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Xpath.ppt

1. The document discusses the XPath specification and data model. It provides examples of XPath queries and explains how XPath expressions are evaluated. 2. Key points covered include the seven node types in XPath (root, element, attribute, text, comment, processing instruction, namespace), location paths composed of axes and node tests, and the use of predicates to filter node sets. 3. Examples demonstrate different XPath axes like child, descendant, and attribute as well as wildcards, predicates, and accessing attribute values. Evaluation of XPath expressions is explained as a multi-step process working from context nodes.

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1 Web Data Management XPath
2 In this lecture • Review of the XPath specification – data model – examples – syntax Resources: A formal semantics of patterns in XSLT by Phil Wadler. XML Path Language (XPath) www.w3.org/TR/xpath
3 XPath • http://www.w3.org/TR/xpath (11/99) • Building block for other W3C standards: – XSL Transformations (XSLT) – XML Link (XLink) – XML Pointer (XPointer) – XML Query • Was originally part of XSL
XPath • An expression language to be used in another host language (e.g., XSLT, XQuery). • Allows the description of paths in an XML tree, and the retrieval of nodes that match these paths. • Can also be used for performing some (limited) operations on XML data. 4
5 Example for XPath Queries <bib> <book> <publisher> Addison-Wesley </publisher> <author> Serge Abiteboul </author> <author> <first-name> Rick </first-name> <last-name> Hull </last-name> </author> <author> Victor Vianu </author> <title> Foundations of Databases </title> <year> 1995 </year> </book> <book price=“55”> <publisher> Freeman </publisher> <author> Jeffrey D. Ullman </author> <title> Principles of Database and Knowledge Base Systems </title> <year> 1998 </year> </book> </bib>
Data Model for XPath • XPath expressions operate over XML trees, which consist of the following node types: – Document: the root node of the XML document; – Element: element nodes; – Attribute: attribute nodes, represented as children of an Element node; – Text: text nodes, i.e., leaves of the XML tree. 6
7 Data Model for XPath bib book book publisher author . . . . Addison-Wesley Serge Abiteboul The root The root element Much like the Xquery data model Processing instruction Comment Attr= “1” element attribute text
Data Model for XPath • The root node of an XML tree is the (unique) Document node; • The root element is the (unique) Element child of the root node; • A node has a name, or a value, or both – an Element node has a name, but no value; – a Text node has a value (a character string), but no name; – an Attribute node has both a name and a value. • Attributes are special! Attributes are not considered as first-class nodes in an XML tree. They must be addressed specifically, when needed. 8
9 XPath: Simple Expressions /bib/book/year Result: <year> 1995 </year> <year> 1998 </year> /bib/paper/year Result: empty (there were no papers)
10 XPath Tree Nodes • Seven nodes types: – root, element, attribute, text, comment, processing instruction and namespace • Namespace and attribute nodes have parent nodes, but are not children of those parent nodes. • The relationship between a parent node and a child node is containment • Attribute nodes and namespace nodes describe their parent nodes
11 Xpath Tree Nodes <?xml version = "1.0"?> <!-- Fig. 11.3 : simple2.xml --> <!-- Processing instructions and namespacess --> <html xmlns = "http://www.w3.org/TR/REC-html40"> <head> <title>Processing Instruction and Namespace Nodes </title> </head> <?deitelprocessor example = "fig11_03.xml"?> <body> <deitel:book deitel:edition = "1" xmlns:deitel = "http://www.deitel.com/xmlhtp1"> <deitel:title>XML How to Program</deitel:title> </deitel:book> </body> </html>
12 XPath Tree Nodes • String-value: The string-value of a text node consists of the character data contained in the node. • Document order: Nodes in an XPath tree have an ordering that is determined by the order in which the nodes appear in the original XML document. • The reverse document order is the reverse ordering of the nodes in a document. • The string-value for the html element node is determined by concatenating the string-values for all of its descendant text nodes in document order. • The string-value for element node html is Processing Instruction and Namespace NodesXML How to Program • Because all whitespace is removed when the text nodes are normalized, there is no space in the concatenation.
13 XPath Tree Nodes • For processing instructions, the string-value consists of the remainder of the processing instruction after the target, including whitespace, but excluding the ending ?> • The string-value for the processing instruction is • example = "fig11_03.xml" • Namespace-node string-values consist of the URI for the namespace. • The string-value for the namespace declaration is http://www.deitel.com/xmlhtpl
14 XPath Tree Nodes • For the root node of the document, the string-value is also determined by concatenating the string-values of its text- node descendents in document order. • The string-value of the root node is therefore identical to the string-value calculated for the html element node • The string-value for the edition attribute node consists of its value, which is 1. • The string-value for a comment node consists only of the comment's text, excluding <!-- and -->. • The string-value for the second comment node is therefore: Processing instructions and namespacess.
