Re: c++/7369: weird results with the statement "#define PI 4.*atan(1.)"

[Andrew Pinski <pinskia at physics dot uc dot edu>]

The following reply was made to PR c++/7369; it has been noted by GNATS.

From: Andrew Pinski <pinskia@physics.uc.edu>
To: pel@ucla.edu
Cc: gcc-gnats@gcc.gnu.org
Subject: Re: c++/7369: weird results with the statement "#define PI 4.*atan(1.)"
Date: Sun, 21 Jul 2002 15:26:53 -0400

 No you have to use #define PI (4.0*atan(1.))
 because if you don't the statement becomes 1/4.*atan(1.0) which is
 equal to .25*atan(1.0) not .25*1/atan(1.0).
 
 Please close this bug report the reported behavior is the correct one.
 
 
 Thanks,
 Andrew Pinski
 
 
 On Sunday, July 21, 2002, at 03:03 , pel@ucla.edu wrote:
 
 >
 >> Number:         7369
 >> Category:       c++
 >> Synopsis:       weird results with the statement "#define PI 
 >> 4.*atan(1.)"
 >> Confidential:   no
 >> Severity:       non-critical
 >> Priority:       low
 >> Responsible:    unassigned
 >> State:          open
 >> Class:          sw-bug
 >> Submitter-Id:   net
 >> Arrival-Date:   Sun Jul 21 12:06:01 PDT 2002
 >> Closed-Date:
 >> Last-Modified:
 >> Originator:     Peter Latham
 >> Release:        2.96
 >> Organization:
 >> Environment:
 > Red Hat Linux 7.3
 >> Description:
 > If PI is defined via the statement
 >
 > #define PI 4.*atan(1.)
 >
 > then 1/PI and 1/3.14159 are different.
 >> How-To-Repeat:
 > compile using "g++ main.c". then type "a.out", which
 > prints PI, 1/PI and 1/3.14159. the result is
 >
 >    3.141593 0.196350 0.318310
 >
 > If for some strange reasont the file didn't attach properly,
 > the source code is:
 >
 >
 > #include <stdio.h>
 > #include <math.h>
 >
 > #define PI 4.*atan(1.)
 > //#define PI 3.14159
 >
 > void main(int argc, char** argv)
 > {
 >         fprintf(stdout, "%f %f %f\n", PI, 1/PI, 1/3.14159);
 > }
 >
 >
 >
 >
 >> Fix:
 > the workaround is to not use atan in the define statement,
 >
 > #define PI=3.14159
 >
 > this produces the correct output.
 >
 >> Release-Note:
 >> Audit-Trail:
 >> Unformatted:
 > ----gnatsweb-attachment----
 > Content-Type: application/octet-stream; name="main.c"
 > Content-Transfer-Encoding: base64
 > Content-Disposition: attachment; filename="main.c"
 >
 > CiNpbmNsdWRlIDxzdGRpby5oPgojaW5jbHVkZSA8bWF0aC5oPgoKI2RlZmluZSBQSSA0LiphdGFu
 > KDEuKQovLyNkZWZpbmUgUEkgMy4xNDE1OQoKdm9pZCBtYWluKGludCBhcmdjLCBjaGFyKiogYXJn
 > dikKewoJZnByaW50ZihzdGRvdXQsICIlZiAlZiAlZlxuIiwgUEksIDEvUEksIDEvMy4xNDE1OSk7
 > Cn0K
 >
 >