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Re: Drowning in a teacup?

Path csiph.com!fu-berlin.de!uni-berlin.de!not-for-mail
From Ian Kelly <ian.g.kelly@gmail.com>
Newsgroups comp.lang.python
Subject Re: Drowning in a teacup?
Date Fri, 1 Apr 2016 14:52:10 -0600
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On Fri, Apr 1, 2016 at 2:39 PM, Rob Gaddi
<rgaddi@highlandtechnology.invalid> wrote:
> Fillmore wrote:
>> Nope, wrong! contrary to what I thought I had understood about how
>> parameters are passed in Python, the function is acting on a copy(!) and
>> my original list is unchanged.
>>
>
> Nope, that's not your problem.  Your problem is the line:
>
>>              mylist = [mylist[i]] + mylist[:i] + mylist[i+1:]
>
> Which should be read as: "Take mylist[i], all of mylist before i,
> and all of mylist after i and create a new list from it. Store that
> new list in a variable called mylist, overwriting the previous value."
>
> If instead you were using the .insert and .remove methods, you'd be
> manipulating the existing list instead.  But you don't want to do that,
> because those methods are O(N) on the list length; manipulating the
> middle of lists is slow.

Or use slice assignment. This should work in place of the above:

    mylist[:] = [mylist[i]] + mylist[:i] + mylist[i+1:]

Still O(n), but so will be any implementation that removes something
from an arbitrary list position and inserts it at the front.

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Thread

Drowning in a teacup? Fillmore <fillmore_remove@hotmail.com> - 2016-04-01 16:27 -0400
  Re: Drowning in a teacup? Rob Gaddi <rgaddi@highlandtechnology.invalid> - 2016-04-01 20:39 +0000
    Re: Drowning in a teacup? Ian Kelly <ian.g.kelly@gmail.com> - 2016-04-01 14:52 -0600
  Re: Drowning in a teacup? MRAB <python@mrabarnett.plus.com> - 2016-04-01 21:45 +0100
  Re: Drowning in a teacup? Michael Selik <michael.selik@gmail.com> - 2016-04-01 20:46 +0000
  Re: Drowning in a teacup? Steven D'Aprano <steve@pearwood.info> - 2016-04-02 09:46 +1100
  Re: Drowning in a teacup? Fillmore <fillmore_remove@hotmail.com> - 2016-04-02 00:03 -0400
  Re: Drowning in a teacup? Vito De Tullio <vito.detullio@gmail.com> - 2016-04-02 07:45 +0200
  Re: Drowning in a teacup? Michael Selik <michael.selik@gmail.com> - 2016-04-02 05:51 +0000
  Re: Drowning in a teacup? Vito De Tullio <vito.detullio@gmail.com> - 2016-04-02 08:26 +0200
  Re: Drowning in a teacup? Peter Otten <__peter__@web.de> - 2016-04-02 12:31 +0200
  Re: Drowning in a teacup? Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-04-02 12:53 +0100
  Re: Drowning in a teacup? Michael Selik <michael.selik@gmail.com> - 2016-04-02 14:36 +0000
  Re: Drowning in a teacup? Ned Batchelder <ned@nedbatchelder.com> - 2016-04-02 12:28 -0700
    Re: Drowning in a teacup? Random832 <random832@fastmail.com> - 2016-04-02 15:54 -0400
      Re: Drowning in a teacup? Steven D'Aprano <steve@pearwood.info> - 2016-04-03 12:33 +1000
    Re: Drowning in a teacup? Ethan Furman <ethan@stoneleaf.us> - 2016-04-02 16:15 -0700
    Re: Drowning in a teacup? Random832 <random832@fastmail.com> - 2016-04-02 19:19 -0400

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