Functions

Boolean Functions

XPath boolean construction and logic functions

#Boolean Functions

XPath has four boolean functions. XPath defines "effective boolean value" rules. These rules convert other types to a boolean automatically in conditional contexts.

For C# developers: C# has no equivalent implicit conversion to boolean. XPath's effective boolean value rules convert an empty sequence, an empty string, or a zero value to false automatically. XPath's true and false are function calls, not literal keywords like C#'s true.

#boolean()

Converts a value to boolean using XPath's effective boolean value rules.

Signature: boolean($value as item()*) as xs:boolean

boolean("hello")    => true     (: non-empty string :)
boolean("")         => false    (: empty string :)
boolean(42)         => true     (: non-zero number :)
boolean(0)          => false    (: zero :)
boolean(())         => false    (: empty sequence :)
boolean((1, 2))     => true     (: non-empty sequence starting with a node or value :)
                                    

Effective boolean value rules:

C# equivalent: None. See the callout above.

Why this matters: In XPath, you can write if (//error) then .... The sequence of error elements converts to true automatically when any exist. You do not need if (count(//error) > 0).

#true()

Returns true. Needed because XPath doesn't have boolean literals.

Signature: true() as xs:boolean

true()   => true
                                      

Note: This is a function call with parentheses, not a literal value. XPath 4.0 also allows the keyword true without parentheses in some contexts.

#false()

Returns false.

Signature: false() as xs:boolean

false()   => false
                                        

#not()

Logical negation.

Signature: not($value as item()*) as xs:boolean

not(true())               => false
not(false())              => true
not(())                   => true    (: empty sequence is "falsy" :)
not(//error)              => true if no error elements exist
not(contains("abc", "x")) => true
                                          

C# equivalent: !value

Common pattern:

//book[not(@out-of-print)]          (: books without the out-of-print attribute :)
//item[not(price > 100)]            (: items not over $100 :)