15 XPath Tree Nodes • Expanded-name: Certain nodes (i.e., element, attribute, processing instruction and namespace) also have an expanded-name that can be used to locate specific nodes in the XPath tree. • Expanded-names consist of both a local part and a namespace URI. • If there is a prefix for the element node, the namespace URI of the expanded-name is the URI to which the prefix is bound.
16 XPath Tree Nodes Node Type string-value expanded-name Description root Determined by concatenating the string-values of all text-node descendents in document order. None Represents the root of an XML document. This node exists only at the top of the tree and may contain element, comment or processor- instruction children. element Determined by concatenating the string-values of all text-node descendents in document order. The element tag, including the namespace prefix (if applicable). Represents an XML element and may contain element, text, comment or processor-instruction children. attribute The normalized value of the attribute. The name of the attribute, including the namespace prefix (if applicable). Represents an attribute of an element. text The character data contained in the text node. None. Represents the character data content of an element comment The content of the comment (not including <!-- and -->). None. Represents an XML comment processing instruction The part of the processing instruction that follows the target and any whitespace The target of the processing instruction. Represents an XML processing instruction namespace The URI of the namespace The namespace prefix. Represents an XML namespace
17 XPath: Axes • A location path is an expression that specifies how to navigate an XPath tree from one node to another. • A location path is composed of location steps, each of which is composed of an "axis," a "node test" and an optional "predicate." • Searching through an XML document begins at a context node in the XPath tree. • Searches through the XPath tree are made relative to this context node. • An axis indicates which nodes, relative to the context node, should be included in the search. • The axis also dictates the ordering of the nodes in the set. • Axes that select nodes that follow the context node in document order are called forward axes. • Axes that select nodes that precede the context node in document order are called reverse axes.
XPath Steps • The basic component of XPath expression are steps, of the form: axis::node-test[P1][P2]. . . [Pn] – axis is an axis name indicating what the direction of the step in the XML tree is (child is the default). – node-test is a node test, indicating the kind of nodes to select. – Pi is a predicate, that is, any XPath expression, evaluated as a boolean, indicating an additional condition. There may be no predicates at all. • A step is evaluated with respect to a context, and returns a node list. 18
Path Expressions • A path expression is of the form: [/]step1/step2/. . . /stepn – A path that begins with / is an absolute path expression; – A path that does not begin with / is a relative path expression. • Examples – /A/B is an absolute path expression denoting the Element nodes with name B, children of the root named A; – ./B/descendant::text() is a relative path expression which denotes all the Text nodes descendant of an Element B, itself child of the context node; – /A/B/@att1[.> 2] denotes all the Attribute nodes @att1 whose value is greater than 2. 19
Evaluation of Path Expressions • Each stepi is interpreted with respect to a context; its result is a node list. • A step stepi is evaluated with respect to the context of stepi−1. More precisely: – For i = 1 (first step) if the path is absolute, the context is a singleton, the root of the XML tree; else (relative paths) the context is defined by the environment; – For i > 1 if N = < N1,N2, · · · ,Nn > is the result of step stepi−1, stepi is successively evaluated with respect to the context [N,Nj ], for each j  [1,n]. • The result of the path expression is the node set obtained after evaluating the last step. 20
Evaluation of Path Expressions • Evaluation of /A/B/@att1 – The path expression is absolute: the context consists of the root node of the tree. • The first step, A, is evaluated with respect to this context. 21
Evaluation of /A/B/@att1 • The result is A, the root element. • A is the context for the evaluation of the second step, B. 22
Evaluation of /A/B/@att1 • The result is a node list with two nodes B[1], B[2]. • @att1 is first evaluated with the context node B[1]. 23
Evaluation of /A/B/@att1 • The result is the attribute node of B[1]. 24
Evaluation of /A/B/@att1 • @att1 is also evaluated with the context node B[2]. 25
Evaluation of /A/B/@att1 • The result is the attribute node of B[2]. 26
Evaluation of /A/B/@att1 • Final result: the node set union of all the results of the last step, @att1. 27
28 XPath: Axes Axes Ordering Description Self None The context node itself. Parent Reverse The context node's parent, if one exists. Child Forward The context node's children, if they exist. Ancestor Reverse The context node's ancestors, if they exist. ancestor-or-self Reverse The context node's ancestors and also itself. Descendant Forward The context node's descendants. descendant-or-self Forward The context node's descendants and also itself. Following Forward The nodes in the XML document following the context node, not including descendants. following-sibling Forward The sibling nodes following the context node. Preceding Reverse The nodes in the XML document preceding the context node, not including ancestors. preceding-sibling Reverse The sibling nodes preceding the context node. Attribute Forward The attribute nodes of the context node. Namespace Forward The namespace nodes of the context node.
