DTD - Attributes

In a DTD, Attributes are declared with an ATTLIST declaration.

Declaring Attributes

An attribute declaration has the following syntax:

<!ATTLIST element-name attribute-name 
 attribute-type default-value>

example:

DTD example:
<!ATTLIST payment type CDATA "check">

XML example:
<payment type="check" />

The attribute-type can have the following values:

The default-value can have the following values:

Attribute declaration example

DTD example:
<!ELEMENT square EMPTY>
  <!ATTLIST square width CDATA "0">

XML example:
<square width="100" />

In the example above, the element square is defined to be an empty element with a width attribute of  type CDATA. If no width attribute is given, the width attribute has a default value of 0.

Default attribute value

Syntax:
<!ATTLIST element-name attribute-name
 attribute-type "default-value">

DTD example:
<!ATTLIST payment type CDATA "check">

XML example:
<payment type="check" />

Specifying a default value for an attribute ensures that the attribute will get a value even if the author of the XML document didn't include it.

Implied attribute

Syntax:
<!ATTLIST element-name attribute-name 
 attribute-type #IMPLIED>

DTD example:
<!ATTLIST contact fax CDATA #IMPLIED>

XML example:
<contact fax="555-667788" />

Use an implied attribute if you don't want to force the author to include an attribute, and you don't have an option for a default value.

Required attribute

Syntax:
<!ATTLIST element-name attribute_name 
 attribute-type #REQUIRED>

DTD example:
<!ATTLIST person number CDATA #REQUIRED>

XML example:
<person number="5677" />

Use a required attribute if you don't have an option for a default value, but still want to force the attribute to be present.

Fixed attribute value

Syntax:
<!ATTLIST element-name attribute-name 
 attribute-type #FIXED "value">

DTD example:
<!ATTLIST sender company CDATA #FIXED "Microsoft">


XML example:
<sender company="Microsoft" />

Use a fixed attribute value when you want an attribute to have a fixed value without allowing the author to change it. If an author includes another value, the XML parser will return an error.

Enumerated attribute values

Syntax:
<!ATTLIST element-name 
 attribute-name (en1|en2|..) default-value>

DTD example:
<!ATTLIST payment type (check|cash) "cash">

XML example:
<payment type="check" />
or
<payment type="cash" />

Use enumerated attribute values when you want the attribute values to be one of a fixed set of legal values.

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