Re: Information Systems Annex (usefulness of Decimal Floats)

[dewar@gnat.com (Robert Dewar)]

"Robert C. Leif" <rleif@rleif.com> wrote in message news:<mailman.1029611105.24616.comp.lang.ada@ada.eu.org>...
> From: Robert C. Leif
> To: Robert Dewar et al.

> Decimal fixed point is sufficient only if you have a 
> common intermediate
> reference, such as the Dollar, Euro, or gold. However, 
> one can get into
> a combinatorial catastrophe employing multiple conversion 
> factors.

Sorry, this just seems like FUD. I have written a lot of
financial code of some complexity in COBOL, and never run
into any requirement like this. And as I pointed out before
clearly the COBOL community has not considered this a problem or this
facility would have been provided. Remember
that fixed-point can always be used for any calculations
if you are willing to worry carefully about scaling.

> Complex scientific and engineering calculations require a 
> floating type. I still believe that given the existence 
> of gigahertz processors, that
> there is a benefit in performing calculations in the most 
> human understandable format, decimal. 

This is wrong for two reasons.

1. The efficiency hit on modern processors between binary
floating-point (IEEE 754) and simulated decimal floating
point (IEEE 854) is simply huge. Probably a factor of 200
in practice at least, perhaps more with modern pipelined
machines. That means your gigahertz processor is crawling
along at the equivalent of 5 megahertz (like the original
PC-1), totally hopeless for "complex scientific and engineering
calculations".

2. There are no gains in practice over binary floating-point. Yes, it
may avoid surprises for naive users, but naive users of floating-point
should be banned
from doing "complex scientific and engineering calculations" since
they will make a complete mess of it.
Such applications [I have even more experience in that
sphere] require a proper understanding of how floating-point works,
and how errors propagate, and there is nothing inherently better about
decimal fpt.

The one area where decimal fpt has turned out to be an advantage is in
hand held calculators, where abysmal performance does not matter at
all, and you avoid surprises for naive users :-)

Note that once you start talking about "complex scientific
and engineering" applications, you have completely shifted
the sphere of argument away from the subject material. This has
nothing whatever to do with the Information Systems Annex.