29 XPath: Axes • An axis has a principal node type that corresponds to the type of node the axis may select. • For attribute axes, the principal node type is attribute. • For namespace axes, the principal node type is namespace. • All other axes have a element principal node type.
XPath: Axes • Child axis: denotes the Element or Text children of the context node. • Important: An Attribute node has a parent (the element on which it is located), but an attribute node is not one of the children of its parent. • Example: child::D 30
XPath: Axes • Parent axis: denotes the parent of the context node. – Always a Element or Document node, or an empty node-set (if the parent does not match the node test). – .. is an abbreviation for parent::node(): the parent of the context • Example: parent::node() 31
XPath: Axes • Attribute axis: denotes the attributes of the context node. – The node test is either the attribute name, or * which matches all the names. • Example: attribute::* 32
XPath: Axes • Descendant axis: all the descendant nodes, except the Attribute nodes. – The node test is either the node name (for Element nodes), or * (any Element node) or text() (any Text node) or node() (all nodes). – The context node does not belong to the result: use descendant-or-self instead. • Example: descendant::node() 33
XPath: Axes • Example: descendant::* 34
XPath: Axes • Ancestor axis: all the ancestor nodes. – The node test is either the node name (for Element nodes), or node() (any Element node, and the Document root node). – The context node does not belong to the result: use ancestor-or-self instead. • Example: ancestor::node() 35
XPath: Axes • Following axis: all the nodes that follows the context node in the document order. – Attribute nodes are not selected. – The node test is either the node name or node(). – The axis preceding denotes all the nodes that precede the context node. • Example: following::node() 36
XPath: Axes • Following sibling axis: all the nodes that follows the context node, and share the same parent node. – Same node tests as descendant or following. – The axis preceding-sibling denotes all the nodes the precede the context node. • Example: following-sibling::node() 37
38 Location Path Abbreviations Location path Abbreviation child:: This location path is used by default if no axis is supplied and may therefore be omitted attribute:: @ /descendant-or-self::node()/ // self::node() (.) parent::node() (..)
39 XPath: Node Tests • The set of selected nodes is refined with node tests. • node tests rely upon the principal node type of an axis for selecting nodes in a location path Node Test Description * Selects all nodes of the same principal node type. node() Selects all nodes, regardless of their type. text() Selects all text nodes. comment() Selects all comment nodes. processing-instruction() Selects all processing-instruction nodes. node name Selects all nodes with the specified node name.
40 XPath: Axes • Location Paths Using Axes and Node Tests – Location paths are composed of sequences of location steps. – A location step contains an axis and a node test separated by a double-colon (::) and, optionally, a "predicate" enclosed in square brackets ([ ]).
