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| Date | Fri, 9 Nov 2012 17:59:36 +1100 |
| Subject | Re: Multi-dimensional list initialization |
| From | Chris Angelico <rosuav@gmail.com> |
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On Fri, Nov 9, 2012 at 5:37 PM, Steven D'Aprano
<steve+comp.lang.python@pearwood.info> wrote:
> On Fri, 09 Nov 2012 17:07:09 +1100, Chris Angelico wrote:
>> Can you enlighten us as to how this is better than either:
>> x := -x
>> or
>> x := 0 - x
>> ? I'm not seeing it.
>
> I'm hoping that Mark intended it as an example of crappy code he has
> spotted in some other language rather than a counter-example of something
> you would do.
Ohh. Yeah, that figures. Huh.
> To be pedantic... there may very well be some (rare) cases where you
> actually do want x -= x rather than just x = 0. Consider the case where x
> could be an INF or NAN. Then x -= x should give x = NAN rather than zero.
> That may be desirable in some cases.
>
> At the very least, the compiler should NOT optimize away x = x - x to
> x = 0 if x could be a float, complex or Decimal.
Yep. In the specific case of integers, though, and in the specific
instance of CPU registers in assembly language, it's reasonable to
optimize it the *other* way - MOV reg,0 is a one-byte opcode and 1, 2,
or 4 bytes of immediate data, while SUB reg,reg (or XOR reg,reg) is a
two-byte operation regardless of data size. But that's
microoptimization that makes, uhh, itself-subtracted-from-itself sense
in Python.
>> And I'm not seeing any nonnumeric that would
>> benefit from being subtracted from itself twice (strings, arrays, sets,
>> you can subtract them from one another but not usefully more than once).
>
> How do you subtract strings?
The same way you subtract sets. Same with arrays. Python doesn't do
either, but Python also doesn't do the ":=" operator that the example
code demonstrated, so I didn't assume Python.
Pike v7.8 release 700 running Hilfe v3.5 (Incremental Pike Frontend)
> "Hello, world!"-"l";
(1) Result: "Heo, word!"
> ({1,2,3,3,2,3,1,2,1})-({2});
(2) Result: ({ /* 6 elements */
1,
3,
3,
3,
1,
1
})
Python spells it differently:
>>> "Hello, world!".replace("l","")
'Heo, word!'
Not sure how to do array subtraction other than with filter:
>>> list(filter(lambda x: x!=2,[1,2,3,3,2,3,1,2,1]))
[1, 3, 3, 3, 1, 1]
But there's probably a way (list.remove only takes out the first
occurrence, so it's not equivalent).
In any case, subtracting something from _itself_ is only going to give
you an empty string, array, set, or whatever, and doing so a second
time is going to achieve nothing. Hence my comment.
But poor code we will always have with us, to paraphrase the Gospel of Matthew.
ChrisA
Back to comp.lang.python | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread
Multi-dimensional list initialization Demian Brecht <demianbrecht@gmail.com> - 2012-11-04 22:27 -0800
Re: Multi-dimensional list initialization Hans Mulder <hansmu@xs4all.nl> - 2012-11-05 10:13 +0100
Re: Multi-dimensional list initialization Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2012-11-06 01:32 +0000
Re: Multi-dimensional list initialization Chris Angelico <rosuav@gmail.com> - 2012-11-06 13:01 +1100
Re: Multi-dimensional list initialization Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2012-11-06 02:30 +0000
Re: Multi-dimensional list initialization Andrew Robinson <andrew3@r3dsolutions.com> - 2012-11-05 21:51 -0800
Re: Multi-dimensional list initialization Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-11-06 09:04 +0000
RE: Multi-dimensional list initialization "Shambhu Rajak" <shambhu.rajak@calsoftinc.com> - 2012-11-06 18:57 +0530
Re: Multi-dimensional list initialization Andrew Robinson <andrew3@r3dsolutions.com> - 2012-11-06 14:41 -0800
Re: Multi-dimensional list initialization Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-11-07 01:55 +0000
Re: Multi-dimensional list initialization Demian Brecht <demianbrecht@gmail.com> - 2012-11-06 22:56 -0800
Re: Multi-dimensional list initialization wxjmfauth@gmail.com - 2012-11-07 00:57 -0800
Re: Multi-dimensional list initialization Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-11-07 22:27 +0000
Re: Multi-dimensional list initialization Ian Kelly <ian.g.kelly@gmail.com> - 2012-11-07 16:39 -0700
Re: Multi-dimensional list initialization Ian Kelly <ian.g.kelly@gmail.com> - 2012-11-08 00:09 -0700
Re: Multi-dimensional list initialization Ian Kelly <ian.g.kelly@gmail.com> - 2012-11-08 08:58 -0700
Re: Multi-dimensional list initialization Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-11-09 01:39 +0000
