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

[Tung Vu Xuan <toilatung90@gmail.com>]

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Hi,

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

      let top_float = Big_int.float_of_big_int top in
      (* interval [2, 2] *)
      let two_I = {low=2.;high=2.} in
      (* interval: [2, 2]**n *)
      let two_I_to_n = pow_I_i two_I n in

      (* compute upper bound of mantissa, represented as an intv *)
      (* interval arithmetic is needed because rounding error might occur *)
      let mantissa_upper_bound_intv = {low=top_float;high=top_float} +$
one_I in

      (* intv of mantissa *)
      let mantissa_intv = {low=top_float; high=mantissa_upper_bound_intv.high}
in

      (* compute final interval *)
      mantissa_intv *$ two_I_to_n
    end

[1] Alliot, J.M., Gotteland, J.B., Vanaret, C., Durand, N., Gianazza, D.:
Implementing an interval computation library for OCaml on x86/amd64
architectures. In: ICFP. ACM (2012)

Thanks again,
Tung.

On Tue, Oct 25, 2016 at 5:28 PM, Soegtrop, Michael <
michael.soegtrop@intel.com> wrote:

> Dear Jacques-Henri,
>
>
>
> if the number is cut off after n digits, the upper and lower bounds I
> suggested are the (truncated integer)*10^(#cut-off-digits) and (truncated
> integer+1)*10^(#cut-off-digits). The true number is obviously between
> these two. Since the integer has higher precision than the mantissa in the
> case of cut off, this is only a fraction of a mantissa bit. The errors you
> get by multiplying with the powers of 10 is likely larger in most cases.
>
>
>
> In the case you mentioned, 2^70=1180591620717411303424 and 32 bit one
> would truncate after
>
>
>
> 1180591620 * 10^12
>
>
>
> Maxima gives
>
>
>
> is(1180591620 * 10^12 <= 2^70);
>
> true
>
>
>
> is(1180591621 * 10^12 >= 2^70);
>
> true
>
>
>
> As I said, this method doesn’t give the tightest bounds possible, but it
> gives you true upper and lower bounds without multi precision arithmetic.
> It gives 0 intervals e.g. for integers which fit in the mantissa, but not
> for large exact powers of 2.
>
>
>
> > Moreover, it is not possible to implement interval arithmetic with
> OCaml, since you cannot change the rounding mode without a bit of C...
>
>
>
> It should be possible to make the powers of 10 tables such that this works
> even with round to nearest, but it would likely be easier to make a C
> library for this.
>
>
>
> Of cause it is a good question if I/O is the right place to save
> performance and waste precision. On the other hand you lose only 2 or 3
> bits and you know what the precision is. I would think most applications
> are such that the required precision is not exactly an IEEE precision, so
> that these 2 or 3 bits are ok. I used this method once in an embedded
> platform, where multi precision arithmetic was not an option, and this was
> a good compromise.
>
>
>
> Best regards,
>
>
>
> Michael
>
> Intel Deutschland GmbH
> Registered Address: Am Campeon 10-12, 85579 Neubiberg, Germany
> Tel: +49 89 99 8853-0, www.intel.de
> Managing Directors: Christin Eisenschmid, Christian Lamprechter
> Chairperson of the Supervisory Board: Nicole Lau
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> Commercial Register: Amtsgericht Muenchen HRB 186928
>



-- 
Tung Vu Xuan,
Japan Advanced Institute of Science and Technology,
Mobile: (+81) 080 4259 9135 -  (+84) 01689934381
Hometown: Bac Ninh
Email: toilatung90@gmail.com

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