41 XPath: Wildcard //author/child::* or //author/* Result: <first-name> Rick </first-name> <last-name> Hull </last-name> * Matches any element
42 XPath: Axes and Node Tests • child::text() – selects all text-node children of the context node • Combining two location steps to form the location path – child::*/child::text() – selects all text-node grandchildren of the context node
43 XPath: Node Tests /bib/book/author/text() Result: Serge Abiteboul Victor Vianu Jeffrey D. Ullman Rick Hull doesn’t appear because he has firstname, lastname /bib/book/author/*/text() Result: Rick Hull
44 XPath: Restricted Kleene Closure • select all author element nodes in an entire document /descendent-or-self::node()/child::author • Instead use the abbreviation: //author Result:<author> Serge Abiteboul </author> <author> <first-name> Rick </first-name> <last-name> Hull </last-name> </author> <author> Victor Vianu </author> <author> Jeffrey D. Ullman </author> /bib//first-name Result: <first-name> Rick </first-name>
45 XPath: Attribute Nodes /bib/book/@price Result: “55” @price means that price has to be an attribute
XPath: Predicates • Boolean expression, built with tests and the Boolean connectors and/or. • a test is – either an XPath expression, whose result is converted to a Boolean; – a comparison or a call to a Boolean function. • Important: predicate evaluation requires several rules for converting nodes and node sets to the appropriate type. 46
Predicate Evaluation • A step is of the form axis::node-test[P] • First axis::node-test is evaluated: one obtains an intermediate result I • Second, for each node in I, P is evaluated: the step result consists of those nodes in I for which P is true. /A/B/descendant::text()[1] 47
Predicate Evaluation • Here the result consists of those Text nodes, first descendant (in the document order) of a node B. • /A/B//text()[1] 48
49 XPath: Predicates <?xml version = "1.0"?> <!-- Fig. 11.9 : books.xml --> <!-- XML book list --> <books> <book> <title>Java How to Program</title> <translation edition="1">Spanish</translation> <translation edition="1">Chinese</translation> <translation edition="1">Japanese</translation> <translation edition="2">French</translation> <translation edition="2">Japanese</translation> </book> <book> <title>C++ How to Program</title> <translation edition="1">Korean</translation> <translation edition="2">French</translation> <translation edition="2">Spanish</translation> <translation edition="3">Italian</translation> <translation edition="3">Japanese</translation> </book> </books>
50 XPath: Predicates • Select the title element node for each book that has a Japanese translation /books/book/translation[. = 'Japanese']/../title • A predicate is a Boolean expression used as part of a location path to filter nodes from the search. • Select the edition attribute node for books with Japanese translations /books/book/translation[. = 'Japanese']/@edition
XPath 1.0 Type System • Four primitive types: • The boolean(), number(), string() functions convert types into each other (no conversion to nodesets is defined), but this conversion is done in an implicit way most of the time. • Rules for converting to a Boolean: – A number is true if it is neither 0 nor NaN(not a number). – A string is true if its length is not 0. – A nodeset is true if it is not empty. 51 Type Description Literals Examples Boolean Boolean values None true() Number Floating-point 12, 12.5 1 div 33 String Ch. Strings "to", ’ti’ concat(’Hello’,’!’) Nodeset Node set None /a/b[c=1 or @e]/d
XPath 1.0 Type System • Rules for converting a nodeset to a string: – The string value of a nodeset is the string value of its first item in document order. – The string value of an element or document node is the concatenation of the character data in all text nodes below. – The string value of a text node is its character data. – The string value of an attribute node is the attribute value. • Examples 52 <a toto="3"> <b titi=’tutu’><c /></b> <d>tata</d> </a> string(/) "tata" string(/a/@toto) "3" boolean(/a/b) true() boolean(/a/e) false()
53 Operators • Node-set operators allow to manipulate the node sets to form other node sets. Node-set Operators Description pipe (|) union of node-sets (Example: node()|@*) slash (/) Separates location steps double-slash (//) Abbreviation for the location path /descendant-or-self::node()/ +, -, *, div, mod standard arithmetic operators or, and Boolean operators (Example: @a and c=3) <, <=, >=, > relational operators (Example: ($a<2) and ($a>0))
XPath Examples • child::A/descendant::B : B elements, descendant of an A element, itself child of the context node; • child::*/child::B : all the B grand-children of the context node • descendant-or-self::B : elements B descendants of the context node, plus the context node itself if its name is B. • child::B[position()=last()] : the last child named B of the context node. Abbreviated to B[last()]. • following-sibling::B[1] : the first sibling of type B (in the document order) of the context node 54
XPath 2.0 • An extension of XPath 1.0 • Improved data model tightly associated with XML Schema. –  a new sequence type, representing ordered set of nodes and/or values, with duplicates allowed. • More powerful new operators (loops) and better control of the output (limited tree restructuring capabilities) • Extensible Many new built-in functions; possibility to add user-defined functions. 55
Path expressions in XPath 2.0 • New node tests in XPath 2.0: • Nested paths expressions: Any expression that returns a sequence of nodes can be used as a step 56 /book/(author | editor)/name Node tests Description item() any node or atomic value element() any element (eq. to child::* in XPath 1.0) element(author) any element named author element(*, xs:person) any element of type xs:person attribute() any attribute
XPath 1.0 Implementations • libxml2 Free C library for parsing XML documents, supporting XPath. • java.xml.xpath Java package, included with JDK versions starting from 1.5. • System.Xml.XPath .NET classes for XPath. • XML::XPath Free Perl module, includes a command-line tool. • DOMXPath PHP class for XPath, included in PHP5. • PyXML Free Python library for parsing XML documents, supporting XPath. 57

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Xpath.ppt

  • 1.
