An HTML Example The Same Example in XML HTML versus XML ...

Chapter 2 Structured Web Documents in XML

An HTML Example
Nonmonotonic Reasoning: ContextDependent Reasoning
by V. Marek and M. Truszczynski
Springer 1993
ISBN 0387976892

Grigoris Antoniou Frank van Harmelen

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Chapter 2

A Semantic Web Primer

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The Same Example in XML

HTML versus XML: Similarities

Nonmonotonic Reasoning: ContextDependent Reasoning V. Marek M. Truszczynski Springer 1993 0387976892

z

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Both use tags (e.g.
and ) z Tags may be nested (tags within tags) z Human users can read and interpret both HTML and XML representations quite easily … But how about machines?

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Problems with Automated Interpretation of HTML Documents

HTML vs XML: Structural Information

An intelligent agent trying to retrieve the names of the authors of the book z Authors’ names could appear immediately after the title z or immediately after the word by z Are there two authors? z Or just one, called “V. Marek and M. Truszczynski”? 5

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z

z

– – –

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A machine processing the XML document would be able to deduce that – –

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the author element refers to the enclosing book element rather than by proximity considerations



The HTML representation provides more than the XML representation: –

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The formatting of the document is also described

he main use of an HTML document is to display information: it must define formatting XML: separation of content from display –

E.g. a year must be a number of four digits

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XML allows the definition of constraints on values –

Every piece of information is described. Relations are also defined through the nesting structure. E.g., the tags appear within the tags, so they describe properties of the particular book.

HTML vs XML: Formatting

HTML vs XML: Structural Information (2) z

HTML documents do not contain structural information: pieces of the document and their relationships. XML more easily accessible to machines because

same information can be displayed in different ways Chapter 2


HTML vs XML: Another Example z

z

HTML vs XML: Different Use of Tags

In HTML
Relationship matter-energy
E = M × c2 In XML Relationship matter energy E M × c2

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z z z z z

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– – –

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1. 2. 3.


Introduction Detailed Description of XML Structuring a) b)

mathematics (MathML) bioinformatics (BSML) human resources (HRML) … Chapter 2

Chapter 2

Lecture Outline

Web applications must agree on common vocabularies to communicate and collaborate Communities and business sectors are defining their specialized vocabularies –

HTML tags define display: color, lists … XML tags not fixed: user definable tags XML meta markup language: language for defining markup languages

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XML Vocabularies z

In both HTML docs same tags In XML completely different

4. 5. 6. A Semantic Web Primer

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DTDs XML Schema

Namespaces Accessing, querying XML documents: XPath Transformations: XSLT Chapter 2


The XML Language

Prolog of an XML Document

An XML document consists of z a prolog z a number of elements z an optional epilog (not discussed)

The prolog consists of z an XML declaration and z an optional reference to external structuring documents

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XML Elements z z

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E.g. books, authors, publishers

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An element consists of: – – –


XML Elements (2)

The “things” the XML document talks about –

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an opening tag the content a closing tag

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Tag names can be chosen almost freely. The first character must be a letter, an underscore, or a colon No name may begin with the string “xml” in any combination of cases –

E.g. “Xml”, “xML”

David Billington 15

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Content of XML Elements z

XML Attributes

Content may be text, or other elements, or nothing

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David Billington +61 í 7 í 3875 507 z

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–

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If there is no content, then the element is called empty; it is abbreviated as follows: for Chapter 2


An empty element is not necessarily meaningless

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It may have some properties in terms of attributes

An attribute is a name-value pair inside the opening tag of an element Chapter 2


XML Attributes: An Example

The Same Example without Attributes

23456 John Smith October 15, 2002 a528 1 c817 3

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XML Elements vs Attributes z z z

Further Components of XML Docs

Attributes can be replaced by elements When to use elements and when attributes is a matter of taste But attributes cannot be nested

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Comments – –

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Processing Instructions (PIs) – –

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z

– –

Only one outermost element (called root element) Each element contains an opening and a corresponding closing tag Tags may not overlap. The following is forbidden: z

– –

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Lee Hong

Attributes within an element have unique names Element and tag names must be permissible Chapter 2


Chapter 2


Where is your draft? Grigoris, where is the draft of the paper you promised me last week?

