XPath
Kanda Runapongsa
([email protected])
Dept. of Computer Engineering
Khon Kaen University
What is XPath?
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XPath is a language designed to address
specific parts of an XML document
It was designed to be used by both XSLT
and XPointer
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XSLT: transforms an XML document into any
text-based format, such as HTML
XPointer: the basis for a fragment identifies
only for text/xml & application/xml media
types
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XPath Expressions
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The XPath data model views a
document as a tree of nodes
An instance of the XPath language is
called an expression
A path expression is an expression
used for selecting a node set by
following a path or steps
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XPath Expressions (Cont.)
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XPath use path expressions to
identify nodes in an XML document
These path expressions look very
much like the expressions you see
when you work with a computer file
system
Documents/courses/168493/xpath.ppt
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Sample Document
<?xml version=“1.0”?>
<catalog>
<cd country=“UK”>
<title>Hide your heart</title>
<artist>Bonnie Tyler</artist>
<price>9.90</price>
</cd>
<cd country=“USA”>
<title>Greatest Hits</title>
<artist>Dolly Parton</artist>
<price>10.90</price>
</cd>
</catalog>
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Path Expressions Examples
/catalog
This XPath expression selects the ROOT
element catalog
 /catalog/cd
This XPath expression selects all the cd
elements of the catalog element
 /catalog/cd/price
This XPath expression selects all the price
elements of all the cd elements of the
catalog element
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/ and //
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/ select the root node of the current
document
Example: /catalog
Select the root element catalog
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An absolute path to an element
// select nodes that are the descendant
nodes of context node
Example: //cd
Select the cd elements in the document
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Selecting Unknown Elements
Wildcards (*) can be used to select
unknown elements
 /catalog/cd/*
This XPath expression selects all the
child elements of all the cd elements
of the catalog element
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Selecting Unknown Elements (Cont.)
/catalog/*/price
This expression selects all the price
elements that are grandchild
elements of the catalog element
 /*/*/price
This expression selects all price
elements which have 2 ancestors
 How to select all elements?

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Selecting Branches
By using square brackets in an XPath
expression you can specify an
element further
 /catalog/cd[1]
The XPath expression selects the first
cd child element of the catalog
element
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Selecting Branches (Cont.)
/catalog/cd[last()]
The expression selects the last cd
element of the catalog element
 /catalog/cd[price]
The expression selects all the cd
elements of the catalog element that
have a price element
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Selecting Branches (Cont.)
/catalog/cd[price=10.90]
Selects all the cd elements of the catalog
element that have a price element with a
value of 10.90
 /catalog/cd[price=10.90]/price
Selects all the price elements of all the cd
elements of the catalog element that have
a price element with a value of 10.90

