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Xref: | news2.mv.net comp.lang.c:54599 comp.lang.c++:70107 comp.os.msdos.djgpp:690 gnu.g++.help:4446 gnu.gcc.help:5657 |
From: | wfeldt AT physik DOT uni-leipzig DOT de (Steffen Winterfeldt) |
Newsgroups: | comp.lang.c,comp.lang.c++,gnu.gcc.help,gnu.g++.help,comp.os.msdos.djgpp |
Subject: | Re: float != float and floats as return types |
Followup-To: | comp.lang.c,comp.lang.c++,gnu.gcc.help,gnu.g++.help,comp.os.msdos.djgpp |
Date: | 30 Jan 1996 18:21:39 GMT |
Organization: | Uni Leipzig |
Lines: | 32 |
Message-ID: | <4elnjj$er4@server2.rz.uni-leipzig.de> |
References: | <4ej9lb$mpc AT fu-berlin DOT de> |
NNTP-Posting-Host: | tph100.physik.uni-leipzig.de |
To: | djgpp AT delorie DOT com |
DJ-Gateway: | from newsgroup comp.os.msdos.djgpp |
Axel Thimm (axl AT zedat DOT fu-berlin DOT de) wrote: : Hello, : I am getting confused, about how C/C++ manage float binary operations, : in particular multiplication. The next C++ example gives me surprising : results: : *** cut here: begin file t_prec.cc : #include <iostream.h> : #include <iomanip.h> : #include <math.h> : float quad( float ); : int main() { : for( int i=0; i<10; ++i ) { : float a, b, c; : a = i/13.123123; : b = a*a; : c = quad(a); : cout << (b - c) << '\t'; : cout << (b - a*a) << '\t'; : cout << (c - quad(a)) << '\n'; : } : return 0; : } : float quad( float x ) { return x*x; } : [...] (c - quad(a)) is not zero, because quad's return value is in a floating point register and so has higher precision than c. BTW, with higher optimization (say, -O3), even the second column becomes zero. Steffen
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