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XPath Help

Learn how to extract static parts of XPath expressions, visualize XPath in a schema, and combine sequences using union, intersection, and except operators. Explore the meanings of XPath expressions and strategies. Dive into the data model and EBNF notations for XPath. Practice combining node sequences with union, intersect, and except operators. Understand the contexts and meanings of various XPath expressions, sequences, and operations. Experiment with context node types and mixing strategies with XPath for efficient data extraction and manipulation.

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XPath Help

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  1. XPath Help Karl Lieberherr

  2. Source of information / Plan • http://www.w3.org/TR/2005/WD-xpath20-20050915/ • Complex definition • We want to extract the “static” part of XPath • Objective: Use / and /+/ (//) to build sequences and use operators: union, intersection and except to combine them.

  3. Mismatches • XPath can work without schema. • But we want to visualize XPath expression in schema. • Works only for “static” XPath expressions • XPath views meaning of expression as a sequence. • Strategies views the meaning of an expression as a function that maps objects to object graph slices. Traversing slice: sequence of nodes.

  4. Mismatches • But we can also view the meaning of a strategy and a class graph as another graph: TraversalGraph

  5. XPath data model • In the data model, a value is always a sequence. • A sequence is an ordered collection of zero or more items. • An item is either an atomic value or a node. • singleton, empty sequence. Sequences are never nested.

  6. EBNF for XPath • [26]   PathExpr   ::=   ("/" RelativePathExpr?)| ("//" RelativePathExpr)| RelativePathExpr • [27]   RelativePathExpr   ::=   StepExpr (("/" | "//") StepExpr)* • [28]   StepExpr   ::=   AxisStep | FilterExpr

  7. Class dictionaries / EBNF • exercise3.txt • class dictionaries are both a simplification and extension of EBNF.

  8. EBNF for XPath • [30]   ForwardStep   ::=   (ForwardAxisNodeTest) | AbbrevForwardStep • [31]   ForwardAxis   ::=   <"child" "::">| <"descendant" "::">| <"attribute" "::">| <"self" "::">| <"descendant-or-self" "::">| <"following-sibling" "::">| <"following" "::">| <"namespace" "::">

  9. EBNF for XPath • 3.3.3 Combining Node Sequences • [14]   UnionExpr   ::=   IntersectExceptExpr ( ("union" | "|") IntersectExceptExpr )* • [15]   IntersectExceptExpr   ::=   InstanceofExpr ( ("intersect" | "except") InstanceofExpr )* • [16]   InstanceofExpr::=   PathExpr

  10. sequence operations • XPath provides the following operators for combining sequences of nodes: • The union and | operators are equivalent. They take two node sequences as operands and return a sequence containing all the nodes that occur in either of the operands. • The intersect operator takes two node sequences as operands and returns a sequence containing all the nodes that occur in both operands. • The except operator takes two node sequences as operands and returns a sequence containing all the nodes that occur in the first operand but not in the second operand.

  11. sequence operations $seq1 is bound to (A, B) $seq2 is bound to (A, B) $seq3 is bound to (B, C) Then: $seq1 union $seq2 evaluates to the sequence (A, B). $seq2 union $seq3 evaluates to the sequence (A, B, C). $seq1 intersect $seq2 evaluates to the sequence (A, B). $seq2 intersect $seq3 evaluates to the sequence containing B only. $seq1 except $seq2 evaluates to the empty sequence. $seq2 except $seq3 evaluates to the sequence containing A only.

  12. Declare context node type • what is meaning of / B and /+/ B?

  13. Mixing strategies and XPath?? • what would the meaning be of: • s1 : from A via B to C • s2 : A /+/ D • union (s1 s2)

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