XPath tutorial - basics

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Introduction

Prerequisites

Editing XML (being able to use a simple DTD)
Introductory XSLT (xsl:template, xsl:apply-templates and xsl:value-of)
Know about the role of XPath with respect to XSLT

Objectives

Better understand XPath expressions
Learn some XSLT programming constructions (conditions and loops)
Being able to cope with most XML to HTML transformations

Disclaimer

There may be typos (sorry) and mistakes (sorry again)
Please also consult a textbook !

Introduction to XML Path Language

Definition and history

XPath is a language for addressing parts of an XML document
In support of this primary purpose, it also provides basic facilities for manipulation of strings, numbers and booleans.
XPath uses a compact non-XML syntax (to facilitate use of XPath within URIs and XML attribute values).
XPath gets its name from its use of a path notation as in URLs for navigating through the hierarchical structure of an XML document.
XPath was defined at the same time as XSLT (nov 1999)
Initally, it was developped to support XSLT and XPointer (XML Pointer Language used for XLink, XInclude, etc.)

Specifications

XPath 1.0 http://www.w3.org/TR/xpath (nov 1999)
- Used by XSLT 1.0
XPath 2.0 http://www.w3.org/TR/xpath20/ (Jan 2007)
XPath 2.0 Functions and Operators http://www.w3.org/TR/xquery-operators/
- XPath 2.0 is a superset of XPath 1.0
- Used by XSLT 2.0 and XQuery ... and other specifications

XSLT, XQuery and XPath

Each time a given XSLT or XQuery instruction needs to address (refer to) parts of an XML document we use XPath expressions.
XPath expressions also can contain functions, simple math and boolean expressions

With XSLT, XPath expressions are typicially used in match , select and test attributes:

Xpath expressions in an XSLT template

The XPath Syntax and the document model

Xpath Syntax

XPath expressions (at least the simple ones) look a bit like file paths. E.g.

/section/title

means: find all title nodes below section nodes

Primary (relatively simple) XPath expressions

Result of an Xpath

Can be various things, e.g. sets of nodes, a single node, a number, etc
Most often the result is a set of nodes

There are two notations for location paths

abbreviated (less available options)
- e.g. para is identical to child::para
unabbreviated (not presented in these slides !!)
- e.g. " child::para " would define the para element children of the current context node.

The formal specification of an XML Path

is very complex, i.e. has about 39 clauses and is very difficult to understand
Some expressions shown here are beyound the scope of this tutorial, don't panic !

2.2 The document model of XPath

XPath sees an XML document as a tree structure
Each information (XML elements, attributes, text, etc.) is called a node. This is fairly similar to the W3C DOM model an XML or XSLT processor would use.

Nodes that XPath can see

root node
- ATTENTION: The root is not necessarily the XML root element. E.g. processing instructions like a stylesheet declaration are also nodes.
Elements and attributes
Special nodes like comments, processing instructions, namespace declarations.

Nodes XPath can't see

XPath looks at the final document, therefore it can't see entities and document type declarations....

The XML context

What a given XPath expression means, is always defined by a given XML context, i.e. the current node in the XML tree

2.3 Element Location Paths

We present a few expressions for locating nodes. This is not complete and we will use abbreviated syntax.

Document root node: returns the document root (which is not necessarily the XML root!)

/

Direct child element

XML_element_name

Direct child of the root node

/XML_element_name

Child of a child

XML_element_name/XML_element_name

Descendant of the root

//XML_element_name

Descendant of a node

XML_element_name//XML_element_name

Parent of a node

../

A far cousin of a node

../../XML_element_name/XML_element_name/XML_element_name

Example Extracting titles from an XML file with XSLT

An XML document (file: xpath-jungle.xml used throughout the rest)


