XML in a Nutshell, 3rd Edition

Boolean Functions

The Boolean functions are few in number and quite straightforward. They all return a Boolean that has the value true or false. The true() function always returns true. The false( ) function always returns false. These substitute for Boolean literals in XPath.

The not( ) function reverses the sense of its Boolean argument. For example, not(@val>400) is almost always equivalent to (@val<=400). (NaN is a special case.)

The boolean() function converts its single argument to a Boolean and returns the result. If the argument is omitted, then it converts the context node. Numbers are converted to false if they’re zero or NaN. All other numbers are true. Node-sets are false if they’re empty and true if they contain at least one node. Strings are false if they have zero length, otherwise they’re true. Note that according to this rule, the string "false" is in fact true.