Re: [Caml-list] Approximations when converting from string to float

[Xavier Leroy <Xavier.Leroy@inria.fr>]

On 10/25/2016 06:58 PM, Tung Vu Xuan wrote:

> Thanks for suggestions. Since I am using a library for interval arithmetic
> [1], your idea becomes easier for me.
> let intv_of_big_int bi = 
>     let n = Big_int.num_bits_big_int bi in
>     if n <= 53 then 
>       let bi_float = Big_int.float_of_big_int bi in
>       {low=bi_float;high=bi_float}
>     else begin
>       let n = n - 53 in
>       (* Extract top 53 bits of x *)
>       let top = Big_int.shift_right_big_int bi n in

Yes, that's how I would go about it, too.  But probably you don't need
the interval arithmetic in the end: both "top" and "top + 1" convert
to floats without rounding (*), and the scaling by 2^n can be achieved
with "ldexp".

(*) If "top" is 2^53 - 1, "top + 1" is 2^53, which converts exactly.
Otherwise, "top + 1" has at most 53 digits and converts exactly as well.

Jacques-Henri Jourdan adds:

> But really, you should use ZArith, which is much more modern, faster
> and well-maintained than Big_int.

Agreed.  And if you must use Big_int.float_of_big_int, make sure
you're using OCaml 4.03.0 or later, because older versions were flaky.

- Xavier Leroy