The XPath Language 1 XPath Expressions l l

The XPath Language 1

XPath Expressions l l Flexible notation for navigating around trees A basic technology that is widely used l l uniqueness and scope in XML Schema pattern matching an selection in XSLT relations in XLink and XPointer computations on values in XSLT and XQuery 2

Location Paths l l l A location path evaluates to a sequence of nodes The sequence is sorted in document order The sequence will never contain duplicates 3

Locations Steps l l The location path is a sequence of steps A location step consists of l l l an axis a nodetest some predicates axis : : nodetest [ Exp 1] [ Exp 2] … 4

Evaluating a Location Path l l A step maps a context node into a sequence This also maps sequences to sequences l l l each node is used as context node and is replaced with the result of applying the step The path then applies each step in turn 5

An Example A B C C E F F B D C F F E E E F F 6

An Example l Context node A B C C E F F B D C F F E E E F F 7

An Example l descendant: : C A B C C E F F B D C F F E E E F F 8

An Example l descendant: : C/child: : E A B C C E F F B D C F F E E E F F 9

An Example l descendant: : C/child: : E/child: : F A B C C E F F B D C F F E E E F F 10

Contexts l The context of an XPath evaluation consists of l l l l a context node (a node in an XML tree) a context position and size (two nonnegative integers) a set of variable bindings a function library a set of namespace declarations The application determines the initial context If the path starts with ‘/’ then l l the initial context node is the root the initial position and size are 1 11

Axes l l An axis is a sequence of nodes located relative to the context node XPath supports 12 different axes l child l descendant l parent l ancestor l following-sibling l preceding-sibling l attribute l following l preceding l self l descendant-or-self l ancestor-or-self 12

Axis Directions l l Each axis has a direction Forwards means document order: l l Backwards means reverse document order: l l child, descendant, following-sibling, following, self, descendant-or-self parent, ancestor, preceding-sibling, preceding Stable but depends on the implementation: l attribute 13

The parent Axis 14

The child Axis 15

The descendant Axis 16

The ancestor Axis 17

The following-sibling Axis 18

The preceding-sibling Axis 19

The following Axis 20

The preceding Axis 21

Node Tests l l l l text() comment() processing-instruction() node() * l selects all nodes QName l selects the nodes with the qualified name *: NCName l selects the nodes with the given name in any namespace (nocolon name) NCName: * 22

Example: Selecting Attribute Nodes <wdg: widget type="gadget" xmlns: wdg="http: //www. widget. inc"> <wdg: head size="medium"/> <wdg: big><wdg: subwidget ref="gizmo"/></wdg: big> <wdg: info xmlns: xhtml="http: //www. w 3. org/TR/xhtml 1"> <xhtml: head> <xhtml: title>Description of gadget</xhtml: title> </xhtml: head> <xhtml: body> <xhtml: h 1>Gadget</xhtml: h 1> A gadget contains a big gizmo </xhtml: body> </wdg: info> </wdg: widget> /child: : wdg: widget/child: : wdg: big/child: : wdg: subwidget/attribute: : ref 23

Predicates l l l General XPath expressions Evaluated with the current node as context Result is coerced into a boolean l l l a number yields true if it equals the context position a string yields true if it is not empty a sequence yields true if it is not empty 24

Examples l l The recipes contain sugar as an ingredient: /descendant: : rcp: recipe[descendant: : rcp: ingredient[attribu te: : name='sugar']] The ingredients whose name is sugar: /descendant: : rcp: recipe/descendant: : rcp: ingredient[attribu te: : name='sugar'] The third ingredient: /descendant: : rcp: ingredient[position()=3][position()=1] Empty sequence: /descendant: : rcp: ingredient[position()=1][position()=3] 25

Typical Location Paths l l l The title of each recipe: /descendant: : rcp: recipe/child: : rcp: title The name of each ingredient: /descendant: : rcp: recipe/descendent: : rcp: ingredient /attribute: : name All character data in the collection: /descendant: : rcp: */child: : text() 26

Predicate Examples l l l Test existence of an atribute: [attribute: : amount] Test equality of an attribute value: [attribute: : name='flour'] Test inequality of an attribute value : [attribute: : name!='flour'] Test two things at once: [attribute: : amount<0. 5 and attribute: : unit='cup'] Test position of the context node: [position()=2] Probe the subtree below: [descendant: : rcp: ingredient] 27

Abbreviations l l /child: : rcp: collection/child: : rcp: recipe/child: : rcp: ingredient /rcp: collection/rcp: recipe/rcp: ingredient /child: : rcp: collection/child: : rcp: recipe/child: : rcp: ingredient/ attribute: : amount /rcp: collection/rcp: recipe/rcp: ingredient/@amount /descendant-or-self: : node()/ // self: : node() . l parent: : node() . . 28

Examples l l l The amounts of flour being used: //rcp: ingredient[@name="flour"]/@amount The ingredients of which half a cup are used: //rcp: ingredient[@amount='0. 5' and @unit='cup']/@name The second step in preparing stock for Cailles en Sarcophages: //rcp: ingredient[@name="stock"]/rcp: preparation/step[po sition()=2]/text() 29

XPath visualization l l l Using IE 6, it is easy to experiment with XPath expressions. The XPath Visualizer provides an interactive XPath evaluator that additionally visualizes the resulting node set. (http: //www. vbxml. com/xpathvisualizer/) This tool is implemented as an ordinary HTML page that makes heavy use of XSLT and Java. Script. 30

More Examples l l l para selects the para element children of the context node * selects all element children of the context node text() selects all text node children of the context node @name selects the name attribute of the context node @* selects all the attributes of the context node para[1] selects the first para child of the context node 31

More Examples (cont. ) l l para[fn: last()] selects the last para child of the context node */para selects all para grandchildren of the context node /book/chapter[5]/section[2] selects the second section of the fifth chapter of the book whose parent is the document node that contains the context node chapter//para selects the para element descendants of the chapter element children of the context node 32

More Examples (cont. ) l l l //para selects all the para descendants of the root document node and thus selects all para elements in the same document as the context node //@version selects all the version attribute nodes that are in the same document as the context node //list/member selects all the member elements in the same document as the context node that have a list parent 33

More Examples (cont. ) l l l . //para selects the para element descendants of the context node. . selects the parent of the context node. . /@lang selects the lang attribute of the parent of the context node para[@type="warning"] selects all para children of the context node that have a type attribute with value warning para[@type="warning"][5] selects the fifth para child of the context node that has a type attribute with value warning 34

More Examples (cont. ) l l l para[5][@type="warning"] selects the fifth para child of the context node if that child has a type attribute with value warning chapter[title="Introduction"] selects the chapter children of the context node that have one or more title children whose typed value is equal to the string Introduction chapter[title] selects the chapter children of the context node that have one or more title children 35

More Examples (cont. ) l l l employee[@secretary and @assistant] selects all the employee children of the context node that have both a secretary attribute and an assistant attribute book/(chapter|appendix)/section selects every section element that has a parent that is either a chapter or an appendix element, that in turn is a child of a book element that is a child of the context node. If E is any expression that returns a sequence of nodes, then the expression E/. returns the same nodes in document order, with duplicates eliminated based on node identity. 36