Dear Quan Zhu
On 18 Nov 98 at 15:06, you wrote:
>
> I was not able to post questions to the root discussion list, and I
> hope you can shed some light on the following problem:
>
> If I run the follow fairly simple test code using root:
>
> {
> Float_t x = 888.45;
> Float_t y = 888.46;
> printf("%f\n", y - x);
>
> Float_t r = x - (Int_t)x;
> printf("%f\n", r);
> }
>
> on Windows NT machine, I get the followng output:
> 0.010010
> 0.450012
>
> which is clear wrong.
I didn't understand what is wrong with the result you present.
As for me it is perfect one. You will get "exactly" the same with any
32-bits machine. Doesn't matter whether it is UNIX/Windows/VMS/MS
DOS.
> This got me ***REALLY*** worried since many
> root classes use Float_t types a lot, and I only have access to
> Windows NT machines. (However, if one changes the above code to
> Double_t, then everything appear to be ok!)
SURE, because you will double the precision of your calculations.
That why the "double" was introduced.
>
Below you will find the result I got with SunSparc machine.
root [0] Float_t x = 888.45;
root [1] Float_t y = 888.46;
root [2] printf("%f\n", y - x);
0.010010
The precision of the IEEE floating point representation is about
1.0e-7 . Apparently the result you got does fit that.
> Any ideas ?
Please, find and have a read some textbook on "Numerical
calculations with computers"
Hope this helps,
With my best regards,
Valery
=================================================================
Dr. Valeri Faine (Fine)
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Dr. Valery Fine Telex : 911621 dubna su
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