linux i-86 problem with rounding (gcc-3.1.1 & 3.2)

[Martin Dickopp]

On Wed, Sep 18, 2002 at 10:06:51AM -0400, Hillel (Sabba) Markowitz wrote:
> The following test program give incorrect results in a cast from
> double to int when compiled without an optimization option but
> correct results when compiled with an optimization option.
> 
> int main(int argc, char **argv)
> {
>   int i = 128000;
>   double f = 0.0075;
>   double g;
>   unsigned int u;
> 
>   u = (unsigned int) (i*f);
>   printf("unsigned int result of i*f: %u\n", u);
> 
>   g = i*f;
>   u = (unsigned int)g;
>   printf("double result: %lf\n", g);
>   printf("unsigned int result: %u\n", u);
> 
>   return(0);
> }
> 
> 
> %gcc testfloat.c -o testfloat
> %./testfloat
> 
> unsigned int result of i*f: 959
> double result: 960
> unsigned int result: 960

You incorrectly assume that calculations involving floating-point
numbers are infinitely precise; in reality, rounding is inevitable.
The details of the rounding may depend, amongst other things, on the
processor type and optimization level.

Solution:

  u = (unsigned int) (i*f + 0.5);

This rounds i*f to the nearest unsigned integer, instead of always
rounding down. Therefore, you obtain your expected result even if
i*f is slightly less than 960.0.

Martin