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Selecting Several Branches
By using the | operator in an XPath
expression you can select several
paths
 /catalog/cd/title | /catalog/cd/artist
Selects all the title and artist elements
of the cd element of the catalog
element
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Selecting Attributes
In XPath all attributes are specified
by the @ prefix
 //@country
Selects all attributes named country
 //cd[@country]
Selects all cd elements which have an
attribute named country
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Selecting Attributes (Cont.)
//cd[@*]
Selects all cd elements which have any
attribute
 //cd[@country=‘UK’]
Selects all cd elements which have an
attribute named country with a value
of ‘UK’
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A Location Path
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It is the most important kind of
expressions in the XPath notation
It can be absolute or relative
An absolute path starts with a slash
(/) and a relative location path does
not start with a slash
The location path consists of one or
more location steps, each separated
by a slash
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A Location Path (Cont.)
An absolute location path:
/step/step/…
 A relative location path:
step/step/…
 The location steps are evaluated in
order one at a time, from left to right
 Each step is evaluated against the
nodes in the current node-set
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A location Path (Cont.)
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The current node-set that is being
selected is called the set of the context
nodes
Each step of a location path (a location
step) has three following parts:
 An axis – specifies the tree relationship
between the nodes selected by the
location step and the current node
 A node test – specifies the node type
 Zero or more predicates – use
expressions to further refine the set of
nodes
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A Location Step
The syntax for a location step is:
axisname::nodetest[predicate]
 Example:
child::cd[price=9.90]
 An axis defines a node-set relative to
the current node
 A node test is used to identify a node
with an axis
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Axis Names
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ancestor: contains all ancestors
(parent, grandparent, etc.) of the
current node
ancestor-or-self: contains the current
node plus all its ancestors (parent,
grandparent, etc.)
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What is the node that this axis always
includes?
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Axis Names (Cont.)
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attribute: contains all attributes of
the current node
child: contains all children of the
current node
descendant: contains all descendants
(children, grandchildren, etc.) of the
current node
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Does this axis contain attribute nodes?
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Axis Names (Cont.)
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descendant-or-self: contains the
current node plus all its descendants
following: contains everything in the
document after the closing tag of the
current node, except descendants
following-sibling: contains all siblings
after the current node
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Axis Names (Cont.)
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namespace: contains all namespace
nodes of the current node
parent: contains the parent of the
current node
preceding: contains everything in the
document that is before the starting
tag of the current node, except
ancestors
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Axis Names (Cont.)
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preceding-sibling: contains all siblings
before the current node
self: contains the current node
For any given context node v, the
four major axes specify a partitioning
of the document containing v
v/descendants U v/ancestors U
v/following U v/preceding U {v}
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Examples

child:cd
Selects all cd elements that are
children of the current node
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attribute::src
Selects the src attribute of the current
node
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Examples (Cont.)
child::cd[position() < 6]
Selects the first five cd children of
the current node
 child::cd[attribute::type=“classic”]
Selects all cd children of the current
node that have a type attribute
with value classic
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Unabbreviated vs. Abbreviated
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The name of the location path is long
Location paths can be expressed
using abbreviated syntax
The most important abbreviation is
child:: which can be omitted from a
location step
Example:
cd is short for child::cd
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Abbreviated Syntax
@ is short for attributes::
Example:
cd[@type=“classic”] is short for

child::cd[attribute::type=“classic”]
. is short for self::node()
Example::
./cd is short for

self::node()/child::cd
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Abbreviated Syntax (Cont.)
.. is short for parent::node
Example:
../cd is short for
parent::node()/child::cd
 // is short for /descendant-or-self::node()/
Example::
//cd is short for
/descendant-or-self::node()/child::cd
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Examples
cd[@type=“classic”]
Selects all cd children of the current
node that has a type attribute with
value classic
 cd[@type=“classic”][5]
Selects the fifth cd child of the current
node that has a type attribute with
value classic
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Examples
cd[5][@type=“classic”]
Selects the fifth cd child of the current
node if that child has a type attribute
with value classic
 cd[@type and @country]
Selects all the cd children of the
current node that have both a type
attribute and a country attribute
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Node Set Functions
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Function count() counts the number
of selected elements
Function name() returns name of the
element
Function position() returns the
position in the node list of the node
that is currently being processed
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String Functions
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Function contains() returns true if the 1st
argument string contains the 2nd argument
string
Example: contains(‘XML’, ‘M’)
Result: true
Function string-length() returns the
number of characters in the string
Example: string-length(‘XML’)
Result: 3
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String Functions (Cont.)
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Function substring() returns the
substring of the first argument
starting at the position specified in
the 2nd argument with length
specified in the 3rd argument
Example: substring(‘Students’,4,4)
Result: ‘dent’
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String Functions (Cont.)
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Function starts-with() returns true if the
first argument starts with the second
argument
Example: starts-with(‘XML’, ‘X’)
Result: true
Function string() converts the value
argument to a string
Example: string(314)
Result: ‘314’
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Number Functions
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celing() returns the smallest integer
that is not less than the number
argument
Example: ceiling(3.14)  4
sum() returns the total value of a set
of numeric values in a node-set
Example: sum(/cd/price)
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