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XPATH
ch03

Originally based on XML Visual Quickstart Guide (chpt.03) by Kevin Howard Goldberg, and notes therefrom by Jack Davis (jcdavis@radford.edu).

In the last chapter XSL templates were described and examined. When you apply a template, you use an expression to specify the node set that should be processed. You write both patterns and expressions using XML Path Language (XPath) syntax.

XPath is a language for selecting nodes and node sets by specifying their location paths in the XML document. This chapter will describe how to specify XPath location paths in detail. XPath can be used in XSLT instructions to further process given node sets to return values instead of nodes. XPath has built-in functions to do math, process strings, and test conditions in an XML document. The next chapter, XPath Functions, will describe these functions.

Here is a nice website: an xpath interpreter: chris.photobooks.com/xml/

highlights

• xpath is a convenient way to talk about specific nodes within a tree (a "nodeset")
• like a file-structure: use / to go down “into” a tag
• If starts with /, it's an absolute path; otherwise it's a relative path
  (every xpath expression is evaluated within a context; we already saw <xsl:template match='/'> whose context is the root, and xsl:for-each whose body-context will be the node being currently-processed.
  Just as in your filesystem, be aware of the difference between absolute and relative paths!
• like unix file-system: . and .. mean current and parent node, respectively.
• Use […] to filter a node set
• You can do further refinement after filtering

• Locating Nodes -
  XPath location paths to refer to a node or node set. Remember that a node is an individual piece of the XML document (such as an element, an attribute, or some text content). A location path uses relationships to describe the location of a node or set of nodes relative to a given node. When translating location paths, XPath considers all XML documents as tree structures. Specifically, they are considered node trees, which are a hierarchical structure of nodes.
  more example nodes
  
• XML Node Tree --
  In an XML node tree, everything in the tree is a node, and every node is in some way related to another node in the tree. At the top of the node tree is the root node. The root node, or document node, can have any number of child nodes. To these child nodes, the root node is the parent node. These child nodes can have any number of child nodes themselves, and so on, and so on. Child nodes with the same parent are called sibling nodes. Descendent nodes are a node's child nodes, its children's child nodes, and so forth. Ancestor nodes are a node's parent node, grandparent nodes, etc. Through XPath location paths, you can access any of these nodes from any other simply by knowing the relationship between the two.
• Location Paths --
  
  • relative location path -
    consists of a sequence of location steps separated by / (a forward slash). Each step selects a node or node set relative to the current node. Then, each node in that set is used as the current node for the following step, and so on.
  • absolute location path -
    consists of / (a forward slash), optionally followed by a relative location path. A / by itself selects the root node of the XML document. If it is by a relative location path, then the location path is a relative location path starting at the root node. The choice of whether to use a relative or an absolute location path depends on the circumstance. Relative location paths are most commonly used, because they generate the resulting node set relative to the current node.
• Working on a subset of nodes: The history of each wonder:
  all wonders: summary output
  all wonders xsl
  
• In XPath, there three common types of nodes, plus four peripheral ones:
  • element nodes (tags); use / to access the node from its parent Element nodes may themselves have child-elements.
  • attribute nodes; use /@ to access the node from its parent; is a leaf (having just its attribute-value with it, but no children).
  • text nodes; XSLT uses xsl:value-of to extract a text node from its parent. Is a leaf.
  As well as four others we won't particularly bother ourselves with in this course:
  • root nodes (empty tree has no children from root; legal XML has exactly one element-node as a child.)
  • comment nodes (a leaf)
  • processing instruction nodes (a leaf)
  and
  • namespace nodes (a leaf).
  For each node type, there is a way of retrieving its value. For elements (and the root), that value is based on its descendant nodes. For the others, the value is part of the node (e.g. attributes have two parts, but no child-nodes).
• Building XPath location paths --
  
  • . - refers to the context or current node
  • .. - refers to the parent of the current node
  • ./* - select all of the children of the current node
  • /@attrName refers to the attribute with the indicated name.
  • /@ refers to all the current node's attributes
  • Sometimes you don't want all children nodes, but just a subset of them. Use square-brackets ([, ]) to filter the current node set:

    /* the name of every wonder, but only those with a language * attribute equal to 'English'*/ /ancient_wonders/wonder/name[@language='English'] /* the name node of the last wonder. */ /ancient_wonders/wonder[position()==last()]/name

    Predicates can compare values, test for existence, do math, and more. You use XPath Functions (next chapter) to create more complex conditions.

    Task: What is an xpath expression to find all wonder nodes which have both a greek and english name?

  • A double-slash selects all descendents: .// - select all descendants of the current node — no matter how deeply nested!

    As a corollary: If processing the root-node, then //elementName matches all the nodes named elementName in the entire document.

  Usually, you use relative paths. Beware when using absolute paths inside a loop — it'll always return the same value!
  xpath-ch03-17.xml(bug: absolute xpath in loop)
  xpath-ch03-17.xsl(bug: absolute xpath in loop)
  

• An example where the xpath refers to an individual attribute, not even a full html-tag:
  xpath-all-file-attrs.xml
  xpath-all-file-attrs.xsl
  

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