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Groups > comp.lang.python > #46570
| Newsgroups | comp.lang.python |
|---|---|
| Date | 2013-05-30 20:11 -0700 |
| References | (3 earlier) <51a4b5a1$0$29966$c3e8da3$5496439d@news.astraweb.com> <04b90c02-833a-4bad-88ad-ab71178b8f79@googlegroups.com> <48519aa0-d0cd-4ffc-a2f5-2107465321d8@qn4g2000pbc.googlegroups.com> <mailman.2365.1369847746.3114.python-list@python.org> <51a6b969$0$29966$c3e8da3$5496439d@news.astraweb.com> |
| Message-ID | <987fed2a-e7c7-4bcc-8aaf-9608e21eff26@googlegroups.com> (permalink) |
| Subject | Re: Short-circuit Logic |
| From | 88888 Dihedral <dihedral88888@gmail.com> |
Steven D'Aprano於 2013年5月30日星期四UTC+8上午10時28分57秒寫道： > On Wed, 29 May 2013 10:50:47 -0600, Ian Kelly wrote: > > > > > On Wed, May 29, 2013 at 8:33 AM, rusi <rustompmody@gmail.com> wrote: > > >> 0.0 == 0.0 implies 5.4 == 5.4 > > >> is not a true statement is what (I think) Steven is saying. 0 (or if > > >> you prefer 0.0) is special and is treated specially. > > > > > > It has nothing to do with 0 being special. A floating point number will > > > always equal itself (except for nan, which is even more special), and in > > > particular 5.4 == 5.4. But if you have two different calculations that > > > produce 0, or two different calculations that produce 5.4, you might > > > actually get two different numbers that approximate 0 or 5.4 thanks to > > > rounding error. If you then compare those two ever-so-slightly > > > different numbers, you will find them unequal. > > > > EXACTLY! > > > > The problem does not lie with the *equality operator*, it lies with the > > calculations. And that is an intractable problem -- in general, floating > > point is *hard*. So the problem occurs when we start with a perfectly > > good statement of the facts: > > > > "If you naively test the results of a calculation for equality without > > understanding what you are doing, you will often get surprising results" > > > > which then turns into a general heuristic that is often, but not always, > > reasonable: > > > > "In general, you should test for floating point *approximate* equality, > > in some appropriate sense, rather than exact equality" > > > > which then gets mangled to: > > > > "Never test floating point numbers for equality" > > > > and then implemented badly by people who have no clue what they are doing > > and have misunderstood the nature of the problem, leading to either: > > > > * de facto exact equality testing, only slower and with the *illusion* of > > avoiding equality, e.g. "abs(x-y) < sys.float_info.epsilon" is just a > > long and slow way of saying "x == y" when both numbers are sufficiently > > large; > > > > * incorrectly accepting non-equal numbers as "equal" just because they > > happen to be "close". > > > > > > The problem is that there is *no one right answer*, except "have everyone > > become an expert in floating point, then judge every case on its merits", > > which will never happen. > > > > But if nothing else, I wish that we can get past the rank superstition > > that you should "never" test floats for equality. That would be a step > > forward. > > > > > > > > -- > > Steven The string used to represent a floating number in a computer language is normally in the decimal base of very some limited digits. Anyway with the advances of A/D-converters in the past 10 years which are reflected in the anttena- transmitter parts in phones, the long integer part in Python can really beat the low cost 32- 64 bit floating computations in scientific calculations.
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Short-circuit Logic Ahmed Abdulshafy <abdulshafy@gmail.com> - 2013-05-26 04:11 -0700
Re: Short-circuit Logic Roy Smith <roy@panix.com> - 2013-05-26 07:38 -0400
Re: Short-circuit Logic Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-05-26 12:13 +0000
Re: Short-circuit Logic Ahmed Abdulshafy <abdulshafy@gmail.com> - 2013-05-27 13:11 -0700
Re: Short-circuit Logic Nobody <nobody@nowhere.com> - 2013-05-28 01:10 +0100
Re: Short-circuit Logic Ahmed Abdulshafy <abdulshafy@gmail.com> - 2013-05-28 01:39 -0700
RE: Short-circuit Logic Carlos Nepomuceno <carlosnepomuceno@outlook.com> - 2013-05-28 12:32 +0300
Re: Short-circuit Logic Mark Lawrence <breamoreboy@yahoo.co.uk> - 2013-05-28 12:45 +0100
Re: Short-circuit Logic Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-05-28 13:51 +0000
Re: Short-circuit Logic Grant Edwards <invalid@invalid.invalid> - 2013-05-28 15:14 +0000
Re: Short-circuit Logic Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-05-28 15:55 +0000
Re: Short-circuit Logic Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-05-28 13:48 +0000
Re: Short-circuit Logic Chris Angelico <rosuav@gmail.com> - 2013-05-29 00:01 +1000
Re: Short-circuit Logic Ahmed Abdulshafy <abdulshafy@gmail.com> - 2013-05-29 07:27 -0700
Re: Short-circuit Logic Chris Angelico <rosuav@gmail.com> - 2013-05-30 00:32 +1000
