On Feb 9, 2009, at 6:30 PM, Michael Ash wrote:
Not really sure what you mean by this. It's true that a constant such
as 11.2f is a less precise representation of 11.2 than the non-float
version. But aside from actually defining the constants (and note that
all exact integers under 2^23 and many common fractional values will
be *exactly* represented and thus suffer no precision loss) there's no
trouble to be had.
Actually, there is trouble, for small values of "trouble". Try running
this code in GDB:
int main(int argc, char **argv)
{
double bigDouble =
20.0356412345678901234567890123456789012345678901234567890;
double test1 = bigDouble / 2.345678;
double test2 = bigDouble / 2.345678f;
double test3 = bigDouble / 2.345678;
return 0;
}
When I try this, test1 comes out to about 8.5415138968638882, whereas
test2 comes out to 8.541513565318839. And just to eliminate one other
possibility, test3 is identical to test1. Apparently test2 lost
precision by being divided by a float constant. I've been unable to
Google why this is happening, so I can only guess that either GCC is
downscan-converting the results of the division by the float constant,
or GDB is playing games with me.
I discovered this a while back while working on a brand new 32/64-bit
hybrid project that did a lot of floating point math, that I ended up
scrapping. Originally I did all the math using float constants, then
abandoned that for double constants and -fsingle-precision-constant
when I realized some precision was being lost...
Nick Zitzmann
<http://www.chronosnet.com/>
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