
atan(+Y, +X, -Result)

   Computes the arc tangent function of two variables and unifies the resulting value with Result.

Arguments
   Y                   A number.
   X                   A number.
   Result              Output: float or breal.

Type
   Arithmetic

Description
   This predicate is used by the ECLiPSe compiler to expand evaluable
   arithmetic expressions.  So the call to atan(Y, X, Result) is
   equivalent to

    Result is atan(Y, X)

    which should be preferred for portability.

    It is similar to calculating the arc tangent of Y/X, except that the
    signs of both arguments are used to determine the quadrant of the result.
    The result lies in the interval -pi..pi. The operation is valid even if
    X is zero, in which case the result is pi/2 or -pi/2.  One application
    is the conversion of cartesian to polar coordinates, where this function
    computes the angle component (in radians).

   In coroutining mode, if X or Y is uninstantiated, the call to atan/3 is
   delayed until both variables are instantiated.




Modes and Determinism
   atan(+, +, -) is det

Exceptions
     4 --- X or Y is not instantiated (non-coroutining mode only).
    24 --- Number is not of a numeric type.

Examples
   
Success:
    Result is atan( 0.0, -1.0).		% gives Result =  3.141592
    Result is atan( 1.0, -1.0).		% gives Result =  2.356194
    Result is atan( 1.0,  0.0).		% gives Result =  1.570796
    Result is atan( 1.0,  1.0).		% gives Result =  0.785398
    Result is atan( 0.0,  0.0).		% gives Result =  0.0
    Result is atan(-1.0,  1.0).		% gives Result = -0.785398
    Result is atan(-1.0,  0.0).		% gives Result = -1.570796
    Result is atan(-1.0, -1.0).		% gives Result = -2.356194
    Result is atan(-0.0, -1.0).		% gives Result = -3.141592

    Result is atan( 7.0,  7.0).		% gives Result =  0.785398


See Also
   is / 2
