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Groups > comp.lang.forth > #44081
| From | anton@mips.complang.tuwien.ac.at (Anton Ertl) |
|---|---|
| Newsgroups | comp.lang.forth |
| Subject | Re: D>F and S>F first draft |
| Date | 2015-12-05 11:25 +0000 |
| Organization | Institut fuer Computersprachen, Technische Universitaet Wien |
| Message-ID | <2015Dec5.122557@mips.complang.tuwien.ac.at> (permalink) |
| References | (6 earlier) <n3eg5m$5sq$1@speranza.aioe.org> <n3fk5e$92g$1@dont-email.me> <n3u1dg$5gf$1@speranza.aioe.org> <2d978ba0-d332-447c-b484-31563cd72e4d@googlegroups.com> <55f0b46f-36be-4587-8014-bc5cede778ec@googlegroups.com> |
mhx@iae.nl writes:
>On Saturday, December 5, 2015 at 8:58:33 AM UTC+1, Julian Fondren wrote:
>> On Saturday, December 5, 2015 at 12:49:25 AM UTC-6, HAA wrote:
>> > I gave you a real example:
>> >
>> > 2147483647. d>f f. 2147484000. ok
>> >
>> > In your view is the above 'rounding'?
If the FP number that is printed as "2147484000." is the nearest FP
number, then this is rounding and would be the correct result
according to the proposal.
>> > Or is it an 'ambiguous condition' which
>> > the Standard supports with the system throw code:
>> >
>> > -41 "loss of precision"
In the Forth-2012 it is an ambiguous condition, which means that the
system can do anything. It can throw -41 (but no system does AFAIK),
it can return 2147484000e, it can return something that is not the
nearest FP number, or it can do something else. OTOH, if the proposal
is accepted, the programmer can check whether the conversion was exact
or did round, and throw if it was not exact.
>To one looking at bits and bytes "2147483647. d>f f. 2147484000."
>is, given a 64bit system with 80bit floats, the wrong answer,
It's the wrong answer for both the current and the proposed F>D,
because 2147483647 can be represented exactly in 80-bit floats (which
have a 64-bit mantissa).
> as is
>
>1152921504606846976 1 + s>f f. 1152921504606850000. ok
>compared to
>1152921504606846976 1 + s>f f. 1.152921504606846977e18 ok
Again, 1152921504606846977 can be represented exactly in 80-bit FP
numbers, so on such a system the correct result is again the same
without and with this proposal.
OTOH, if you have a system with IEEE 64-bit floats (53-bit mantissa),
1152921504606846977 cannot be represented exactly as floating-point
number. With the current S>F, the programmer cannot rely on anything
in that case; a program that does that is not a standard program, the
system can do anything, and the programmer has no portable way to
prevent that from happening. With the proposed change, the programmer
is guaranteed to get the closest FP number, so for a system above the
following
1152921504606846977 s>f f>s .
outputs 1152921504606846976 (using "F>S ." avoids the problem of f. and
other FP output words being somewhat approximate).
>"Rounding" these results only results in added confusion, especially
>as it still needs to be convincingly explained where a 'rounded'
>result helps (especially when relied on unthinkingly).
If it does not help, at least you can determine that the result is not
exact, which you cannot do with an ambiguous result.
I remember one case of using D>F: to get the square root of an integer
with D>F FSQRT F>D. IIRC I did make an analysis to determine that any
roundings introduced in this process do not cause problems for my
application.
- anton
--
M. Anton Ertl http://www.complang.tuwien.ac.at/anton/home.html
comp.lang.forth FAQs: http://www.complang.tuwien.ac.at/forth/faq/toc.html
New standard: http://www.forth200x.org/forth200x.html
EuroForth 2015: http://www.rigwit.co.uk/EuroForth2015/
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RfD: D>F and S>F first draft C G Montgomery <cgm@physics.utoledo.edu> - 2015-11-21 17:59 -0500
Re: D>F and S>F first draft "HAA" <someone@microsoft.com> - 2015-11-22 18:25 +1100
Re: D>F and S>F first draft krishna.myneni@ccreweb.org - 2015-11-22 06:20 -0800
Re: D>F and S>F first draft "HAA" <someone@microsoft.com> - 2015-11-23 10:58 +1100
Re: D>F and S>F first draft Bernd Paysan <bernd.paysan@gmx.de> - 2015-11-23 02:57 +0100
Re: D>F and S>F first draft "HAA" <someone@microsoft.com> - 2015-11-24 13:16 +1100
Re: D>F and S>F first draft Paul Rubin <no.email@nospam.invalid> - 2015-11-23 18:49 -0800
Re: D>F and S>F first draft "HAA" <someone@microsoft.com> - 2015-11-25 11:51 +1100
Re: D>F and S>F first draft Paul Rubin <no.email@nospam.invalid> - 2015-11-24 17:26 -0800
Re: D>F and S>F first draft Bernd Paysan <bernd.paysan@gmx.de> - 2015-11-25 02:43 +0100
Re: D>F and S>F first draft "HAA" <someone@microsoft.com> - 2015-11-26 12:16 +1100
Re: D>F and S>F first draft Paul Rubin <no.email@nospam.invalid> - 2015-11-25 17:47 -0800
Re: D>F and S>F first draft "HAA" <someone@microsoft.com> - 2015-11-26 15:47 +1100
