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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 invalidity on diverse computer systems. 2. Theatan2() 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). 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) 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 4.1 January 15, 2015 DragonFly 4.1

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