Re: Lucky numbers in Python

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From	Ian Kelly <ian.g.kelly@gmail.com>
Date	Wed, 29 Apr 2015 21:08:36 -0600
Subject	Re: Lucky numbers in Python
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On Wed, Apr 29, 2015 at 6:11 PM, Steven D'Aprano
<steve+comp.lang.python@pearwood.info> wrote:
> On Thu, 30 Apr 2015 05:57 am, Ian Kelly wrote:
>
>> On Wed, Apr 29, 2015 at 12:24 PM, Cecil Westerhof <Cecil@decebal.nl>
>> wrote:
>>> I was wondering if there is a way to do this:
>>>             for del_index in range((sieve_len // skip_count) * skip_count
>>>             - 1,
>>>                                   skip_count - 2, -skip_count):
>>>                 del sieve[del_index]
>>> in a more efficient way.
>>
>> You can delete using slices.
>>
>> del sieve[(sieve_len // skip_count) * skip_count - 1 : skip_count - 2
>> : -skip_count]
>>
>> Now you no longer need to do the iteration in reverse, which makes the
>> slicing simpler:
>>
>> del sieve[skip_count - 1 : (sieve_len // skip_count) * skip_count :
>> skip_count]
>
> True, but *probably* at the expense of speed. When you delete items from a
> list, the remaining items have to be moved, which takes time, especially
> for very large lists.
>
> Most of the time, rather than deleting items, it is faster to set them to a
> placeholder (for example None) and then copy the ones which aren't None in
> a separate loop:

You're correct, but I think this would be difficult to apply to the
OP's algorithm since the list indexing depends on the items from
previous iterations having been removed.

Thread

Lucky numbers in Python Cecil Westerhof <Cecil@decebal.nl> - 2015-04-29 20:24 +0200
  Re: Lucky numbers in Python Ian Kelly <ian.g.kelly@gmail.com> - 2015-04-29 13:57 -0600
    Re: Lucky numbers in Python Cecil Westerhof <Cecil@decebal.nl> - 2015-04-29 23:45 +0200
      Re: Lucky numbers in Python Ian Kelly <ian.g.kelly@gmail.com> - 2015-04-29 16:38 -0600
        Re: Lucky numbers in Python Cecil Westerhof <Cecil@decebal.nl> - 2015-04-30 02:01 +0200
          Re: Lucky numbers in Python Ian Kelly <ian.g.kelly@gmail.com> - 2015-04-29 20:55 -0600
            Re: Lucky numbers in Python Cecil Westerhof <Cecil@decebal.nl> - 2015-04-30 08:34 +0200
      Re: Lucky numbers in Python Chris Kaynor <ckaynor@zindagigames.com> - 2015-04-29 15:56 -0700
    Re: Lucky numbers in Python Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-04-30 10:11 +1000
      Re: Lucky numbers in Python Ian Kelly <ian.g.kelly@gmail.com> - 2015-04-29 21:08 -0600
  Re: Lucky numbers in Python Cecil Westerhof <Cecil@decebal.nl> - 2015-04-30 20:55 +0200
    Re: Lucky numbers in Python Dave Angel <davea@davea.name> - 2015-04-30 15:28 -0400

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