Re: D>F and S>F first draft Paul Rubin <no.email@nospam.invalid> - 2015-11-25 23:07 -0800
Re: D>F and S>F first draft anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2015-11-26 07:27 +0000
Re: D>F and S>F first draft anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2015-11-26 09:19 +0000
Re: D>F and S>F first draft "HAA" <someone@microsoft.com> - 2015-11-28 04:26 +1100
Re: D>F and S>F first draft anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2015-11-28 11:31 +0000
Re: D>F and S>F first draft "HAA" <someone@microsoft.com> - 2015-11-29 20:23 +1100
Re: D>F and S>F first draft Bernd Paysan <bernd.paysan@gmx.de> - 2015-11-29 20:39 +0100
Re: D>F and S>F first draft "HAA" <someone@microsoft.com> - 2015-12-05 17:48 +1100
Re: D>F and S>F first draft Julian Fondren <julian.fondren@gmail.com> - 2015-12-04 23:58 -0800
Re: D>F and S>F first draft mhx@iae.nl - 2015-12-05 02:19 -0800
Re: D>F and S>F first draft anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2015-12-05 11:25 +0000
Re: D>F and S>F first draft Albert van der Horst <albert@spenarnc.xs4all.nl> - 2015-12-05 13:11 +0000
Re: D>F and S>F first draft mhx@iae.nl - 2015-12-05 05:27 -0800
Re: D>F and S>F first draft anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2015-12-05 15:33 +0000
Re: D>F and S>F first draft mhx@iae.nl - 2015-12-07 10:10 -0800
Re: D>F and S>F first draft anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2015-12-09 16:58 +0000
Re: D>F and S>F first draft krishna.myneni@ccreweb.org - 2015-12-06 10:23 -0800
Re: D>F and S>F first draft Bernd Paysan <bernd.paysan@gmx.de> - 2015-12-06 21:37 +0100
Re: D>F and S>F first draft Bernd Paysan <bernd.paysan@gmx.de> - 2015-12-05 19:08 +0100
Re: D>F and S>F first draft anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2015-11-30 08:33 +0000
Re: D>F and S>F first draft "Elizabeth D. Rather" <erather@forth.com> - 2015-11-25 21:43 -1000
Re: D>F and S>F first draft "HAA" <someone@microsoft.com> - 2015-11-28 19:28 +1100
Re: D>F and S>F first draft Albert van der Horst <albert@spenarnc.xs4all.nl> - 2015-11-26 11:37 +0000
Re: D>F and S>F first draft anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2015-11-22 15:10 +0000
Re: D>F and S>F first draft Bernd Paysan <bernd.paysan@gmx.de> - 2015-11-22 20:48 +0100
Re: D>F and S>F first draft "Elizabeth D. Rather" <erather@forth.com> - 2015-11-22 13:42 -1000
Re: D>F and S>F first draft anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2015-11-23 11:50 +0000
Re: D>F and S>F first draft "Elizabeth D. Rather" <erather@forth.com> - 2015-11-23 12:46 -1000
Forth libraries Lars Brinkhoff <lars.spam@nocrew.org> - 2015-11-24 08:55 +0100
Re: RfD: D>F and S>F first draft krishna.myneni@ccreweb.org - 2015-11-22 06:33 -0800
Re: RfD: D>F and S>F first draft anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2015-11-22 16:00 +0000
Re: RfD: D>F and S>F first draft krishna.myneni@ccreweb.org - 2015-11-22 08:53 -0800
Re: RfD: D>F and S>F first draft krishna.myneni@ccreweb.org - 2015-11-22 09:11 -0800
Re: RfD: D>F and S>F first draft anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2015-11-22 18:24 +0000
Re: RfD: D>F and S>F first draft Paul Rubin <no.email@nospam.invalid> - 2015-11-22 09:07 -0800
Re: RfD: D>F and S>F first draft stephen@mpeforth.com (Stephen Pelc) - 2015-11-22 18:10 +0000
Re: RfD: D>F and S>F first draft anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2015-11-22 18:26 +0000
Re: RfD: D>F and S>F first draft rickman <gnuarm@gmail.com> - 2015-11-22 13:58 -0500
Re: RfD: D>F and S>F first draft anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2015-11-23 12:34 +0000
Re: RfD: D>F and S>F first draft Albert van der Horst <albert@spenarnc.xs4all.nl> - 2015-11-23 16:39 +0000
Re: RfD: D>F and S>F first draft Bernd Paysan <bernd.paysan@gmx.de> - 2015-11-24 03:00 +0100
Re: RfD: D>F and S>F first draft Lars Brinkhoff <lars.spam@nocrew.org> - 2015-11-23 13:48 +0100
Re: RfD: D>F and S>F first draft mhx@iae.nl - 2015-11-23 05:19 -0800
Re: RfD: D>F and S>F first draft mhx@iae.nl - 2015-11-23 16:09 -0800
Re: RfD: D>F and S>F first draft Bernd Paysan <bernd.paysan@gmx.de> - 2015-11-22 20:42 +0100
Re: RfD: D>F and S>F first draft C G Montgomery <cgm@physics.utoledo.edu> - 2015-11-22 16:43 -0500
Re: RfD: D>F and S>F first draft krishna.myneni@ccreweb.org - 2015-11-22 18:40 -0800
Re: RfD: D>F and S>F first draft krishna.myneni@ccreweb.org - 2015-11-22 18:51 -0800
Re: RfD: D>F and S>F first draft C G Montgomery <cgm@physics.utoledo.edu> - 2015-11-25 15:47 -0500
Re: RfD: D>F and S>F first draft anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2015-11-22 14:29 +0000
Re: RfD: D>F and S>F first draft "David N. Williams" <williams@umich.edu> - 2015-11-22 10:41 -0600
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