was it a design error with xml? Is there something I am missing here?

The only reason I could think of for using attributes would be for one to one relationships. ie: name. But it would have to be a one to one relationship to something that is a primitive (or string). Because it would be important that in the future you would not want to break it up into several differnt sections. ie:

<date> May 23, 2001 </date>

to:

<date>
   <month> May </month>
   <d> 23 </d>
   <yr> 2001 </yr>
</date>

Because this would not be possible with an attribute.

Bonus question: In the date example would it be possible to do something like this:

<date>
   <default> May 23, 200 </default>
   <month> May </month>
   <d> 23 </d>
   <yr> 2001 </yr>
</date>

To give future applications more (or different) information while still offering existing apps the same format? Or would you have to do this:

<date> May 23, 2001 </date>
<NEWdate>
   <month> May </month>
   <d> 23 </d>
   <yr> 2001 </yr>
</NEWdate>

Answer 1

Attributes are good when you want to attach information to other information, perhaps to describe how the information should be interpreted. For example:

<speed unit="mph">65</speed>

Answer 2

The points you list about elements are correct, and I would add the following:

• elements generally make prettier (more readable) diffs when you need to compare revisions of a file

But sometimes using an element to model a data point is overkill -- particularly when you have a lot of small, heterogeneous data points within a single parent element. Using attributes for simple things can improve readability. Some will probably argue that XML isn't readable or meant to be read/edited by humans... but I do it all the time.

Consider this example (basic hyperlink):

<a href="http://www.htmlhelp.com/" title="Help Information" target="_top">Web Design Group</a>

Would you like it if you had to write or read it this way instead?

<a>
    <href>http://www.htmlhelp.com/</href>
    <title>Help Information</title>
    <target>_top</target>
    <text>Web Design Group</text>
</a>

To me that looks like a lot of noise.