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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>
To: python-list@python.org
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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