Re: Short-circuit Logic rusi <rustompmody@gmail.com> - 2013-05-29 07:33 -0700
Re: Short-circuit Logic Ian Kelly <ian.g.kelly@gmail.com> - 2013-05-29 10:50 -0600
Re: Short-circuit Logic Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-05-30 02:28 +0000
Re: Short-circuit Logic Chris Angelico <rosuav@gmail.com> - 2013-05-30 13:45 +1000
Re: Short-circuit Logic Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-05-30 05:42 +0000
Re: Short-circuit Logic Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2013-05-30 10:22 +0300
Re: Short-circuit Logic Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-05-30 08:29 +0000
Re: Short-circuit Logic Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2013-05-30 12:07 +0300
Re: Short-circuit Logic Nobody <nobody@nowhere.com> - 2013-05-30 23:55 +0100
Re: Short-circuit Logic Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2013-05-30 19:31 -0400
Re: Short-circuit Logic Stefan Drees <stefan@drees.name> - 2013-05-31 17:34 +0200
Re: Short-circuit Logic Roy Smith <roy@panix.com> - 2013-05-30 08:48 -0400
Re: Short-circuit Logic Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2013-05-30 19:38 -0400
Re: Short-circuit Logic Nobody <nobody@nowhere.com> - 2013-05-31 02:10 +0100
Re: Short-circuit Logic Roy Smith <roy@panix.com> - 2013-05-30 21:21 -0400
Re: Short-circuit Logic Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2013-05-30 21:57 -0400
Re: Short-circuit Logic Michael Torrie <torriem@gmail.com> - 2013-05-30 21:33 -0600
RE: Short-circuit Logic Carlos Nepomuceno <carlosnepomuceno@outlook.com> - 2013-05-31 03:03 +0300
Re: Short-circuit Logic Chris Angelico <rosuav@gmail.com> - 2013-05-30 18:29 +1000
RE: Short-circuit Logic Carlos Nepomuceno <carlosnepomuceno@outlook.com> - 2013-05-31 00:03 +0300
Re: Short-circuit Logic Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-05-31 05:13 +0000
RE: Short-circuit Logic Carlos Nepomuceno <carlosnepomuceno@outlook.com> - 2013-05-31 09:42 +0300
Re: Short-circuit Logic Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-05-31 08:11 +0000
Re: Short-circuit Logic Chris Angelico <rosuav@gmail.com> - 2013-05-31 17:09 +1000
Re: Short-circuit Logic Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-05-31 08:45 +0000
Re: Short-circuit Logic Roy Smith <roy@panix.com> - 2013-05-31 09:20 -0400
RE: Short-circuit Logic Carlos Nepomuceno <carlosnepomuceno@outlook.com> - 2013-06-01 10:23 +0300
Re: Short-circuit Logic 88888 Dihedral <dihedral88888@gmail.com> - 2013-05-30 20:11 -0700
Re: Short-circuit Logic Dave Angel <davea@davea.name> - 2013-05-29 20:23 -0400
Re: Short-circuit Logic Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-05-30 05:20 +0000
Re: Short-circuit Logic Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-05-30 05:10 +0000
Re: Short-circuit Logic Chris Angelico <rosuav@gmail.com> - 2013-05-30 15:22 +1000
Re: Short-circuit Logic Roy Smith <roy@panix.com> - 2013-05-30 08:40 -0400
Re: Short-circuit Logic Chris Angelico <rosuav@gmail.com> - 2013-05-30 22:58 +1000
Re: Short-circuit Logic rusi <rustompmody@gmail.com> - 2013-05-30 09:58 -0700
Re: Short-circuit Logic Chris Angelico <rosuav@gmail.com> - 2013-05-31 03:23 +1000
Re: Short-circuit Logic Rick Johnson <rantingrickjohnson@gmail.com> - 2013-05-30 17:13 -0700
Re: Short-circuit Logic Chris Angelico <rosuav@gmail.com> - 2013-05-31 12:29 +1000
Re: Short-circuit Logic Ethan Furman <ethan@stoneleaf.us> - 2013-05-30 08:02 -0700
Re: Short-circuit Logic Chris Angelico <rosuav@gmail.com> - 2013-05-31 01:56 +1000
Re: Short-circuit Logic Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-05-30 16:40 +0000
Re: Short-circuit Logic Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-05-30 19:22 +0000
Re: Short-circuit Logic Chris Angelico <rosuav@gmail.com> - 2013-05-31 07:46 +1000
Re: Short-circuit Logic Ethan Furman <ethan@stoneleaf.us> - 2013-05-30 09:30 -0700
Re: Short-circuit Logic Neil Cerutti <neilc@norwich.edu> - 2013-05-30 19:30 +0000
Re: Short-circuit Logic Ian Kelly <ian.g.kelly@gmail.com> - 2013-05-30 13:49 -0600
Re: Short-circuit Logic Terry Jan Reedy <tjreedy@udel.edu> - 2013-05-26 16:19 -0400
Re: Short-circuit Logic Roy Smith <roy@panix.com> - 2013-05-26 16:22 -0400
Re: Short-circuit Logic Terry Jan Reedy <tjreedy@udel.edu> - 2013-05-26 17:28 -0400
Re: Short-circuit Logic Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-05-27 00:40 +0000
Re: Short-circuit Logic Cameron Simpson <cs@zip.com.au> - 2013-05-27 11:57 +1000
Re: Short-circuit Logic rusi <rustompmody@gmail.com> - 2013-05-26 21:44 -0700
Re: Short-circuit Logic Vito De Tullio <vito.detullio@gmail.com> - 2013-05-27 06:59 +0200
Re: Short-circuit Logic Nobody <nobody@nowhere.com> - 2013-05-27 18:52 +0100
Re: Short-circuit Logic Ahmed Abdulshafy <abdulshafy@gmail.com> - 2013-05-27 13:08 -0700
Re: Short-circuit Logic Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2013-05-27 21:36 -0400
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