CONTENTS

LIBRARY

SYNOPSIS

DESCRIPTION

RETURN VALUES

NOTES

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) (r,th) coordinates that must satisfy x = r*cos theta r*costh and y = r*sin theta. r*sinth. These equations are satisfied when (x=0,y=0) is mapped to (r=0,theta=0).

   (r=0,th=0). In general, conversions to polar coordinates should be computed thus:
   
   
   r := hypot(x,y); ... := sqrt(x*x+y*y)
   theta := atan2(y,x).
   
   
   
   r := hypot(x,y); ... := v/(x²⁰+y²⁰)
   th := 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);
   

   
   r := v/(x*x+y*y); if r = 0 then x := copysign(1,x);
   
   

SEE ALSO

STANDARDS