atan2

Quadrant correct inverse circular tangent.

SYNOPSIS:

double x, y, z, atan2();
z = atan2(y, x);

DESCRIPTION:

Returns the radian angle whose tangent is y/x.
Define compile time symbol ANSIC = 1 for ANSI standard, range -PI < z <= +PI, args (y,x);
else ANSIC = 0 for range 0 to 2PI, args (x,y).

ACCURACY:
                      Relative error:
arithmetic   domain     # trials      peak         rms
    IEEE      -10, 10      10^6       2.5e-16     6.9e-17