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ATAN2(3) DragonFly Library Functions Manual ATAN2(3)## NAME

atan2,atan2f,atan2l-- arc tangent functions of two variables## SYNOPSIS

#include<math.h>doubleatan2(doubley,doublex);floatatan2f(floaty,floatx);longdoubleatan2l(longdoubley,longdoublex);## DESCRIPTION

Theatan2() function computes the principal value of the arc tangent ofy/x, using the signs of both arguments to determine the quadrant of the return value. Theatan2f() function is a single precision version ofatan2(). Theatan2l() function is an extended precision version ofatan2().## RETURN VALUES

Theatan2(),atan2f() andatan2l() functions, if successful, return the arc tangent ofy/xin the range [-pi, +pi] radians. If bothxandyare zero, the global variableerrnois set to EDOM. On the VAX:atan2(y,x) :=atan(y/x) ifx> 0, sign(y)*(pi -atan(|y/x|)) ifx< 0, 0 ifx=y= 0, or sign(y)*pi/2 ifx= 0,y!= 0.## NOTES

The functionatan2() defines "if x > 0,"atan2(0,0) = 0 on a VAX despite that previouslyatan2(0,0) may have generated an error message. The reasons for assigning a value toatan2(0,0) are these: 1. Programs that test arguments to avoid computingatan2(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. Theatan2() 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) andatan2() provided for such a machine are designed to handle all cases. That is whyatan2(+-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);## SEE ALSO

acos(3), asin(3), atan(3), cos(3), cosh(3), sin(3), sinh(3), tan(3), tanh(3)## STANDARDS

Theatan2() function conforms to ANSI X3.159-1989 (``ANSI C89''). DragonFly 4.7 January 15, 2015 DragonFly 4.7

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