Re: Multi-dimensional list initialization Chris Angelico <rosuav@gmail.com> - 2012-11-09 17:07 +1100
Re: Multi-dimensional list initialization Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-11-09 06:37 +0000
Re: Multi-dimensional list initialization Chris Angelico <rosuav@gmail.com> - 2012-11-09 17:59 +1100
Re: Multi-dimensional list initialization Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-11-09 07:27 +0000
Re: Multi-dimensional list initialization rusi <rustompmody@gmail.com> - 2012-11-09 07:05 -0800
Re: Multi-dimensional list initialization Chris Angelico <rosuav@gmail.com> - 2012-11-10 02:23 +1100
Re: Multi-dimensional list initialization Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2012-11-09 14:34 -0500
RE: Multi-dimensional list initialization "Prasad, Ramit" <ramit.prasad@jpmorgan.com> - 2012-11-09 20:31 +0000
Re: Multi-dimensional list initialization Ethan Furman <ethan@stoneleaf.us> - 2012-11-09 13:49 -0800
RE: Multi-dimensional list initialization "Prasad, Ramit" <ramit.prasad@jpmorgan.com> - 2012-11-06 23:39 +0000
Re: Multi-dimensional list initialization Ian Kelly <ian.g.kelly@gmail.com> - 2012-11-06 16:52 -0700
Re: Multi-dimensional list initialization MRAB <python@mrabarnett.plus.com> - 2012-11-07 00:23 +0000
Re: Multi-dimensional list initialization rusi <rustompmody@gmail.com> - 2012-11-06 20:11 -0800
Re: Multi-dimensional list initialization Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-11-07 05:05 +0000
Re: Multi-dimensional list initialization Roy Smith <roy@panix.com> - 2012-11-07 00:12 -0500
Re: Multi-dimensional list initialization Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2012-11-07 18:32 +1300
RE: Multi-dimensional list initialization "Prasad, Ramit" <ramit.prasad@jpmorgan.com> - 2012-11-07 15:57 +0000
Re: Multi-dimensional list initialization Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2012-11-07 10:52 +0200
Re: Multi-dimensional list initialization MRAB <python@mrabarnett.plus.com> - 2012-11-07 17:17 +0000
Re: Multi-dimensional list initialization Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-11-07 22:36 +0000
Re: Multi-dimensional list initialization Ethan Furman <ethan@stoneleaf.us> - 2012-11-07 07:23 -0800
Re: Multi-dimensional list initialization Ian Kelly <ian.g.kelly@gmail.com> - 2012-11-07 14:01 -0700
Re: Multi-dimensional list initialization Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-11-08 00:00 +0000
Re: Multi-dimensional list initialization Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2012-11-08 00:30 +0000
Re: Multi-dimensional list initialization Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-11-08 03:47 +0000
Re: Multi-dimensional list initialization Andrew Robinson <andrew3@r3dsolutions.com> - 2012-11-07 20:51 -0800
Re: Multi-dimensional list initialization wrw@mac.com - 2012-11-08 08:26 -0500
Re: Multi-dimensional list initialization Andrew Robinson <andrew3@r3dsolutions.com> - 2012-11-07 16:24 -0800
Re: Multi-dimensional list initialization Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-11-08 04:20 +0000
Re: Multi-dimensional list initialization Chris Angelico <rosuav@gmail.com> - 2012-11-06 17:07 +1100
Re: Multi-dimensional list initialization Andrew Robinson <andrew3@r3dsolutions.com> - 2012-11-06 00:21 -0800
Re: Multi-dimensional list initialization Ian Kelly <ian.g.kelly@gmail.com> - 2012-11-06 02:19 -0700
RE: Multi-dimensional list initialization "Prasad, Ramit" <ramit.prasad@jpmorgan.com> - 2012-11-06 17:32 +0000
Re: Multi-dimensional list initialization Andrew Robinson <andrew3@r3dsolutions.com> - 2012-11-06 13:14 -0800
Re: Multi-dimensional list initialization Andrew Robinson <andrew3@r3dsolutions.com> - 2012-11-06 13:19 -0800
Re: Multi-dimensional list initialization Ian Kelly <ian.g.kelly@gmail.com> - 2012-11-06 15:46 -0700
Re: Multi-dimensional list initialization Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2012-11-07 18:34 +1300
Re: Multi-dimensional list initialization Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2012-11-07 14:00 +0000
Re: Multi-dimensional list initialization Ethan Furman <ethan@stoneleaf.us> - 2012-11-07 06:47 -0800
Re: Multi-dimensional list initialization Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2012-11-07 23:06 +0000
Re: Multi-dimensional list initialization Greg Ewing <greg.ewing@canterbury.ac.nz> - 2012-11-08 14:29 +1300
Re: Multi-dimensional list initialization wxjmfauth@gmail.com - 2012-11-05 01:55 -0800
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