Re: -infinity and REPRESENT

[Anton Ertl]

On Tue, May 05, 2020 at 01:03:31AM +0200, address@hidden wrote:
> On 2020-05-04 17:24, Anton Ertl wrote:
> > One issue is how to deal with -infinity (e.g., the result of -1e 0e f/).
> > Previously Gforth 0.7.2+glibc produced "-inf" and f1=0 (positive),
> > f2=0 (not valid); other libc implementations might lead to different
> > results.
> > 
> > gforth 0.7.9_20200423 uses an ecvt_r supplied by us and produces
> > "-inf" and f1=-1 (negative), f2=0 (not valid); the new REPRESENT
> > currently works in the same way (except that it gives you "-infinity",
> > space permitting).
> > 
> > In both systems F. and FS. outputs -in.
> > 
> > iForth behaves like gforth 0.7.2.
> > 
> > VFX produces "Inf" and f1=-1, f2=0.
> > 
> > The VFX behaviour looks like the most sensible one to me, but it might
> > affect code that uses REPRESENT; we can change our own code
> > accordingly, but what about your code?  Do you have code that uses
> > REPRESENT and that has been designed for a specific behaviour when
> > getting -infinity?
> 
> ( Bernd clarified f1 and f2. )
> 
> > VFX produces "Inf" and f1=-1, f2=0.
> 
> I think I will fix iForth (in due time) to give "-INF", f1=-1 f2=-1.
> It does not make sense to me to claim f2=0 when the item has a sign and
> gives valid output for many operations.

I think that putting the sign in f1, not in the string is most
appropriate, because that's the same as for finite numbers (-1e
produces "10..."  and f1=-1).

As for f2, with IEEE arithmetic there is no number that's strictly
invalid; IEEE 754 defines a meaning for every bit pattern.  So what
should we return as f2?

1) Always return -1?  That's not useful, and contrary to existing
practice.

2) Return -1 for finite and infinite numbers, and 0 for NaNs.  That's
arguable, but contrary to existing practice.  Moreover, what would be
the use of this behaviour?

3) Return -1 for finite numbers, and 0 for infinities and NaNs.
That's existing practice, and is useful for indicating that the that
the string result consists of digits and that the n (exponent) result
of REPRESENT is actually meaningful.

- anton