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>
DTD example:
<!ATTLIST payment type CDATA "check">
XML example:
<payment type="check" />
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The attribute-type can be one of the following:
| Type |
Description |
| CDATA |
The value is character data |
| (en1|en2|..) |
The value must be one from an enumerated list |
| ID |
The value is a unique id |
| IDREF |
The value is the id of another element |
| IDREFS |
The value is a list of other ids |
| NMTOKEN |
The value is a valid XML name |
| NMTOKENS |
The value is a list of valid XML names |
| ENTITY |
The value is an entity |
| ENTITIES |
The value is a list of entities |
| NOTATION |
The value is a name of a notation |
| xml: |
The value is a predefined xml value |
The default-value can be one of the following:
| Value |
Explanation |
| value |
The default value of the attribute |
| #REQUIRED |
The attribute is required |
| #IMPLIED |
The attribute is not required |
| #FIXED value |
The attribute value is fixed |
A Default Attribute Value
DTD:
<!ELEMENT square EMPTY>
<!ATTLIST square width CDATA "0">
Valid XML:
<square width="100" /> |
In the example above, the "square" element is defined to be an empty element with
a "width" attribute of type CDATA. If no width is specified, it has a default
value of 0.
#REQUIRED
Syntax
| <!ATTLIST element-name attribute-name attribute-type #REQUIRED> |
Example
DTD:
<!ATTLIST person number CDATA #REQUIRED>
Valid XML:
<person number="5677" />
Invalid XML:
<person /> |
Use the #REQUIRED keyword if you don't have an option for a default value, but still want to force the attribute to be present.
#IMPLIED
Syntax
| <!ATTLIST element-name attribute-name attribute-type #IMPLIED> |
Example
DTD:
<!ATTLIST contact fax CDATA #IMPLIED>
Valid XML:
<contact fax="555-667788" />
Valid XML:
<contact /> |
Use the #IMPLIED keyword if you don't want to force the author to include an attribute, and you don't have an option for a default value.
#FIXED
Syntax
| <!ATTLIST element-name attribute-name attribute-type #FIXED "value"> |
Example
DTD:
<!ATTLIST sender company CDATA #FIXED "Microsoft">
Valid XML:
<sender company="Microsoft" />
Invalid XML:
<sender company="W3Schools" /> |
Use the #FIXED keyword 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> |
Example
DTD:
<!ATTLIST payment type (check|cash) "cash">
XML example:
<payment type="check" />
or
<payment type="cash" /> |
Use enumerated attribute values when you want the attribute value to be one of a fixed set of legal values.
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