Syntactically correct documents Some syntactic rules: –

Define procedural attachments

The Tree Model of XML Documents: An Example

Well-Formed XML Documents z

A piece of text that is to be ignored by parser

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The Tree Model of XML Documents: An Example (2)

The Tree Model of XML Docs z

The tree representation of an XML document is an ordered labeled tree: – – – – – –

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Lecture Outline 1. 2. 3.

b)

4. 5. 6.

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z z

DTDs XML Schema

Define all the element and attribute names that may be used Define the structure – –



Structuring XML Documents

Introduction Detailed Description of XML Structuring a)

There is exactly one root There are no cycles Each non-root node has exactly one parent Each node has a label. The order of elements is important … but the order of attributes is not important


z

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what values an attribute may take which elements may or must occur within other elements, etc.

If such structuring information exists, the document can be validated Chapter 2


Structuring XML Dcuments (2) z

–

it is well-formed respects the structuring information it uses

There are two ways of defining the structure of XML documents: – –

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David Billington +61 í 7 í 3875 507

An XML document is valid if –

z

DTD: Element Type Definition

DTD for above element (and all lecturer elements):

DTDs (the older and more restricted way) XML Schema (offers extended possibilities)

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z z z

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The element types lecturer, name, and phone may be used in the document A lecturer element contains a name element and a phone element, in that order (sequence) A name element and a phone element may have any content In DTDs, #PCDATA is the only atomic type for elements

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DTD: Disjunction in Element Type Definitions

The Meaning of the DTD z

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z

We express that a lecturer element contains either a name element or a phone element as follows:

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A lecturer element contains a name element and a phone element in any order.

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Example of an XML Element

The Corresponding DTD

ID CDATA CDATA

#REQUIRED #REQUIRED #REQUIRED>

ID CDATA CDATA

#REQUIRED #REQUIRED #IMPLIED>

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Comments on the DTD z z

– – –

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?: appears zero times or once *: appears zero or more times +: appears one or more times No cardinality operator means exactly once

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In addition to defining elements, we define attributes This is done in an attribute list containing: – –

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Comments on the DTD (2)

The item element type is defined to be empty + (after item) is a cardinality operator: –

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Name of the element type to which the list applies A list of triplets of attribute name, attribute type, and value type

Attribute name: A name that may be used in an XML document using a DTD Chapter 2


DTD: Attribute Types z z

Similar to predefined data types, but limited selection The most important types are – – –

– –

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DTD: Attribute Value Types z

–

CDATA, a string (sequence of characters) ID, a name that is unique across the entire XML document IDREF, a reference to another element with an ID attribute carrying the same value as the IDREF attribute IDREFS, a series of IDREFs (v1| . . . |vn), an enumeration of all possible values


#IMPLIED

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#FIXED "value"

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"value"

–

–

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Attribute must appear in every occurrence of the element type in the XML document

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–

Limitations: no dates, number ranges etc. Chapter 2

#REQUIRED

The appearance of the attribute is optional Every element must have this attribute This specifies the default value for the attribute

Chapter 2


Referencing with IDREF and IDREFS

An XML Document Respecting the DTD

Bob Marley Bridget Jones Mary Poppins Peter Marley

Chapter 2

#REQUIRED #IMPLIED #IMPLIED #IMPLIED>


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A DTD for an Email Element

A DTD for an Email Element (2)

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Interesting Parts of the DTD z

– – –

z

–

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z

a from element at least one to element zero or more cc elements a subject element

–

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the name attribute is not required the address attribute is always required Chapter 2

A body element contains –

In from, to, and cc elements –



Interesting Parts of the DTD (2)

A head element contains (in that order): –

Chapter 2

The encoding attribute of an attachment element must have either the value “mime” or “binhex” –

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a text element possibly followed by a number of attachment elements

“mime” is the default value Chapter 2


Remarks on DTDs z

A DTD can be interpreted as an Extended Backus-Naur Form (EBNF) – –

z

Lecture Outline 1. 2.

is equivalent to email ::= head body

3.

a) b)

Recursive definitions possible in DTDs –

4.

5. 6.