  • 2.
    2 In this lecture •Review of the XPath specification – data model – examples – syntax Resources: A formal semantics of patterns in XSLT by Phil Wadler. XML Path Language (XPath) www.w3.org/TR/xpath
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    3 XPath • http://www.w3.org/TR/xpath (11/99) •Building block for other W3C standards: – XSL Transformations (XSLT) – XML Link (XLink) – XML Pointer (XPointer) – XML Query • Was originally part of XSL
  • 4.
    XPath • An expressionlanguage to be used in another host language (e.g., XSLT, XQuery). • Allows the description of paths in an XML tree, and the retrieval of nodes that match these paths. • Can also be used for performing some (limited) operations on XML data. 4
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    5 Example for XPathQueries <bib> <book> <publisher> Addison-Wesley </publisher> <author> Serge Abiteboul </author> <author> <first-name> Rick </first-name> <last-name> Hull </last-name> </author> <author> Victor Vianu </author> <title> Foundations of Databases </title> <year> 1995 </year> </book> <book price=“55”> <publisher> Freeman </publisher> <author> Jeffrey D. Ullman </author> <title> Principles of Database and Knowledge Base Systems </title> <year> 1998 </year> </book> </bib>
  • 6.
    Data Model forXPath • XPath expressions operate over XML trees, which consist of the following node types: – Document: the root node of the XML document; – Element: element nodes; – Attribute: attribute nodes, represented as children of an Element node; – Text: text nodes, i.e., leaves of the XML tree. 6
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    7 Data Model forXPath bib book book publisher author . . . . Addison-Wesley Serge Abiteboul The root The root element Much like the Xquery data model Processing instruction Comment Attr= “1” element attribute text
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    Data Model forXPath • The root node of an XML tree is the (unique) Document node; • The root element is the (unique) Element child of the root node; • A node has a name, or a value, or both – an Element node has a name, but no value; – a Text node has a value (a character string), but no name; – an Attribute node has both a name and a value. • Attributes are special! Attributes are not considered as first-class nodes in an XML tree. They must be addressed specifically, when needed. 8
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    9 XPath: Simple Expressions /bib/book/year Result:<year> 1995 </year> <year> 1998 </year> /bib/paper/year Result: empty (there were no papers)
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    10 XPath Tree Nodes •Seven nodes types: – root, element, attribute, text, comment, processing instruction and namespace • Namespace and attribute nodes have parent nodes, but are not children of those parent nodes. • The relationship between a parent node and a child node is containment • Attribute nodes and namespace nodes describe their parent nodes
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    11 Xpath Tree Nodes <?xmlversion = "1.0"?> <!-- Fig. 11.3 : simple2.xml --> <!-- Processing instructions and namespacess --> <html xmlns = "http://www.w3.org/TR/REC-html40"> <head> <title>Processing Instruction and Namespace Nodes </title> </head> <?deitelprocessor example = "fig11_03.xml"?> <body> <deitel:book deitel:edition = "1" xmlns:deitel = "http://www.deitel.com/xmlhtp1"> <deitel:title>XML How to Program</deitel:title> </deitel:book> </body> </html>
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    12 XPath Tree Nodes •String-value: The string-value of a text node consists of the character data contained in the node. • Document order: Nodes in an XPath tree have an ordering that is determined by the order in which the nodes appear in the original XML document. • The reverse document order is the reverse ordering of the nodes in a document. • The string-value for the html element node is determined by concatenating the string-values for all of its descendant text nodes in document order. • The string-value for element node html is Processing Instruction and Namespace NodesXML How to Program • Because all whitespace is removed when the text nodes are normalized, there is no space in the concatenation.
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    13 XPath Tree Nodes •For processing instructions, the string-value consists of the remainder of the processing instruction after the target, including whitespace, but excluding the ending ?> • The string-value for the processing instruction is • example = "fig11_03.xml" • Namespace-node string-values consist of the URI for the namespace. • The string-value for the namespace declaration is http://www.deitel.com/xmlhtpl
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    14 XPath Tree Nodes •For the root node of the document, the string-value is also determined by concatenating the string-values of its text- node descendents in document order. • The string-value of the root node is therefore identical to the string-value calculated for the html element node • The string-value for the edition attribute node consists of its value, which is 1. • The string-value for a comment node consists only of the comment's text, excluding <!-- and -->. • The string-value for the second comment node is therefore: Processing instructions and namespacess.
