Re: inconsistent for() and while() behavior when using floating point

On Mon, Jan 15, 2018 at 08:38:15PM +0300, Mehmet Erol Sanliturk:
> 
> 
> On Mon, Jan 15, 2018 at 5:38 PM, Yuri Pankov <yuripv_at_icloud.com 
> <mailto:yuripv_at_icloud.com>> wrote:
> 
>     Hi,
> 
>     Looking at https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=217149
>     <https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=217149>, I
>     noticed that it isn't a seq(1) problem per se, rather for() and
>     while() loops behaving inconsistently while using floating point, i.e.:
> 
>              double i;
> 
>              for (i = 1; i <= 2.00; i += 0.1)
>                      printf("%g\n", i);
> 
>     would produce:
> 
>              1
>              ...
>              1.9
> 
>     but:
> 
>              double i;
> 
>              for (i = 1; i <= 2; i += 0.2)
>                      printf("%g\n", i);
> 
>     would correctly end with 2:
> 
>              1
>              ...
>              2
> 
>     $ cc -v
>     FreeBSD clang version 6.0.0 (branches/release_60 321788) (based on
>     LLVM 6.0.0)
>     Target: x86_64-unknown-freebsd12.0
>     Thread model: posix
>     InstalledDir: /usr/bin
> 
>     though gcc 4.4.4 on illumos behaves the same.
> 
>     Is this a known problem with loops and floating point numbers?
>     _______________________________________________
> 
> 
> 
> 
> When you perform floating point computations , it may be useful to 
> remember that , the last bits of floating point numbers may be 
> considered to be "noise" .
> For that reason , the same "for" or "while" loops may behave differently 
> in different times and places .
> 
> To make floating point related loops more deterministic , the useful 
> steps may be to compute "step size" and "number of steps" , and use 
> integer variables for counting loop steps with multiplication of "loop 
> counter"  and "step size" during loop steps :  For floating point loop 
> counter T = "integer loop counter" * "step size" .

Indeed, exactly as I did in a patch for that PR.

> A statement  like  T = T + "step size" will/may produce wrong results if 
> number of steps is sufficiently large .
> 
> 
> Computer arithmetic and theoretical arithmetic are not the same .
> For example , addition is not associative in computer arithmetic : a + ( 
> b + c ) is not always equal to ( a + b ) + c .

Thanks to all replies, I now clearly see the problem.