ATAN2(3)						   BSD Library Functions Manual 						  ATAN2(3)

NAME

     atan2, atan2f, atan2l, carg, cargf, cargl -- arc tangent and complex phase angle functions

LIBRARY

     Math Library (libm, -lm)

SYNOPSIS

     #include <math.h>

     double
     atan2(double y, double x);

     float
     atan2f(float y, float x);

     long double
     atan2l(long double y, long double x);

     #include <complex.h>

     double
     carg(double complex z);

     float
     cargf(float complex z);

     long double
     cargl(long double complex z);

DESCRIPTION

     The atan2(), atan2f(), and atan2l() 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 carg(), cargf(), and cargl() functions compute the complex argument (or phase angle) of z.  The complex argument is the number theta such
     that z = r * e^(I * theta), where r = cabs(z).  The call carg(z) is equivalent to atan2(cimag(z), creal(z)), and similarly for cargf() and
     cargl().

RETURN VALUES

     The atan2(), atan2f(), and atan2l() functions, if successful, return the arc tangent of y/x in the range [-pi, +pi] radians.  Here are some
     of the special cases:

     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.

NOTES

     The function atan2() defines "if x > 0," atan2(0, 0) = 0 despite that previously atan2(0, 0) may have generated an error message.	The rea-
     sons 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).  In general, conversions 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);

SEE ALSO

     acos(3), asin(3), atan(3), cabs(3), cos(3), cosh(3), math(3), sin(3), sinh(3), tan(3), tanh(3)

STANDARDS

     The atan2(), atan2f(), atan2l(), carg(), cargf(), and cargl() functions conform to ISO/IEC 9899:1999 (``ISO C99'').

BSD
								   July 31, 2008							       BSD