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

[Son To <son@wharton.upenn.edu>]

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
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