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THE XML REVOLUTION  -  TECHNOLOGIES FOR THE FUTURE WEB

Predicates

A predicate filters a node-set by evaluating the predicate expression on each node in the set with

• that node as the context node,
• the size of the node-set as the context size, and
• the position of the node in the node-set wrt. the axis ordering as the context position.

Example:

child::section[position()<6] / descendant::cite[attribute::href="there"]

selects all cite elements with href="there" attributes in the first 5 sections of an article document.

(Compare with the earlier example.)

The XPath predicate language is very large, but these are the essential ones to know

• [attribute::name="flour"]: test equality of an attribute
• [attribute::name!="flour"]: test inequality of an attribute
• [attribute::amount='0.5' and attribute::unit='cup']: test two things at once (also or)
• [position()=2]: test position among siblings
• [attribute::amount<'0.5']: a syntax error
• [attribute::amount&lt;'0.5']: a useless test of lexicographical order
• [number(attribute::amount)&lt;number('0.5')]: what you meant to write instead!

An entire location path may be used as a predicate

• start at the current node
• the predicate is true if the location path hits some result positions
• it is false otherwise

• [attribute::amount]: the node has an amount attribute
• [descendant::ingredient]: the node has a nested ingredient

COPYRIGHT © 2000-2003 ANDERS MØLLER & MICHAEL I. SCHWARTZBACH

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