<?xml version="1.0"?>
<project>
 <title>The Xpath project</title>
 <participants>
  <participant>
    <FirstName>Daniel</FirstName>
    <qualification>8</qualification>
    <description>Daniel will be the tutor</description>
    <FoodPref picture="dolores_001.jpg">Sea Food</FoodPref>
  </participant>
  <participant>
    <FirstName>Jonathan</FirstName>
    <qualification>5</qualification>
    <FoodPref picture="dolores_002.jpg">Asian</FoodPref>
  </participant>
  <participant>
   <FirstName>Bernadette</FirstName>
   <qualification>8</qualification>
    <description>Bernadette is an arts major</description>
  </participant>
  <participant>
   <FirstName>Nathalie</FirstName>
   <qualification>2</qualification>
  </participant>
 </participants>
 <problems>
  <problem>
    <title>Initial problem</title>
    <description>We have to learn something about Location Path</description>
    <difficulty level="5">This problem should not be too hard</difficulty>
  </problem>
  <solutions>
   <item val="low">Buy a XSLT book</item>
   <item val="low">Find an XSLT website</item>
   <item val="high">Register for a XSLT course and do exercices</item>
  </solutions>
   <problem>
    <title>Next problem</title>
    <description>We have to learn something about predicates</description>
    <difficulty level="6">This problem is a bit more difficult</difficulty>
  </problem>
  <solutions>
   <item val="low">Buy a XSLT book</item>
   <item val="medium">Read the specification and do some exercises</item>
   <item val="high">Register for a XPath course and do exercices</item>
  </solutions>
 </problems>
</project>

Task

We would like to get a simple list of problem titles

Solution

XSLT template (file: xpath-jungle-1.xsl)

<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                version="1.0">

  <xsl:output method="html"/>

  <xsl:template match="/project">
    <html>
      <body bgcolor="#FFFFFF">
        <h1><xsl:value-of select="title" /></h1>
        Here are the titles of our problems: <ul>
        <xsl:apply-templates select="problems/problem" /> 
      </ul>
      </body>
    </html>
  </xsl:template>

<xsl:template match="problems/problem">
  <li><xsl:value-of select="title" /></li>
</xsl:template>

</xsl:stylesheet>

(1) XSLT template for the root element

The XPath of the "match" means: applies to project element node, descendant of root node
Execution context of this template is therefore the element "project "
xsl:apply-templates will select a rule for descendant " problem ".

(2) XSLT template for the problem element

The second rule will be triggered by the first rule, because problems/problem is indeed a descendant of the project element

(3) Result HTML

<html>
   <body bgcolor="#FFFFFF">
      <h1>The Xpath project</h1>
      Here are the titles of our problems:
      <ul>
         <li>Initial problem</li>
         <li>Next problem</li>
      </ul>
   </body>
</html>

Attribute Location Paths

Top find an attribute of a child element of the current context

@attribute_name

Example:

@val

Find attributes of an element in a longer location path starting from root

/element_name/element_name/@attribute_name

Example:

/project/problems/solutions/item/@val

Find attributes in the whole document

//@attribute_name

Example 2-2: Make an html img link from an attribute

XML fragment

<participants>

  <participant>

    <FirstName>Daniel</FirstName>

    <qualification>8</qualification>

    <description>Daniel will be the tutor</description>

    <FoodPref picture="dolores_001.jpg">Sea Food</FoodPref>

  </participant>

  <participant>

    <FirstName>Jonathan</FirstName>

    <qualification>5</qualification>

    <FoodPref picture="dolores_002.jpg">Asian</FoodPref>

  </participant>

  <participant>

   <FirstName>Bernadette</FirstName>

   <qualification>8</qualification>

    <description>Bernadette is an arts major</description>

  </participant>

 .......

Task

Display a list of First Names plus their food preferences

XSLT (File xpath-jungle-2.xsl)

The first rule will just select all participants and create the list container (ul)

 <xsl:template match="/">

    <html>

      <body bgcolor="#FFFFFF">

        <h1>What do we know about our participants ?</h1>

        Here are some food preferences:

        <ul>

        <xsl:apply-templates select=".//participant"
 />

        </ul>

      </body>

    </html>

  </xsl:template>

The second rule will display names of participants and launch a template for FoodPref
Note: Not all participants have a FoodPref element. If it is absent it will just be ignored.

