# atan2(3) - NetBSD Manual Pages

Command: Section: Arch: Collection:
```ATAN2(3)                NetBSD Library Functions Manual               ATAN2(3)

NAME
atan2, atan2f, atan2l -- arc tangent function of two variables

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);

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.

RETURN VALUES
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 vari-
able 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.

NOTES
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 inva-
lidity on diverse computer systems.

2.   The atan2() function is used mostly to convert from rectangu-
lar (x,y) to polar (r,theta) coordinates that must satisfy x =
r*cos theta and y = r*sin theta.  These equations are satis-
fied when (x=0,y=0) is mapped to (r=0,theta=0) on a VAX.  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 rea-
sonable 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)

STANDARDS
The atan2() function conforms to ISO/IEC 9899:1999 (``ISO C99'').

NetBSD 10.99                   January 29, 2013                   NetBSD 10.99
```
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