Re: [Caml-list] float pretty-printing precision, once more.

[Damien Doligez <damien.doligez@inria.fr>]

On Tuesday, Dec 10, 2002, at 00:04 Europe/Paris, 
jeanmarc.eber@lexifi.com wrote:

> caml 3.06+1:
[string_of_float loses precision on floating-point numbers]

In the current working version (3.06+18), the precision used by
string_of_float has been increased to 17 digits.  I *think* this
is enough to represent all the double-precision floating-point
numbers:

           Objective Caml version 3.06+18 (2002-11-07)

   # let f = 1. /. 86400.;;
   val f : float = 1.1574074074074073e-05
   # let s = string_of_float f;;
   val s : string = "1.1574074074074073e-05"
   # let f1 = float_of_string s;;
   val f1 : float = 1.1574074074074073e-05
   # f1 = f;;
   - : bool = true
   # f1 -. f;;
   - : float = 0.

However, it has the unfortunate side-effect of revealing this
awful truth about FP numbers: many "interesting" numbers are
impossible to represent in floating-point.  For example:

   # 0.1;;
   - : float = 0.10000000000000001

There is no floating-point number equal to 0.1 and the
best approximation you can get is 0.10000000000000001.
I expect this will quickly become a much-used FAQ entry...

I don't know if we will keep this change for the next release
of O'Caml.  But you can always use (Printf.sprintf "%.17g")
instead of string_of_float, and you'll get all the digits,
significant or not.

-- Damien

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