<xsl:template match="participant">

  <li><xsl:value-of select="FirstName
"/>

  <xsl:apply-templates select="FoodPref
"/>

  </li>

</xsl:template>

This rule displays the text (contents) of FoodPref and then makes an HTML img tag

<xsl:template match="FoodPref">

  prefers <xsl:value-of select="."
/>.

  <img src="{@picture}
"/> <br clear="all"/>

</xsl:template>

Parts of the result

  <h1>What do we know about our participants ?</h1>

      Here are some food preferences:

      <ul>

         <li>Daniel prefers Sea Food.

            <img src="dolores_001.jpg"><br clear="all"></li>

         <li>Jonathan

            prefers Asian.

            <img src="dolores_002.jpg"><br clear="all"></li>

         <li>Bernadette</li>

         <li>Nathalie</li>

      </ul>

2.5 Location wildcards

Sometimes (but not often!), it is useful to work with wildcards
You have to understand that only one rule will be applied/element. Rules with wildcards have less priority and this is why "your rules" are applied before the system defaults.

Find all child nodes of type XML element

*

Find all child nodes (including comments, etc.)

node()

Find all element attributes

@*

Find all text nodes

text()

FYI: The system built-in (default) rules rely on wildcards

This rule applies to the document root and all other elements (see [xml-xpath.html#23299 2.8 ��%G�₁��%@��Union of XPaths��%G�₁��%@ [25]])

<xsl:template match="*|/">

  <xsl:apply-templates/>

</xsl:template>

Text and attribute values are just copied (see

<xsl:template match="text()|@*">

  <xsl:value-of select="."/>

</xsl:template>

2.6 XPaths with predicates

A predicate is an expression that can be true or false
It is appended within [...] to a given location path and will refine results
More than one predicate can be appended to and within (!) a location path
Expressions can contain mathematical or boolean operators

Find element number N in a list

XML_element_name [ N ]

/project/participants/participant[2]

/project/participants/participant[2]
/FirstName

Find elements that have a given attribute

XML_element_name [ @attribute_name ]

Find elements that have a given element as child

XML_element_name [ XML_element_name ]

//participant[FoodPref]

Mathematical expressions

Use the standard operators, except div instead of / ")

- * div mod

mod is interesting if you want to display a long list in table format

5 mod 2 returns 1, "7 mod 2" and "3 mod 2" too

Boolean operators (comparison, and, or)

List of operators (according to precedence)

<=, <, >=, >

=, !=

and

or

Examples

Return all exercise titles with a grade bigger than 5.

//exercise[note>5]/title

Find elements that have a given attribute with a given value

XML_element_name [ @attribute_name = 'value']

//solutions/item[@val="low"]

Example XSLT template that will match all item elements with val="low".

<xsl:template match=" //item[@val='low'] ">

<xsl:value-of select="." />

</xsl:template>

Note: Usually expression also contain functions (see [xml-xpath.html#21068 2.7 ��%G�₁��%@��XPath functions��%G�₁��%@ [21]])

Return last five elements of a list

author [(last() - 4) <= position()) and (position() <= last())]

Return all Participant nodes with a contents of FirstName bigger than 7 characters:

"//Participant[string-length(FirstName)>=8]"

Example 2-3: Retrieve selected elements

The XSLT stylesheet (file xpath-jungle-3.xsl)

<xsl:template match="/">

  <html>

    <body bgcolor="#FFFFFF">

      <h1>Retrieve selected elements</h1>

      Here is the name of participant two:

        <ul><li><xsl:value-of select=".//participant[2]/FirstName
"/></li></ul>

      Here are all participant's firstnames that have a food preference:

      <ul><xsl:apply-templates select=".//participant[FoodPref]
"/></ul>

      Here are all items that have a value of "high"

