RE: [Caml-list] Strange behaviour of string_of_float

["David Allsopp" <dra-news@metastack.com>]

> > Richard gave you the reason.

Erm, please correct me if I'm wrong but every single possible floating point
value (on the same architecture) has a string representation that will be
reparsed to the same floating point value (on the same architecture). It's
the reverse that isn't true because floating point numbers are only an
approximation. 

> > If you can serialize to binary, you can acheive what you want by
> > serializing the 64 bits integers you get with Int64.bits_of_float and
> > applying Int64.float_of_bits to the integers you deserialize.
>
> Just posted a useless message :-)
>
> This is *exactly* what I was searching for, thanks Daniel.

This is of course a better, more reliable and faster way of serialising, but
the real cause for your original spurious result is down to how
string_of_float is defined in pervasives.ml:

# pi;;
- : float = 3.1415926535897931

# string_of_float pi;;
- : string = "3.14159265359"

In other words, (pi |> string_of_float |> float_of_string) is never going to
be equal to your original pi. For some reason, string_of_float is defined
as:

let string_of_float f = valid_float_lexem (format_float "%.12g" f);;

Perhaps Xavier can say why it's only "%.12g" in the format (I imagine
there's a historical reason) but if you increase it to 16 then you'll get
the answer you expected (0.). All that said, the values given by
string_of_float cannot always be fed back to float_of_string anyway (e.g.
float_of_string (string_of_float nan))


David