XPath 2.0 reference

XPaths  |  functions

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abs

abs($arg)
     $arg: numeric
     returns: numeric

Returns the absolute value of $arg. If $arg is negative returns -$arg otherwise returns $arg. If type of $arg is one of the four numeric types xs:float, xs:double, xs:decimal or xs:integer the type of the result is the same as the type of $arg. If the type of $arg is a type derived from one of the numeric types, the result is an instance of the base numeric type.

Example 1:

Input:	abs(10.5)
Output:	10.5

Example 2:

Input:	abs(-10.5)
Output:	10.5

Locate in the official documentation

XPath 2.0 reference