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z z z

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An XML schema is an element with an opening tag like z Structure of schema elements

not necessary to write separate tools Expand or delete already existent schemas

Sophisticated set of data types, compared to DTDs (which only supports strings) Chapter 2


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Reuse and refinement of schemas –

Chapter 2

XML Schema (2)

Significantly richer language for defining the structure of XML documents Tts syntax is based on XML itself –

DTDs XML Schema

Namespaces Accessing, querying XML documents: XPath Transformations: XSLT

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XML Schema z

Introduction Detailed Description of XML Structuring


–

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Element and attribute types using data types

Chapter 2


Element Types

Attribute Types

Cardinality constraints: z minOccurs="x" (default value 1) z maxOccurs="x" (default value 1) z Generalizations of *,?,+ offered by DTDs

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Already existing data types can be extended by new elements or attributes. Example:

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Resulting Data Type

Data Type Extension (2)

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A hierarchical relationship exists between the original and the extended type –

–

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Instances of the extended type are also instances of the original type They may contain additional information, but neither less information, nor information of the wrong type

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Data Type Restriction z z

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Restriction is not achieved by deleting elements or attributes

The following hierarchical relationship still holds: –

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An existing data type may be restricted by adding constraints on certain values Restriction is not the opposite from extension –

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Example of Data Type Restriction

Instances of the restricted type are also instances of the original type They satisfy at least the constraints of the original type

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Restriction of Simple Data Types

Data Type Restriction: Enumeration

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XML Schema: The Email Example

XML Schema: The Email Example (2)

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XML Schema: The Email Example (3)

Lecture Outline

1.

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2. 3.

b)

5.

Similar for bodyType Chapter 2

6. A Semantic Web Primer

Introduction Detailed Description of XML Structuring a)

4.

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DTDs XML Schema



Namespaces z z

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An Example

An XML document may use more than one DTD or schema Since each structuring document was developed independently, name clashes may appear The solution is to use a different prefix for each DTD or schema –

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DTDs XML Schema



Addressing and Querying XML Documents z

In relational databases, parts of a database can be selected and retrieved using SQL – –

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XPath z z

Same necessary for XML documents Query languages: XQuery, XQL, XML-QL

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The central concept of XML query languages is a path expression –

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Types of Path Expressions

–

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Relative to a context node



Syntactically they begin with the symbol / It refers to the root of the document (situated one level above the root element of the document)

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Chapter 2

An XML Example

Absolute (starting at the root of the tree) –

It operates on the tree data model of XML It has a non-XML syntax

Specifies how a node or a set of nodes, in the tree representation of the XML document can be reached Chapter 2

z

XPath is core for XML query languages Language for addressing parts of an XML document.

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Examples of Path Expressions in XPath

Tree Representation

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Address all author elements /library/author

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Addresses all author elements that are children of the library element node, which resides immediately below the root /t1/.../tn, where each ti+1 is a child node of ti, is a path through the tree representation

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Examples of Path Expressions in XPath (2) z

z

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Examples of Path Expressions in XPath (3)

Address all author elements //author Here // says that we should consider all elements in the document and check whether they are of type author This path expression addresses all author elements anywhere in the document Chapter 2

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z

Address the location attribute nodes within library element nodes /library/@location

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The symbol @ is used to denote attribute nodes

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Examples of Path Expressions in XPath (5)

Address all title attribute nodes within book elements anywhere in the document, which have the value “Artificial Intelligence” //book/@title="Artificial Intelligence"

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Address all books with title “Artificial Intelligence” /book[@title="Artificial Intelligence"] Test within square brackets: a filter expression –

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Difference with query 4. –

–

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Tree Representation of Query 4

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It restricts the set of addressed nodes. Query 5 addresses book elements, the title of which satisfies a certain condition. Query 4 collects title attribute nodes of book elements

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Tree Representation of Query 5

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z

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General Form of Path Expressions

Address the first author element node in the XML document //author[1] Address the last book element within the first author element node in the document //author[1]/book[last()] Address all book element nodes without a title attribute //book[not @title]

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z z

– – –

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General Form of Path Expressions (2) z

–

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Chapter 2

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A node test specifies which nodes to address –

E.g. parent, ancestor, child (the default), sibling, attribute node // is such an axis specifier: descendant or self

Chapter 2

An axis specifier, A node test, and An optional predicate

General Form of Path Expressions (3)

An axis specifier determines the tree relationship between the nodes to be addressed and the context node –

A path expression consists of a series of steps, separated by slashes A step consists of

– –

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The most common node tests are element names E.g., * addresses all element nodes comment() addresses all comment nodes

Chapter 2


Lecture Outline

General Form of Path Expressions (4) z

Predicates (or filter expressions) are optional and are used to refine the set of addressed nodes

3.

4.

XPath has a more complicated full syntax.