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    15 XPath Tree Nodes •Expanded-name: Certain nodes (i.e., element, attribute, processing instruction and namespace) also have an expanded-name that can be used to locate specific nodes in the XPath tree. • Expanded-names consist of both a local part and a namespace URI. • If there is a prefix for the element node, the namespace URI of the expanded-name is the URI to which the prefix is bound.
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    16 XPath Tree Nodes NodeType string-value expanded-name Description root Determined by concatenating the string-values of all text-node descendents in document order. None Represents the root of an XML document. This node exists only at the top of the tree and may contain element, comment or processor- instruction children. element Determined by concatenating the string-values of all text-node descendents in document order. The element tag, including the namespace prefix (if applicable). Represents an XML element and may contain element, text, comment or processor-instruction children. attribute The normalized value of the attribute. The name of the attribute, including the namespace prefix (if applicable). Represents an attribute of an element. text The character data contained in the text node. None. Represents the character data content of an element comment The content of the comment (not including <!-- and -->). None. Represents an XML comment processing instruction The part of the processing instruction that follows the target and any whitespace The target of the processing instruction. Represents an XML processing instruction namespace The URI of the namespace The namespace prefix. Represents an XML namespace
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    17 XPath: Axes • Alocation path is an expression that specifies how to navigate an XPath tree from one node to another. • A location path is composed of location steps, each of which is composed of an "axis," a "node test" and an optional "predicate." • Searching through an XML document begins at a context node in the XPath tree. • Searches through the XPath tree are made relative to this context node. • An axis indicates which nodes, relative to the context node, should be included in the search. • The axis also dictates the ordering of the nodes in the set. • Axes that select nodes that follow the context node in document order are called forward axes. • Axes that select nodes that precede the context node in document order are called reverse axes.
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    XPath Steps • Thebasic component of XPath expression are steps, of the form: axis::node-test[P1][P2]. . . [Pn] – axis is an axis name indicating what the direction of the step in the XML tree is (child is the default). – node-test is a node test, indicating the kind of nodes to select. – Pi is a predicate, that is, any XPath expression, evaluated as a boolean, indicating an additional condition. There may be no predicates at all. • A step is evaluated with respect to a context, and returns a node list. 18
  • 19.
    Path Expressions • Apath expression is of the form: [/]step1/step2/. . . /stepn – A path that begins with / is an absolute path expression; – A path that does not begin with / is a relative path expression. • Examples – /A/B is an absolute path expression denoting the Element nodes with name B, children of the root named A; – ./B/descendant::text() is a relative path expression which denotes all the Text nodes descendant of an Element B, itself child of the context node; – /A/B/@att1[.> 2] denotes all the Attribute nodes @att1 whose value is greater than 2. 19
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    Evaluation of PathExpressions • Each stepi is interpreted with respect to a context; its result is a node list. • A step stepi is evaluated with respect to the context of stepi−1. More precisely: – For i = 1 (first step) if the path is absolute, the context is a singleton, the root of the XML tree; else (relative paths) the context is defined by the environment; – For i > 1 if N = < N1,N2, · · · ,Nn > is the result of step stepi−1, stepi is successively evaluated with respect to the context [N,Nj ], for each j  [1,n]. • The result of the path expression is the node set obtained after evaluating the last step. 20
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    Evaluation of PathExpressions • Evaluation of /A/B/@att1 – The path expression is absolute: the context consists of the root node of the tree. • The first step, A, is evaluated with respect to this context. 21
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    Evaluation of /A/B/@att1 •The result is A, the root element. • A is the context for the evaluation of the second step, B. 22
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    Evaluation of /A/B/@att1 •The result is a node list with two nodes B[1], B[2]. • @att1 is first evaluated with the context node B[1]. 23
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    Evaluation of /A/B/@att1 •The result is the attribute node of B[1]. 24
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    Evaluation of /A/B/@att1 •@att1 is also evaluated with the context node B[2]. 25
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    Evaluation of /A/B/@att1 •The result is the attribute node of B[2]. 26
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    Evaluation of /A/B/@att1 •Final result: the node set union of all the results of the last step, @att1. 27
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    28 XPath: Axes Axes OrderingDescription Self None The context node itself. Parent Reverse The context node's parent, if one exists. Child Forward The context node's children, if they exist. Ancestor Reverse The context node's ancestors, if they exist. ancestor-or-self Reverse The context node's ancestors and also itself. Descendant Forward The context node's descendants. descendant-or-self Forward The context node's descendants and also itself. Following Forward The nodes in the XML document following the context node, not including descendants. following-sibling Forward The sibling nodes following the context node. Preceding Reverse The nodes in the XML document preceding the context node, not including ancestors. preceding-sibling Reverse The sibling nodes preceding the context node. Attribute Forward The attribute nodes of the context node. Namespace Forward The namespace nodes of the context node.
