Re: better float2str() C code, anyone?

[Asko Kauppi <asko.kauppi@...>]

Hmm.. could the behaviour be checked at runtime, and altered?


1.12.2004 kello 23:25, David Jones kirjoitti:


> On Dec 01, 2004, at 14:54, Luiz Henrique de Figueiredo wrote:
>
> > > One issue I've had from time to time is that the default
> > >    #define LUA_NUMBER_FMT        "%.14g"
> > > is not long enough to do reversible conversions on doubles. I believe
> > > you need %.19g for the conversion to be reversible.
> >
> > I think it's %.17g for IEEE doubles. (Despite what DBL_DIG says!)
> > Lua was like that once, but then you get ugly things like this:
> >
> > > =string.format("%.17g",0.34)
> > 0.34000000000000002
> >
> > > =0.34
> > 0.34
> >
> > > =34/100
> > 0.34
> >
> > Users complaint that this is an error in Lua! And it does not help to 
> > point
> > them to docs like http://lua-users.org/wiki/FloatingPoint ...
>
> Can you imagine a document that it would be helpful to point your 
> users at?  If you can, then perhaps we can write it.
>
> As I see it Lua used to do The Right Thing.  It used to use %.17g 
> which would give reliable external and internal conversion.  As long 
> as the C library was "good enough".  glibc and FreeBSD libc were good 
> enough, but many others weren't.  It's a shame that Lua now masks poor 
> quality C implementations at the expense of reliability.  It's not 
> particularly a shame that you changed it, it's a shame that the state 
> of C libraries meant you felt compelled to change it.
>
> David Jones