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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 attribute-value>DTD example:<!ATTLIST payment type CDATA "check">XML example:<payment type="check" />
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 attribute-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 optional |
| #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" />
<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE square [<!ELEMENT square EMPTY><!ATTLIST square width CDATA "0">]><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 />
<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE person [<!ELEMENT person EMPTY><!ATTLIST person number CDATA #REQUIRED>]><person number="5677"/>
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 />
<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE contact [<!ELEMENT contact EMPTY><!ATTLIST contact fax CDATA #IMPLIED>]><contact fax="555-667788"/>
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" />
<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE sender [<!ELEMENT sender EMPTY><!ATTLIST sender company CDATA #FIXED "Microsoft">]><sender company="Microsoft"/>
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" />
<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE payment [<!ELEMENT payment EMPTY><!ATTLIST payment type (check|cash) "cash">]><payment/>
Use enumerated attribute values when you want the attribute value to be one of a fixed set of legal values.
DTD - Attributes