5.

– –

–

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2.

E.g., the expression [1] selects the first node [position()=last()] selects the last node [position() mod 2 =0] selects the even nodes

–

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1.

a) b)

We have only presented the abbreviated syntax Chapter 2


6.

DTDs XML Schema

Namespaces Accessing, querying XML documents: XPath Transformations: XSLT

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Displaying XML Documents

Style Sheets

Grigoris Antoniou University of Bremen [email protected]

z

–

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Chapter 2


E.g. CSS2 (cascading style sheets level 2) XSL (extensible stylesheet language)

XSL includes – – –

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Style sheets can be written in various languages –

may be displayed in different ways: Grigoris Antoniou Grigoris Antoniou University of Bremen University of Bremen [email protected] [email protected]

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Introduction Detailed Description of XML Structuring

a transformation language (XSLT) a formatting language Both are XML applications Chapter 2


XSL Transformations (XSLT) z

XSLT specifies rules with which an input XML document is transformed to – – –

z z

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XSLT (2) z

another XML document an HTML document plain text

z z

The output document may use the same DTD or schema, or a completely different vocabulary XSLT can be used independently of the formatting language Chapter 2


z

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Move data and metadata from one XML representation to another XSLT is chosen when applications that use different DTDs or schemas need to communicate XSLT can be used for machine processing of content without any regard to displaying the information for people to read. In the following we use XSLT only to display XML documents Chapter 2


XSLT Transformation into HTML

Style Sheet Output

An author

An author Grigoris Antoniou
University of Bremen
[email protected]

Chapter 2


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Observations About XSLT z

XSLT documents are XML documents –

z

XSLT resides on top of XML In this case an HTML document, with some placeholders for content to be inserted

xsl:value-of retrieves the value of an element and copies it into the output document –

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An author ...
...
...

The XSLT document defines a template –

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A Template

It places some content into the template Chapter 2


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Auxiliary Templates z z z

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Example of an Auxiliary Template Grigoris Antoniou University of Bremen [email protected] David Billington Griffith University [email protected]

We have an XML document with details of several authors It is a waste of effort to treat each author element separately In such cases, a special template is defined for author elements, which is used by the main template

Chapter 2

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Example of an Auxiliary Template (2)

Example of an Auxiliary Template (3)

Authors

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Affiliation:
Email:

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Multiple Authors Output

Explanation of the Example

Authors
Grigoris Antoniou
Affiliation: University of Bremen
Email: [email protected]

David Billington
Affiliation: Griffith University
Email: [email protected]

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Chapter 2

xsl:apply-templates element causes all children of the context node to be matched against the selected path expression – – –


100

E.g., if the current template applies to /, then the element xsl:apply-templates applies to the root element I.e. the authors element (/ is located above the root element) If the current context node is the authors element, then the element xsl:apply-templates select="author" causes the template for the author elements to be applied to all author children of the authors element

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Explanation of the Example (2) z

It is good practice to define a template for each element type in the document –

–

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Processing XML Attributes z

Even if no specific processing is applied to certain elements, the xsl:apply-templates element should be used E.g. authors

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z

Chapter 2


Transforming an XML Document to Another

Processing XML Attributes (2) z

Wrong solution:

In this way, we work from the root to the leaves of the tree, and all templates are applied Chapter 2

Suppose we wish to transform to itself the element:

Not well-formed because tags are not allowed within the values of attributes We wish to add attribute values into template

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Transforming an XML Document to Another (2)

Transforming an XML Document to Another (3)

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z z

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Points for Discussion in Subsequent Chapters

Summary z

Chapter 2

XML is a metalanguage that allows users to define markup XML separates content and structure from formatting XML is the de facto standard for the representation and exchange of structured information on the Web XML is supported by query languages Chapter 2


z z z z

The nesting of tags does not have standard meaning The semantics of XML documents is not accessible to machines, only to people Collaboration and exchange are supported if there is underlying shared understanding of the vocabulary XML is well-suited for close collaboration, where domain- or community-based vocabularies are used –

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It is not so well-suited for global communication.

Chapter 2


An HTML Example The Same Example in XML HTML versus XML ...

An HTML Example The Same Example in XML HTML versus XML ...

Nonmonotonic Reasoning: Context-. Dependent Reasoning

Nonmonotonic Reasoning: ContextDependent Reasoning

Relationship matter-energy

Grigoris Antoniou

David Billington