ATAN2 (3) | C library functions | Unix Manual Pages | :man

### NAME

atan2, atan2f - arc tangent functions of two variables

Library
Synopsis
Description
Return Values
Notes
Standards

.Lb libm

### SYNOPSIS

.In math.h double atan2 "double y" "double x" float atan2f "float y" "float x"

### DESCRIPTION

The atan2 and the atan2f 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 and the atan2f functions, if successful, return the arc tangent of y/ x in the range
.Bk -words [- pi, + pi]
.Ek radians. Here are some of the special cases:
 atan2 y x := atan y/x if x > 0, sign( y )*(pi - atan "\*(Bay/x\*(Ba") 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 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 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);