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    29 XPath: Axes • Anaxis has a principal node type that corresponds to the type of node the axis may select. • For attribute axes, the principal node type is attribute. • For namespace axes, the principal node type is namespace. • All other axes have a element principal node type.
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    XPath: Axes • Childaxis: denotes the Element or Text children of the context node. • Important: An Attribute node has a parent (the element on which it is located), but an attribute node is not one of the children of its parent. • Example: child::D 30
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    XPath: Axes • Parentaxis: denotes the parent of the context node. – Always a Element or Document node, or an empty node-set (if the parent does not match the node test). – .. is an abbreviation for parent::node(): the parent of the context • Example: parent::node() 31
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    XPath: Axes • Attributeaxis: denotes the attributes of the context node. – The node test is either the attribute name, or * which matches all the names. • Example: attribute::* 32
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    XPath: Axes • Descendantaxis: all the descendant nodes, except the Attribute nodes. – The node test is either the node name (for Element nodes), or * (any Element node) or text() (any Text node) or node() (all nodes). – The context node does not belong to the result: use descendant-or-self instead. • Example: descendant::node() 33
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    XPath: Axes • Example:descendant::* 34
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    XPath: Axes • Ancestoraxis: all the ancestor nodes. – The node test is either the node name (for Element nodes), or node() (any Element node, and the Document root node). – The context node does not belong to the result: use ancestor-or-self instead. • Example: ancestor::node() 35
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    XPath: Axes • Followingaxis: all the nodes that follows the context node in the document order. – Attribute nodes are not selected. – The node test is either the node name or node(). – The axis preceding denotes all the nodes that precede the context node. • Example: following::node() 36
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    XPath: Axes • Followingsibling axis: all the nodes that follows the context node, and share the same parent node. – Same node tests as descendant or following. – The axis preceding-sibling denotes all the nodes the precede the context node. • Example: following-sibling::node() 37
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    38 Location Path Abbreviations Locationpath Abbreviation child:: This location path is used by default if no axis is supplied and may therefore be omitted attribute:: @ /descendant-or-self::node()/ // self::node() (.) parent::node() (..)
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    39 XPath: Node Tests •The set of selected nodes is refined with node tests. • node tests rely upon the principal node type of an axis for selecting nodes in a location path Node Test Description * Selects all nodes of the same principal node type. node() Selects all nodes, regardless of their type. text() Selects all text nodes. comment() Selects all comment nodes. processing-instruction() Selects all processing-instruction nodes. node name Selects all nodes with the specified node name.
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    40 XPath: Axes • LocationPaths Using Axes and Node Tests – Location paths are composed of sequences of location steps. – A location step contains an axis and a node test separated by a double-colon (::) and, optionally, a "predicate" enclosed in square brackets ([ ]).
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    41 XPath: Wildcard //author/child::* or//author/* Result: <first-name> Rick </first-name> <last-name> Hull </last-name> * Matches any element
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    42 XPath: Axes andNode Tests • child::text() – selects all text-node children of the context node • Combining two location steps to form the location path – child::*/child::text() – selects all text-node grandchildren of the context node
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    43 XPath: Node Tests /bib/book/author/text() Result:Serge Abiteboul Victor Vianu Jeffrey D. Ullman Rick Hull doesn’t appear because he has firstname, lastname /bib/book/author/*/text() Result: Rick Hull
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    44 XPath: Restricted KleeneClosure • select all author element nodes in an entire document /descendent-or-self::node()/child::author • Instead use the abbreviation: //author Result:<author> Serge Abiteboul </author> <author> <first-name> Rick </first-name> <last-name> Hull </last-name> </author> <author> Victor Vianu </author> <author> Jeffrey D. Ullman </author> /bib//first-name Result: <first-name> Rick </first-name>
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    45 XPath: Attribute Nodes /bib/book/@price Result:“55” @price means that price has to be an attribute
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    XPath: Predicates • Booleanexpression, built with tests and the Boolean connectors and/or. • a test is – either an XPath expression, whose result is converted to a Boolean; – a comparison or a call to a Boolean function. • Important: predicate evaluation requires several rules for converting nodes and node sets to the appropriate type. 46
  • 47.
