190 lines
4.6 KiB
Groff
190 lines
4.6 KiB
Groff
.\" Copyright (c) 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the University of
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.\" California, Berkeley and its contributors.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)atan2.3 8.1 (Berkeley) 6/4/93
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.\" $Id$
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.\"
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.Dd June 4, 1993
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.Dt ATAN2 3
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.Os
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.Sh NAME
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.Nm atan2
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.Nd arc tangent function of two variables
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.Sh SYNOPSIS
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.Fd #include <math.h>
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.Ft double
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.Fn atan2 "double y" "double x"
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.Sh DESCRIPTION
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The
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.Xr atan2
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function computes the principal value of the arc tangent of
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.Ar y/ Ns Ar x ,
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using the signs of both arguments to determine the quadrant of
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the return value.
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.Sh RETURN VALUES
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The
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.Xr atan2
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function, if successful,
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returns the arc tangent of
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.Ar y/ Ns Ar x
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in the range
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.Bk -words
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.Bq \&- Ns \*(Pi , \&+ Ns \*(Pi
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.Ek
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radians.
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If both
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.Ar x
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and
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.Ar y
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are zero, the global variable
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.Va errno
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is set to
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.Er EDOM .
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On the
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.Tn VAX :
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.Bl -column atan_(y,x)_:=____ sign(y)_(Pi_atan2(Xy_xX))___
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.It Fn atan2 y x No := Ta
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.Fn atan y/x Ta
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if
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.Ar x
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> 0,
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.It Ta sign( Ns Ar y Ns )*(\*(Pi -
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.Fn atan "\\*(Bay/x\\*(Ba" ) Ta
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if
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.Ar x
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< 0,
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.It Ta
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.No 0 Ta
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if x = y = 0, or
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.It Ta
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.Pf sign( Ar y Ns )*\\*(Pi/2 Ta
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if
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.Ar x
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= 0 \*(!=
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.Ar y .
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.El
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.Sh NOTES
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The function
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.Fn atan2
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defines "if x > 0,"
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.Fn atan2 0 0
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= 0 on a
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.Tn VAX
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despite that previously
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.Fn atan2 0 0
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may have generated an error message.
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The reasons for assigning a value to
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.Fn atan2 0 0
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are these:
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.Bl -enum -offset indent
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.It
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Programs that test arguments to avoid computing
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.Fn atan2 0 0
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must be indifferent to its value.
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Programs that require it to be invalid are vulnerable
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to diverse reactions to that invalidity on diverse computer systems.
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.It
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The
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.Fn atan2
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function is used mostly to convert from rectangular (x,y)
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to polar
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.if n\
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(r,theta)
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.if t\
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(r,\(*h)
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coordinates that must satisfy x =
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.if n\
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r\(**cos theta
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.if t\
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r\(**cos\(*h
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and y =
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.if n\
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r\(**sin theta.
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.if t\
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r\(**sin\(*h.
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These equations are satisfied when (x=0,y=0)
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is mapped to
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.if n \
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(r=0,theta=0)
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.if t \
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(r=0,\(*h=0)
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on a VAX. In general, conversions to polar coordinates
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should be computed thus:
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.Bd -unfilled -offset indent
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.if n \{\
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r := hypot(x,y); ... := sqrt(x\(**x+y\(**y)
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theta := atan2(y,x).
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.\}
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.if t \{\
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r := hypot(x,y); ... := \(sr(x\u\s82\s10\d+y\u\s82\s10\d)
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\(*h := atan2(y,x).
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.\}
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.Ed
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.It
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The foregoing formulas need not be altered to cope in a
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reasonable way with signed zeros and infinities
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on a machine that conforms to
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.Tn IEEE 754 ;
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the versions of
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.Xr hypot 3
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and
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.Fn atan2
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provided for
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such a machine are designed to handle all cases.
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That is why
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.Fn atan2 \(+-0 \-0
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= \(+-\*(Pi
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for instance.
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In general the formulas above are equivalent to these:
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.Bd -unfilled -offset indent
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.if n \
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r := sqrt(x\(**x+y\(**y); if r = 0 then x := copysign(1,x);
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.if t \
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r := \(sr(x\(**x+y\(**y);\0\0if r = 0 then x := copysign(1,x);
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.Ed
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.El
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.Sh SEE ALSO
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.Xr acos 3 ,
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.Xr asin 3 ,
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.Xr atan 3 ,
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.Xr cos 3 ,
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.Xr cosh 3 ,
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.Xr math 3 ,
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.Xr sin 3 ,
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.Xr sinh 3 ,
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.Xr tan 3 ,
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.Xr tanh 3
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.Sh STANDARDS
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The
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.Fn atan2
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function conforms to
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.St -ansiC .
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