| 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 10.0 |