    Predicate Evaluation • Astep is of the form axis::node-test[P] • First axis::node-test is evaluated: one obtains an intermediate result I • Second, for each node in I, P is evaluated: the step result consists of those nodes in I for which P is true. /A/B/descendant::text()[1] 47
  • 48.
    Predicate Evaluation • Herethe result consists of those Text nodes, first descendant (in the document order) of a node B. • /A/B//text()[1] 48
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    49 XPath: Predicates <?xml version= "1.0"?> <!-- Fig. 11.9 : books.xml --> <!-- XML book list --> <books> <book> <title>Java How to Program</title> <translation edition="1">Spanish</translation> <translation edition="1">Chinese</translation> <translation edition="1">Japanese</translation> <translation edition="2">French</translation> <translation edition="2">Japanese</translation> </book> <book> <title>C++ How to Program</title> <translation edition="1">Korean</translation> <translation edition="2">French</translation> <translation edition="2">Spanish</translation> <translation edition="3">Italian</translation> <translation edition="3">Japanese</translation> </book> </books>
  • 50.
    50 XPath: Predicates • Selectthe title element node for each book that has a Japanese translation /books/book/translation[. = 'Japanese']/../title • A predicate is a Boolean expression used as part of a location path to filter nodes from the search. • Select the edition attribute node for books with Japanese translations /books/book/translation[. = 'Japanese']/@edition
  • 51.
    XPath 1.0 TypeSystem • Four primitive types: • The boolean(), number(), string() functions convert types into each other (no conversion to nodesets is defined), but this conversion is done in an implicit way most of the time. • Rules for converting to a Boolean: – A number is true if it is neither 0 nor NaN(not a number). – A string is true if its length is not 0. – A nodeset is true if it is not empty. 51 Type Description Literals Examples Boolean Boolean values None true() Number Floating-point 12, 12.5 1 div 33 String Ch. Strings "to", ’ti’ concat(’Hello’,’!’) Nodeset Node set None /a/b[c=1 or @e]/d
  • 52.
    XPath 1.0 TypeSystem • Rules for converting a nodeset to a string: – The string value of a nodeset is the string value of its first item in document order. – The string value of an element or document node is the concatenation of the character data in all text nodes below. – The string value of a text node is its character data. – The string value of an attribute node is the attribute value. • Examples 52 <a toto="3"> <b titi=’tutu’><c /></b> <d>tata</d> </a> string(/) "tata" string(/a/@toto) "3" boolean(/a/b) true() boolean(/a/e) false()
  • 53.
    53 Operators • Node-set operatorsallow to manipulate the node sets to form other node sets. Node-set Operators Description pipe (|) union of node-sets (Example: node()|@*) slash (/) Separates location steps double-slash (//) Abbreviation for the location path /descendant-or-self::node()/ +, -, *, div, mod standard arithmetic operators or, and Boolean operators (Example: @a and c=3) <, <=, >=, > relational operators (Example: ($a<2) and ($a>0))
  • 54.
    XPath Examples • child::A/descendant::B: B elements, descendant of an A element, itself child of the context node; • child::*/child::B : all the B grand-children of the context node • descendant-or-self::B : elements B descendants of the context node, plus the context node itself if its name is B. • child::B[position()=last()] : the last child named B of the context node. Abbreviated to B[last()]. • following-sibling::B[1] : the first sibling of type B (in the document order) of the context node 54
  • 55.
    XPath 2.0 • Anextension of XPath 1.0 • Improved data model tightly associated with XML Schema. –  a new sequence type, representing ordered set of nodes and/or values, with duplicates allowed. • More powerful new operators (loops) and better control of the output (limited tree restructuring capabilities) • Extensible Many new built-in functions; possibility to add user-defined functions. 55
  • 56.
    Path expressions inXPath 2.0 • New node tests in XPath 2.0: • Nested paths expressions: Any expression that returns a sequence of nodes can be used as a step 56 /book/(author | editor)/name Node tests Description item() any node or atomic value element() any element (eq. to child::* in XPath 1.0) element(author) any element named author element(*, xs:person) any element of type xs:person attribute() any attribute
  • 57.
    XPath 1.0 Implementations •libxml2 Free C library for parsing XML documents, supporting XPath. • java.xml.xpath Java package, included with JDK versions starting from 1.5. • System.Xml.XPath .NET classes for XPath. • XML::XPath Free Perl module, includes a command-line tool. • DOMXPath PHP class for XPath, included in PHP5. • PyXML Free Python library for parsing XML documents, supporting XPath. 57