Re: [Plug] Strange floating point behavior with C++

[Morgan Wajda-Levie <mpwl@locke.ccil.org>]

That seems like it would work.  Thanks for the help from both of you.

On Sat, Nov 20, 1999 at 02:31:00PM -0500, Son To wrote:
> how about converting the float to char* with sprintf and counting
> the number of digits after the '.'?
> 
> On Sat, 20 Nov 1999, Morgan Wajda-Levie wrote:
> 
> > I know that this is not a C programming mailing list, but a lot of
> > Linux people know C, and this is C code being written on Linux, so
> > it's somewhat applicable.
> > 
> > I'm writing a function in C++ that takes a floating point number and
> > returns the number digits of precision it has.  I'm doing this with
> > the following code:
> > 
> >   int f_digits(float number)
> >   {
> >     int a;
> >     number -= int(number);
> >     if(!number) return(0);
> >     for(a = 0; int(number / pow(10, a)) != number / pow(10, a); a--)
> >       {
> >       }
> >     return(a);
> >   }
> > 
> > As far as I can tell, the code looks like it should work.  If the
> > number has two digits of precision, it should be evenly divisible by
> > 10^-2, or 10.01.  Instead, it loops infinitely.  By running it through
> > gdb, I find that it claims that number = 0.400024414, when it should
> > be .4.  (123.4 - 123).  I tried dividing this number by 10^-9 in the
> > debugger, and I wrote everything out by hand.  Somehow, it came up
> > with the following:
> > 
> > .400024414 / .000000001
> > $4 = 400024413.99999994
> > 
> > Why can't C++ use floating point numbers accurately?
> > 
> > Also, for system info: I'm running Debian potato with glibc 2.1.2-10
> > and gcc 2.95.2-3.
> > -- 
> > Morgan Wajda-Levie
> > http://www.worldaxes.com/wajdalev
> > PGP fingerprint:
> > A353 C750 660E D8B6 5616  F4D8 7771 DD21 7BF6 221C
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> > encrypted mail preferred
> > 
> 
> 
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-- 
Morgan Wajda-Levie
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