Re: Switch rounding mode

[J.Benoist Leger]

Przemek Klosowski a écrit :
> In my opinion, the motivation for interval computing results from
> physical indeterminacy of inputs (e.g. temperature measured to within
> .01 degree). Such physical uncertainity intervals are significantly
> larger than numerical effects, so the effect of the rounding mode used
> in calculation should be negligible.

I'm agree, but in some case, there are problems with bounds. And
interval computing theory include these rounding mode. But in physics
I'm agree it should be negligible.
> 
> Having said that, C99 and later floating point library standards 
> provide fesetround() functionality, which changes the rounding mode
> (if supported by the FP hardware, of course). In Octave, you could
> write an .oct wrapper that calls fesetround() before your expression:
> 
> 
> fesetround(FE_DOWNWARD) ;  c.min = a.min + b.min
> fesetround(FE_UPWARD)   ;  c.max = a.max + b.max 
> fesetround(FE_DFL_ENV);
> 

Thanks, this is that I wanted.

Now I can that I want with this code :

setround(-1);
c.min=a+b;
setround(1);
c.max=a+b;

And I provide in enclosed file the source code of setround if others
wants use it. (It's my first octave C++ source code).

Thanks,

-- 
J.Benoist Leger

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#include <fenv.h>
#include <octave/oct.h> 

DEFUN_DLD (setround, args, , "-*- texinfo -*-\n\
@deftypefn{Loadable Function} {OLD_MODE =} setround(NEW_MODE)\n\
@cindex setround round rounding\n\
For NEW_MODE = -1 switch rounding towards -inf.\n\n\
For NEW_MODE =  1 switch rounding towards +inf.\n\n\
For NEW_MODE =  0 switch rounding to nearest.\n\
@end deftypefn")
{
    int last = fegetround() ;
    if(args(0).int_value()==1)
        fesetround(FE_UPWARD);
    else if(args(0).int_value()==-1)
        fesetround(FE_DOWNWARD);
    else if(args(0).int_value()==0)
        fesetround(FE_TONEAREST);

    if(last==1024)
        return octave_value(-1);
    else if(last==2048)
        return octave_value(1);
    else
        return octave_value(0);
}