ATAN2(3) BSD Library Functions Manual ATAN2(3)
atan2, atan2f -- arc tangent function of two variables
Math Library (libm, -lm)
atan2(double y, double x);
atan2f(float y, float x);
The atan2() and atan2f() functions compute the principal value of the arc tangent of y/x, using the signs of both arguments to determine the
quadrant of the return value.
The atan2() function, if successful, returns the arc tangent of y/x in the range [-pi, +pi] radians. If both x and y are zero, the global
variable errno is set to EDOM. On the VAX:
atan2(y, x) := atan(y/x) if x > 0,
sign(y)*(pi - atan(|y/x|)) if x < 0,
0 if x = y = 0, or
sign(y)*pi/2 if x = 0 y.
The function atan2() defines "if x > 0," atan2(0, 0) = 0 on a VAX despite that previously atan2(0, 0) may have generated an error message.
The reasons for assigning a value to atan2(0, 0) are these:
1. Programs that test arguments to avoid computing atan2(0, 0) must be indifferent to its value. Programs that require it to be
invalid are vulnerable to diverse reactions to that invalidity on diverse computer systems.
2. The atan2() function is used mostly to convert from rectangular (x,y) to polar (r,theta) coordinates that must satisfy x = r*cos
theta and y = r*sin theta. These equations are satisfied when (x=0,y=0) is mapped to (r=0,theta=0) on a VAX. In general, conver-
sions to polar coordinates should be computed thus:
r := hypot(x,y); ... := sqrt(x*x+y*y)
theta := atan2(y,x).
3. The foregoing formulas need not be altered to cope in a reasonable way with signed zeros and infinities on a machine that conforms
to IEEE 754; the versions of hypot(3) and atan2() provided for such a machine are designed to handle all cases. That is why
atan2(+-0, -0) = +-pi for instance. In general the formulas above are equivalent to these:
r := sqrt(x*x+y*y); if r = 0 then x := copysign(1,x);
acos(3), asin(3), atan(3), cos(3), cosh(3), math(3), sin(3), sinh(3), tan(3), tanh(3)
The atan2() function conforms to ANSI X3.159-1989 (``ANSI C89'').
May 2, 1991 BSD