Re: [Caml-list] string_of_float (0.1 +. 0.2)

[Kenichi Asai <asai@is.ocha.ac.jp>]

Thank you all for the information.

> Nathanaëlle Courant, with help from Julien Lepiller and myself, wrote a
> rather complete interpreter/evaluator for OCaml parsetrees (in OCaml) as
> part of the Camlboot project

Yes, I read the programming paper.  That's an interesting project.

> Kenichi, I don't understand what your own constraints, but
> in general I have the impression that "float" is better than "string"
> to represent double values used for computation.

I am developing an OCaml stepper that executes an OCaml program step
by step.  From the OCaml parsetree for

let a = 0.1 +. 0.2 +. 0.4

I want an OCaml parsetree for

let a = 0.3 +. 0.4

or

let a = 0.300000000000000044 +. 0.4

if this is what OCaml uses internally.  I want to produce an OCaml
parsetree rather than my own parsetree that maintains float as is,
because I could then reuse pretty printer of OCaml.

From Andreas' e-mail, I understand what's happening.  (Thank you!)  I
thought I would use Printf.sprintf "%.18g" in place of string_of_float,
but Daniel's e-mail made me think it could be insufficient, because
even if I use .18 (or .24), it is still an approximation of the float.
(Am I correct?)  I tried to use `Format.sprintf "%h"` in place of
string_of_float, but the OCaml pretty printer produces

let a = 0x1.3333333333334p-2 +. 0.4

which is not suitable for a stepper used by novice programmers.  For
now, I think I will use Printf.sprintf "%.18g" and see if students see
any difference between OCaml execution and stepper execution.

I agree with Francois that it would be nice if the OCaml manual could
mention a potential loss of information.  I first thought that
replacing "%.12g" with "%.18g" solves the problem, but "the" string
representation of a float turned out to be more complicated than I
thought.

Thank you all for the discussion!

Sincerely,

-- 
Kenichi Asai