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Sams Teach Yourself XML in 21 Days

Sep 16, 2005

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Declaring Attributes in DTDs

As in HTML, an attribute is a name-value pair that you can use in a start tag or an empty tag to provide additional information for an element. For example, say you want to add an attribute to an <employee> element named supervisor, which indicates whether an employee is a supervisor and which may be set to "yes" or "no". Here's what that would look like in an XML document:

<document>
    <employee supervisor="no">
        <name>
            <lastname>Kelly</lastname>
            <firstname>Grace</firstname>
        </name>
    .
    .
    .

It's easy enough to add attributes to XML documents, but if you don't declare them in a DTD or an XML schema, your document won't be valid. You can declare a list of attributes for an element by using the <!ATTLIST> element in the DTD. Here's the general syntax of an <!ATTLIST> element:

<!ATTLIST element_name
    
   attribute_name type default_value
    
   attribute_name type default_value
    
   .
    
   .
    
   .
    
   attribute_name type default_value>

Here, element_name is the name of the element for which you're declaring attributes, attribute_name is the name of an attribute you want to declare, type is the attribute's type, and default_value specifies the default value of the attribute. Today's discussion describes what types and kinds of default values are possible in DTDs.

What does an attribute declaration look like? Listing 5.1 shows an example, in ch05_01.xml. In this example, the type of the supervisor attribute is CDATA, which stands for character data, and the default value is #IMPLIED, which means that you can use this attribute or not in <employee> elements.

Example 5.1. A Sample XML Document with an Attribute Declared in a DTD (`ch05_01.xml`)

<?xml version = "1.0" encoding="UTF-8" standalone="yes"?>
<!DOCTYPE document [
<!ELEMENT document (employee)*>
<!ELEMENT employee (name, hiredate, projects)>
<!ELEMENT name (lastname, firstname)>
<!ELEMENT lastname (#PCDATA)>
<!ELEMENT firstname (#PCDATA)>
<!ELEMENT hiredate (#PCDATA)>
<!ELEMENT projects (project)*>
<!ELEMENT project (product, id, price)>
<!ELEMENT product (#PCDATA)>
<!ELEMENT id (#PCDATA)>
<!ELEMENT price (#PCDATA) >
<!ATTLIST employee supervisor CDATA #IMPLIED>
]>
<document>
    <employee supervisor="no">
        <name>
            <lastname>Kelly</lastname>
            <firstname>Grace</firstname>
        </name>
        <hiredate>October 15, 2005</hiredate>
        <projects>
            <project>
                <product>Printer</product>
                <id>111</id>
                <price>$111.00</price>
            </project>
            <project>
                <product>Laptop</product>
                <id>222</id>
                <price>$989.00</price>
            </project>
        </projects>
    </employee>
    <employee supervisor="yes">
        <name>
            <lastname>Grant</lastname>
            <firstname>Cary</firstname>
        </name>
        <hiredate>October 20, 2005</hiredate>
        <projects>
            <project>
                <product>Desktop</product>
                <id>333</id>
                <price>$2995.00</price>
            </project>
            <project>
                <product>Scanner</product>
                <id>444</id>
                <price>$200.00</price>
            </project>
        </projects>
    </employee>
</document>

As its name implies, you can use <!ATTLIST> to declare an entire list of attributes for an element. For example, you might want an <employee> element to have multiple attributes—say, supervisor, division (indicating the division of the company that the employee works in), and fullTime (set to "yes" if the employee is full time, "no" if part time). Listing 5.2 shows how such an example would look, in the valid document ch05_02.xml.

Example 5.2. A Sample XML Document with Multiple Attributes Declared in a DTD (`ch05_02.xml`)

<?xml version = "1.0" encoding="UTF-8" standalone="yes"?>
<!DOCTYPE document [
<!ELEMENT document (employee)*>
<!ELEMENT employee (name, hiredate, projects) >
<!ELEMENT name (lastname, firstname)>
<!ELEMENT lastname (#PCDATA)>
<!ELEMENT firstname (#PCDATA)>
<!ELEMENT hiredate (#PCDATA)>
<!ELEMENT projects (project)*>
<!ELEMENT project (product, id, price)>
<!ELEMENT product (#PCDATA)>
<!ELEMENT id (#PCDATA)>
<!ELEMENT price (#PCDATA)>
<!ATTLIST employee

       supervisor CDATA #IMPLIED

       division CDATA #IMPLIED

       fullTime CDATA #IMPLIED

   >
]>
<document>
    <employee supervisor="no" division="plastics" fullTime="yes">
        <name>
            <lastname>Kelly</lastname>
            <firstname>Grace</firstname>
        </name>
        <hiredate>October 15, 2005</hiredate>
        <projects>
            <project>
                <product>Printer</product>
                <id>111</id>
                <price>$111.00</price>
            </project>
            <project>
                <product>Laptop</product>
                <id>222</id>
                <price>$989.00</price>
            </project>
        </projects>
    </employee>
    <employee supervisor="yes" division="metals" fullTime="yes">
        <name>
            <lastname>Grant</lastname>
            <firstname>Cary</firstname>
        </name>
        <hiredate>October 20, 2005</hiredate>
        <projects>
            <project>
                <product>Desktop</product>
                <id>333</id>
                <price>$2995.00</price>
            </project>
            <project>
                <product>Scanner</product>
                <id>444</id>
                <price>$200.00</price>
            </project>
        </projects>
    </employee>
</document>

These examples provide an introduction to declaring attributes in DTDs. As you can see, all you have to do is use an <!ATTLIST> element to declare the attributes for an element. It's a little more involved to use this element than to use <!ELEMENT>, however, because you're restricted to certain types and default values for attributes in DTDs, as described in the following section.

Using the Legal Default Values and Attribute Types

When you're declaring attributes in DTDs, these are the possible default_value settings you can use in <!ATTLIST> elements:

value — Specifies a text value and must be enclosed in quotes.
#IMPLIED — Makes an attribute optional.
#FIXED value — Sets the attribute's value to value .
#REQUIRED — Means that this attribute is required and must be given a value.

These are the possible type values you can use:

CDATA — Specifies character data, which is just text without markup.
ENTITY — Specifies an entity name.
ENTITIES — Specifies multiple entity names, which are separated by whitespace in the attribute value, like this: entity1 entity2 entity3 .
Enumerated — Specifies one value from a list of values (that is, an enumeration).
ID — Specifies an ID attribute, which holds a proper XML name (which must not be shared by any other attribute of the ID type).
IDREF — Holds the ID value of some other element.
IDREFS — Holds multiple ID values of elements, separated by whitespace.
NMTOKEN — Specifies text made up of XML name characters, or tokens. This text may be made up of one or more letters, digits, hyphens, underscores, colons, and periods.
NMTOKENS — Specifies multiple NMTOKEN items, separated by whitespace.
NOTATION — Specifies a notation name that holds a format description (such as a MIME type).

You'll see these various values at work today.

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