      <ul><xsl:apply-templates select=".//item[@val='high']
"/></ul>

    </body>

  </html>

</xsl:template>

��

<xsl:template match="participant">

  <li><xsl:value-of select="FirstName"/></li>

</xsl:template>

��

<xsl:template match="item">

  <li><xsl:value-of select="."/></li>

</xsl:template>

��

HTML result

<html>

   <body bgcolor="#FFFFFF">

      <h1>Retrieve selected elements</h1>

      Here is the name of participant two:

      <ul>

         <li>Jonathan</li>

      </ul>

      Here are all participant's firstnames that have a food preference:

      <ul>

         <li>Daniel</li>

         <li>Jonathan</li>

      </ul>

      Here are all items that have a value of "high"

      <ul>

         <li>Register for a XSLT course and do exercices</li>

         <li>Register for a XPath course and do exercices</li>

      </ul>

   </body>

</html>

2.7 XPath functions

XPath defines a certain number of functions
- You can recognize a function because it has "()".
Functions are programming constructs that will return various kinds of informations, e.g.
- true / false
- a number
- a string
- a list of nodes
It is not obvious to understand these ....
There are restrictions on how you can use functions (stick to examples or the reference)

last()

last() gives the number or nodes within a context

position()

position() returns the position of an element with respect to other children for a parent

count(node-set)

count gives the number of nodes in a node set (usually found with an XPath).

starts-with(string, string)

returns TRUE if the second string is part of the first and starts off the first

//Participant[starts-with(Firstname,'Berna')]"

contains(string, string)

returns TRUE if the second string is part of the first

//Participant[contains(FirstName,'nat')]

string-length(string)

returns the length of a string

number(string)

transforms a string into a number

sum(node-set)

computes the sum of a given set of nodes.
If necessary, does string conversion with number()

round(number)

round a number, e.g. 1.4 becomes 1 and 1.7 becomes 2

translate(string1, string2, string3)

translates string1 by substituting string2 elements with string3 elements

(See next slides for examples ....)

Example 2-4: Computation of an average

We would like to compute the average of participant's qualifications

<participant><FirstName>Daniel</FirstName>

             <qualification>8</qualification>
   </participant>

The XSLT stylesheet (file xpath-jungle-4.xsl)

We compute the sum of a node-set and then divide by the number of nodes

<xsl:template match="/">

    <html>

      <body bgcolor="#FFFFFF">

        <h1>Qualification level of participants</h1>

        Average is

        <xsl:value-of select=
"sum(.//participant/qualification) div

                              count(.//participant/qualification)"
/>

      </body>

    </html>

  </xsl:template>

HTML result

<html>

   <body bgcolor="#FFFFFF">

      <h1>Qualification level of participants</h1>

      Average is

      5.75

   </body>

</html>

Example 2-5: Find first names containing 'nat'

The XSLT stylesheet (file xpath-jungle-5.xsl)

<xsl:template match="/">

 <html>

  <body bgcolor="#FFFFFF">

  <h1>Do we have a "nat" ?</h1>

  First Names that contain "nat":

  <ul>

  <xsl:apply-templates select=".//participant[contains(FirstName,'nat')
]"/>

  </ul>

  First Names that contain "nat" and "Nat":

  <ul>

   <xsl:apply-templates select=".//participant[contains

                         (translate(FirstName,'N','n'),'nat')]
"/>

   </ul>

  </body>

 </html>

</xsl:template>

��

<xsl:template match="participant">

  <li><xsl:value-of select="FirstName"/></li>

</xsl:template>

2.8 Union of XPaths

Union Xpaths combine more than one XPath (and all the resulting nodes are returned).
A typical example is the default rule which means that the template matches either the root element (i.e. "/" or just any element),

<xsl:template match="*|/">

  <xsl:apply-templates/>

</xsl:template>

Often this is used to simplify apply-templates or even templates themselves. E.g. the following rules applies to both "description" and "para" elements.

