DTD - Attribute
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In DTD, attributes are declared through the ATTLIST declaration.
Declare attribute
Attribute declaration uses the following syntax:
<!ATTLIST element_name attribute_name attribute_type default_value>
DTD instance:
<!ATTLIST payment type CDATA "check">
XML instance:
<payment type="check" />
The following areAttribute typeOptions:
| Type | Description |
|---|---|
| CDATA | The value is character data (character data) |
| (en1|en2|..) | This value is one of the values in an enumeration 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 | This value is the name of a symbol |
| xml: | The value is a predefined XML value |
The default value parameter can use the following values:
| Value | Explanation |
|---|---|
| Value | The default value of the attribute |
| #REQUIRED | The attribute value is required |
| #IMPLIED | The attribute is not required |
| #FIXED value | The attribute value is fixed |
Specify a default attribute value
DTD:
<!ELEMENT square EMPTY> <!ATTLIST square width CDATA "0">
Valid XML:
<square width="100" />
In the above example, "square" is defined as an empty element with a "width" attribute of type CDATA. If the width is not set, its default value is 0.
#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 />
If you do not want to force the author to include the attribute and you do not have a default value option, please use the keyword #IMPLIED.
#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 />
If you do not have a default value option but still want to force the author to submit the attribute, please use the keyword #REQUIRED.
#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="W3School" />
If you want the attribute to have a fixed value and not allow the author to change this value, please use the #FIXED keyword. If the author uses a different value, the XML parser will return an error.
Listed 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" />
If you want the attribute value to be one of a series of fixed valid values, please use listed attribute values.
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