ATAN2(3) | Library Functions Manual | ATAN2(3) |
atan2
, atan2f
,
atan2l
—
#include <math.h>
double
atan2
(double
y, double x);
float
atan2f
(float
y, float x);
long double
atan2l
(long
double y, long double
x);
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.
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. |
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:
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.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,
conversions to polar coordinates should be computed thus:
r := hypot(x,y); ... := sqrt(x∗x+y∗y) theta := atan2(y,x).
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);
atan2
() function conforms to
ISO/IEC 9899:1999 (“ISO C99”).
January 29, 2013 | NetBSD 9.2 |