<xsl:template match="para|description">

  <p><xsl:apply-templates/></p>

</xsl:template>

2.9 List of commonly used XPath expressions

Syntax element	(Type of path)	Example path	Example matches
name	child element name	project	<project> ...... </project>
/	child / child	project/title	<project> <title> ... </title>
		/	(root element)
//	descendant	project//title	<project><problem> <title>....</title>
		//title	<root>... <title>..</title> (any place)
*	"wildcard"	*/title	<bla> <title>..</title> and <bli> <title>...</title>
\|	"or operator	title\|head	<title>...</title> or <head> ...</head>
		\|/\|@	All elements: root, children and attributes
.	current element	.
../	parent element	../problem	<project>
@attr	attribute name	@id	<xyz id="test">...</xyz>
element/@attr	attribute of child	project/@id	<project id="test" ...> ... </project>
@attr='value'	value of attribute	list[@type='ol']	<list type="ol"> ...... </list>
position()	position of element in parent	position()
last()	number of elements within a context	last() position()!=last()

3. XSLT reminder

In most situations, writing simple XSLT rules will do
Do not attempt to use looping constructs etc. when you don't have to
There is no special "magic" for dealing with images, links, stylesheets etc. Simply:
- look at your XML
- figure out how to translate into equivalent HTML (or whatever you are translating into)

Example 3-1: Dealing with pictures

XML file with picture file names (file images.xml)

<?xml version="1.0"?>

<?xml-stylesheet href="images.xsl" type="text/xsl"?>

<page>

 <title>Hello Here are my images</title>

 <list>

   <image>
dolores_001.jpg</image>

   <image>dolores_002.jpg</image>

   <image>dolores_002.jpg</image>

   <image2>scrolls.jpg </image2>

   <image2>scrolls.jpg </image2>

   <image3
 source="dolores_002.jpg">Recipe image</image3>

 </list>

 <comment>Written by DKS.</comment>

</page>

��

XSLT stylesheet (file images.xsl)

<xsl:template match="list">

   Apply templates for image elements:

   <xsl:apply-templates select="image
"/>

   This will only insert the first "image2" element contents it finds:

   <p> <img src="{image2}"/> </p>

   And another template for a tag image3 element (with an attribute)

   <xsl:apply-templates select="image3
"/>

</xsl:template>

This rule will insert the content of the image element into the value of src="".

  <xsl:template match="image">

    <p> <img src="{.}
"/> </p>

  </xsl:template>

This rule will insert the value of the source attribute into the value of src and also insert the contents of the the image3 element.

  <xsl:template match="image3">

    <p> <img src="{@source}
"/><xsl:value-of select="."/>
 </p>

  </xsl:template>

4. Multiple templates for one element

XSLT allows to define multiple rules for the same XPath by using the "mode" attribute
This construct is frequently used to produce table of contents

Parts of the XML file (file douglas-recipes.xml)

<list>

  <recipe id="r1">

    <recipe_name>TACOS</recipe_name>

    <meal>Dinner</meal>

    <ingredients>

      <item>3 taco shells</item>

      <item>1 pkg hamburger 100g</item>

      ........

    </ingredients>

��

    <directions>

      <bullet>Oven on at 180 degrees.</bullet>

      ......

      <bullet>Cut up lettuce.</bullet>

    </directions>

  </recipe>

��

  <recipe id="r2">

 ....

</list>

XSLT fragment (file douglas-recipes.xsl)

<xsl:template match="list">

  <html xmlns="http://www.w3.org/1999/xhtml">

   <head> <title>Recipes</title> </head>

   <body bgcolor="#ffffff">

    <h1>Recipes</h1>

    <h2>Contents</h2>

    <p> <xsl:apply-templates select="recipe" mode="toc"/
> </p>

    <xsl:apply-templates select="recipe
"/>

   </body> </html>

</xsl:template>

We define 2 rules for "recipe": One will have a mode="toc"
The next rule will create the table of contents.
- It will use the recipe_name as link
- It will use the id attribute to create an internal href link

