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sgn(+Number, ?Result)

Unifies Result with the sign of Number which is either -1, 0 or 1.
+Number
A number.
?Result
A variable or an integer.

Description

This predicate is used by the ECLiPSe compiler to expand evaluable arithmetic expressions. So the call to sgn(Number, Result) is equivalent to 
    Result is sgn(Number)
which should be preferred for portability.

sgn/2 gives the integer -1 if Number is negative, 0 if it is zero and 1 if it is greater than zero. It is always true that 

    X =:= sgn(X) * abs(X)
In coroutining mode, if Number is a free variable, the call to sgn/2 is delayed until this variable is instantiated.

Fail Conditions

Fails if the result of the evaluation does not unify with Result.

Resatisfiable

No

Exceptions

(4) instantiation fault
Number is not instantiated (non-coroutining mode only).
(5) type error
Result is a non-integer number.
(24) number expected
Number is not of a numeric type.
(24) number expected
Result is neither a number nor a variable.

Examples

Success:
      sgn(42, 1).
      sgn(-5, Result).             (gives Result = -1)
      sgn(-6.2, Result).           (gives Result = -1)
      sgn(0.0, 0).
Fail:
      sgn(1, 0).
Error:
      sgn(A, 6).                   (Error 4).
      sgn(1, 1.0).                 (Error 5).
      sgn(4 + 2, 6).               (Error 24).
      sgn(1, r).                   (Error 24).

See Also

is / 2