Mark-up Element and Attributes
Let's break down XML markup elements and attributes, the core components that structure and enrich data within an XML document.
1. Elements: The Building Blocks
- Definition: Elements are the fundamental building blocks of an XML document. They define the structure and meaning of the data.
- Structure:
- Start Tag: <elementName> Marks the beginning of an element.
- Content: The data contained within the element. This can be:
- Text
- Other elements (nested elements)
- A mixture of text and other elements
- Nothing (empty element)
- End Tag: </elementName> Marks the end of the element. The end tag must match the start tag exactly (case-sensitive).
- Empty Elements: Elements with no content can be written in two ways:
- <elementName></elementName> (Start tag immediately followed by the end tag)
- <elementName /> (Self-closing tag - a shorthand for empty elements)
- Nesting: Elements can be nested within other elements, creating a hierarchical, tree-like structure. This nesting is crucial for defining relationships between data.
- Root Element: Every well-formed XML document must have exactly one root element that encloses all other elements.
- Naming Rules:
- Must start with a letter or underscore (_).
- Can contain letters, digits, hyphens (-), underscores (_), and periods (.).
- Cannot contain spaces.
- Cannot start with 1 the letters "xml" (or "XML", "Xml", etc.).
- Case-sensitive: <Book> is different from <book>.
- Examples:
XML
<title>The Lord of the Rings</title> <book> <title>Pride and Prejudice</title> <author>Jane Austen</author> </book>
<emptyElement /> <anotherEmpty></anotherEmpty> ```
2. Attributes: Metadata for Elements
- Definition: Attributes provide additional information about an element. They are not the primary data content; they are metadata that modifies or describes the element.
- Placement: Attributes are always placed within the start tag of an element.
- Syntax: attributeName="attributeValue"
- attributeName: The name of the attribute (follows the same naming rules as element names).
- ="attributeValue": The value of the attribute, enclosed in double quotes or single quotes. Consistency is important (use the same type of quotes throughout your document).
- Rules:
- Attribute values must be quoted.
- An element cannot have two attributes with the same name.
- Attribute names are case-sensitive.
- Order of attributes within a tag is not significant.
- Examples:
XML
<book category="fiction" isbn="978-0618260266">
<title>The Lord of the Rings</title>
</book>
- category and isbn are attributes of the book element.
- "fiction" and "978-0618260266" are the attribute values.
XML
<image src="logo.png" width="100" height="50" />
- src, width, and height are attributes of the (empty) image element.
3. Elements vs. Attributes: When to Use Which
This is a crucial design decision in XML. Here are the guidelines:
- Use Elements for:
- The primary data content.
- Data that might contain sub-elements (nested structure).
- Data that might appear multiple times (e.g., multiple <author> elements for a book).
- Data where the order is significant.
- Data that may include mixed content (text and other elements).
- Use Attributes for:
- Metadata about the element (data about the data).
- Identifiers (e.g., id, isbn).
- Data that is unlikely to contain sub-elements.
- Data that is unlikely to appear multiple times for the same element.
- Data where the order is not significant.
- Data that are single values.
- Example: Book Information
Good (Elements for main content, attribute for ID):
XML
<book id="123">
<title>The Lord of the Rings</title>
<author>J.R.R. Tolkien</author>
<year>1954</year>
</book>
Less Good (All attributes):
XML
<book title="The Lord of the Rings" author="J.R.R. Tolkien" year="1954" />
This is less readable and harder to extend. What if you want to add multiple authors, or publisher information?
Also Less Good (Mixing elements and attributes inconsistently):
XML
<book>
<title>The Lord of the Rings</title>
<author name="J.R.R. Tolkien"/>
<year>1954</year>
</book>
4. Well-Formedness Rules (Related to Elements and Attributes)
These rules must be followed for an XML document to be considered well-formed:
- One Root Element: The document must have exactly one root element.
- Matching Tags: Every start tag must have a corresponding end tag (or be self-closing).
- Proper Nesting: Elements must be properly nested (no overlapping).
- Quoted Attribute Values: Attribute values must be enclosed in quotes.
- Unique Attribute Names: An element cannot have two attributes with the same name.
In Summary:
Elements and attributes are the core components of XML. Elements define the structure and contain the primary data, while attributes provide additional metadata about the elements. Understanding the rules for using elements and attributes, and choosing between them appropriately, is essential for creating well-structured and meaningful XML documents. The key principle is to use elements for the main content and attributes for metadata that describes that content.