<xsl:template match="recipe" mode="toc"
>

  <p><a href="#{@id}
"><xsl:value-of select="recipe_name
"/></a></p>

</xsl:template>

This rule is almost "normal", except that we insert a name anchor

<xsl:template match="recipe"
>

  <h1><a name="{@id}
"><xsl:value-of select="recipe_name"/></a> </h1>

  <xsl:apply-templates select="meal"/>

  <xsl:apply-templates select="ingredients"/>

  <xsl:apply-templates select="directions"/> </xsl:template>

HTML result

<?xml version="1.0" encoding="iso-8859-1"?>

��

<!DOCTYPE html

  PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"

         "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">

   <head>

      <title>Recipes</title>

   </head>

   <body bgcolor="#ffffff">

      <h1>Recipes</h1>

      <h2>Contents</h2>

<p>

         <p> <a href="#r1">TACOS</a> 
</p>

         <p> <a href="#r2">VOL AU VENT (one person)</a> 
</p>

      </p>

      <hr/>

      <h1>

         <a name="r1">TACOS</a>

      </h1>

      <p> Type: Dinner</p>

5. Conditional XSLT and whitespaces

Although many conditionals can be expressed with XPath and according selection rules, XSLT provides 2 typical programming constructs to handle "if" and "if-then-else" situations.
When you use XPath functions like position(), you have to be very careful about whitespaces

5.1 Simple if

Syntax for xsl:if:

<xsl:if test = "boolean_expression">

</xsl:if>

Example 5-1: Display lists, the last element differently with xsl:if

Warning: This only works in standards compliant browsers if you either eliminate whitespaces from your XML file or if you add the following XSL instruction at the beginning:

<xsl:strip-space elements="element_name element_name ...."/>

<xsl:strip-space elements="ingredients"/>

XML (file ingredient-list.xml)

<?xml version="1.0" encoding="ISO-8859-1" ?>

<?xml-stylesheet href="ingredient-list.xsl" type="text/xsl"?>

<ingredients>

  <item>3 taco shells</item>

  <item>1 pkg hamburger 100g</item>

  <item>Taco mix</item><item>Lettuce</item></ingredients>

XSL (file ingredient-list.xsl)

 <xsl:template match="/ingredients">

  <html xmlns="http://www.w3.org/1999/xhtml">

   <head> <title>Ingrediant list</title> </head>

   <body bgcolor="#ffffff">

     Here is a list of recipe ingrediants:

    <p> <xsl:apply-templates select="item"/>. </p>

     Here is a list of recipe ingrediants, this time numbered:

    <p> <xsl:apply-templates select="item" mode="numb"/>. </p>

   </body>

  </html>

</xsl:template>

��

<xsl:template match="item">

  <xsl:value-of select="."/>

  <xsl:if test="position()!=last()"
> <xsl:text>, </xsl:text> </xsl:if>

</xsl:template>

��

<xsl:template match="item" mode="numb">

  (<xsl:value-of select="position()
"/>)

  <xsl:value-of select="."/>

  <xsl:if test="position()!=last()"
> <xsl:text>, </xsl:text> </xsl:if>

</xsl:template>

��

</xsl:stylesheet>

HTML results

<html xmlns="http://www.w3.org/1999/xhtml">

   <head>

      <title>Ingrediant list</title>

   </head>

   <body bgcolor="#ffffff">

     Here is a list of recipe ingrediants:

    <p>3 taco shells,
 1 pkg hamburger 100g,
 Taco mix,
 Lettuce.
 </p>

     Here is a list of recipe ingrediants, this time numbered:

<p>

  (1)
 3 taco shells,

  (2) 
1 pkg hamburger 100g,

  (3) Taco mix,

  (4) Lettuce. </p>

   </body>

</html>

Same logic would apply e.g. to building references

<xsl:template match="reference">

  <xsl:apply-templates

       select="author|ref|title|edition|publisher|pubPlace|publicationYear"/>.

 </xsl:template>

 ��

 <xsl:template match="author|edition|publisher|pubPlace|publicationYear">

  <xsl:apply-templates/>

  <xsl:if test="position() !=last()">, </xsl:if>

 </xsl:template>

5.2 If-then-else

Syntax:

<xsl:choose>

</xsl:choose>

<xsl:when test = "boolean-expression">

</xsl:when>

<xsl:otherwise>