Re: [Caml-list] Float precision in OCaml

[Viet Le <vietlq85@gmail.com>]

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Thanks Nicolas. I did check against C++ before writing the email and I can
see OCaml is consistent with C++. I'm aware about IEEE 754. I'm seeking
opinion for maintaining precision using 2 values as shown.

On the other hand I think I should relax this requirement and let my JSON
parser accept this case because realistically I don't see anyone would use
numbers at +/- E308.

Thanks


On Tue, 1 Aug 2017 at 12:18, Nicolás Ojeda Bär <nicolas.ojeda.bar@lexifi.com>
wrote:

> Dear Viet,
>
> I am not sure this is an issue with OCaml (as you can verify using your
> favourite C compiler).  Rather, I think IEEE 754 double-precision binary
> floating-point numbers can only represent numbers between 10^{-308} and
> 10^308 with full decimal digits precision. Numbers smaller than that can
> only be represented with reduced precision.
>
> Best wishes,
> Nicolas
>
> On Tue, Aug 1, 2017 at 12:47 PM, Viet Le <vietlq85@gmail.com> wrote:
>
>> Hi all,
>>
>> I'm writing a JSON parser in pure OCaml and have encountered an issue
>> with small float values:
>>
>> 10 zeros in between
>> # 1.00000000001e-312 ;;
>> - : float = 1.00000000001e-312
>>
>> 11 zeros in between
>> # 1.000000000001e-312 ;;
>> - : float = 1.00000000000341e-312
>>
>> # 5e-324 ;;
>> - : float = 4.94065645841e-324
>>
>>
>> I haven't found precise limit, but as a rule of thumb (*not precise*),
>> for a positive float value to keep its precision, it *should* not be
>> smaller than 1.00000000001e-312. To use JSON as precise serializer, it
>> would be necessary to preserve accuracy.
>>
>> I checked https://github.com/ocaml/Zarith and it supports only big int &
>> quotients, not floating point.
>>
>> For values smaller than the limit above, should I just treat as 2 values:
>> (normalized: float, exponent: float), so we will have:
>>
>> 5e-324 -> (5, -324)
>>
>> Comments and suggestions are appreciated. Thanks.
>>
>> --
>> Kind regards,
>> Viet
>>
>
> --
Kind regards,
Viet

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