Re: [Gambas-user] rounding a half

[Benoît Minisini <benoit.minisini@xxxxxxxxxxxxxxxx>]

Le 09/12/2024 à 21:27, Jussi Lahtinen a écrit :
> Hmmm... no this seems like a fundamental problem in floating points.
> You could do something like this:
>
> RETURN->_float.value = floor((PARAM->_float.value / power + 0.5) * t) / 
> t * power;
>
> Where t is double with a large value. This would hide the problem into 
> smaller decimal places.
> Not sure it's worth it. Probably implementing arbitrary precision 
> datatype would be the right way. Benoit?
>
>
> Jussi

The reason is that Round() rounds in binary, not in decimal.

1.015 and 1.055 do not have an exact representation in binary.

1.015 = 1.000000(11110101110000101000)*
1.055 = 1.0000(11100001010001111010)*

Moreover, Round() sometimes uses a multiplication (when its second 
argument is positive) or a division (when its second argument is 
negative) to round.

So, when the decimal digit where we want to round is "5", sometimes the 
number is rounding down, sometimes up.

Regards,

--
Benoît Minisini.