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Re: List Count

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On 23 April 2013 17:57, Blind Anagram <blindanagram@nowhere.org> wrote:
> On 23/04/2013 15:49, Steven D'Aprano wrote:
>> On Tue, 23 Apr 2013 08:05:53 +0100, Blind Anagram wrote:
>>
>>> I did a lot of work comparing the overall performance of the sieve when
>>> using either lists or arrays and I found that lists were a lot faster
>>> for the majority use case when the sieve is not large.
>>
>> And when the sieve is large?
>
> I don't know but since the majority use case is when the sieve is small,
> it makes sense to choose a list.

That's an odd comment given what you said at the start of this thread:

Blind Anagram wrote:
> I would be grateful for any advice people can offer on the fastest way
> to count items in a sub-sequence of a large list.
>
> I have a list of boolean values that can contain many hundreds of
> millions of elements for which I want to count the number of True values
> in a sub-sequence, one from the start up to some value (say hi).


>> I don't actually believe that the bottleneck is the cost of taking a list
>> slice. That's pretty fast, even for huge lists, and all efforts to skip
>> making a copy by using itertools.islice actually ended up slower. But
>> suppose it is the bottleneck. Then *sooner or later* arrays will win over
>> lists, simply because they're smaller.
>
> Maybe you have not noticed that, when I am discussing a huge sieve, I am
> simply pushing a sieve designed primarily for a small sieve lengths to
> the absolute limit.  This is most definitely a minority use case.
>
> In pushing the size of the sieve upwards, it is the slice operation that
> is the first thing that 'breaks'.  This is because the slice can be
> almost as big as the primary array so the OS gets driven into memory
> allocation problems for a sieve that is about half the length it would
> otherwise be. It still works but the cost of the slice once this point
> is reached rises from about 20% to over 600% because of all the paging
> going on.

You keep mentioning that you want it to work with a large sieve. I
would much rather compute the same quantities with a small sieve if
possible. If you were using the Lehmer/Meissel algorithm you would be
able to compute the same quantity (i.e. pi(1e9)) using a much smaller
sieve with 30k items instead of 1e9. that would fit *very* comfortably
in memory and you wouldn't even need to slice the list. Or to put it
another way, you could compute pi(~1e18) using your current sieve
without slicing or paging. If you want to lift the limit on computing
pi(x) this is clearly the way to go.

>
> The unavailable list.count(value, limit) function would hence allow the
> sieve length to be up to twice as large before running into problems and
> would also cut the 20% slice cost I am seeing on smaller sieve lengths.
>
> So, all I was doing in asking for advice was to check whether there is
> an easy way of avoiding the slice copy, not because this is critical,
> but rather because it is a pity to limit the performance because Python
> forces a (strictly unnecessary) copy in order to perform a count within
> a part of a list.
>
> In other words, the lack of a list.count(value, limit) function makes
> Python less effective than it would otherwise be.  I haven't looked at
> Python's C code base but I still wonder if there a good reason for NOT
> providing this?

If you feel that this is a good suggestion for an improvement to
Python consider posting it on python-ideas. I wasn't aware of the
equivalent functionality on strings but I see that the tuple.count()
function is the same as list.count().


Oscar

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Thread

List Count Blind Anagram <blindanagram@nowhere.org> - 2013-04-22 12:58 +0100
  Re: List Count Dave Angel <davea@davea.name> - 2013-04-22 08:51 -0400
    Re: List Count Blind Anagram <blindanagram@nowhere.org> - 2013-04-22 14:03 +0100
    Re: List Count Blind Anagram <blindanagram@nowhere.org> - 2013-04-22 14:03 +0100
  Re: List Count Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-04-22 13:13 +0000
    Re: List Count Skip Montanaro <skip@pobox.com> - 2013-04-22 08:57 -0500
    Re: List Count Blind Anagram <blindanagram@nowhere.org> - 2013-04-22 15:15 +0100
      Re: List Count Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-04-22 16:14 +0100
        Re: List Count Blind Anagram <blindanagram@nowhere.org> - 2013-04-22 16:50 +0100
          Re: List Count Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-04-22 17:06 +0100
            Re: List Count Blind Anagram <blindanagram@nowhere.org> - 2013-04-22 17:38 +0100
              Re: List Count Skip Montanaro <skip@pobox.com> - 2013-04-22 12:48 -0500
                Re: List Count Blind Anagram <blindanagram@nowhere.org> - 2013-04-22 20:22 +0100
              Re: List Count Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-04-22 21:18 +0100
                Re: List Count Blind Anagram <blindanagram@nowhere.org> - 2013-04-22 22:25 +0100
                Re: List Count Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-04-23 00:06 +0100
                Re: List Count Blind Anagram <blindanagram@nowhere.org> - 2013-04-23 07:45 +0100
                Re: List Count Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-04-22 23:28 +0000
                Re: List Count Blind Anagram <blindanagram@nowhere.org> - 2013-04-23 08:00 +0100
              Re: List Count Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-04-22 22:03 +0100
                Re: List Count Blind Anagram <blindanagram@nowhere.org> - 2013-04-22 22:32 +0100
                Re: List Count Dave Angel <davea@davea.name> - 2013-04-22 21:47 -0400
                Re: List Count Blind Anagram <blindanagram@nowhere.org> - 2013-04-23 08:02 +0100
            Re: List Count Blind Anagram <blindanagram@nowhere.org> - 2013-04-22 17:38 +0100
        Re: List Count Blind Anagram <blindanagram@nowhere.org> - 2013-04-22 16:50 +0100
      Re: List Count Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-04-22 23:01 +0000
        Re: List Count Blind Anagram <blindanagram@nowhere.org> - 2013-04-23 08:05 +0100
          Re: List Count Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-04-23 12:08 +0100
            Re: List Count Blind Anagram <blindanagram@nowhere.org> - 2013-04-23 12:45 +0100
              Re: List Count Terry Jan Reedy <tjreedy@udel.edu> - 2013-04-23 15:01 -0400
          Re: List Count Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-04-23 14:49 +0000
            Re: List Count Blind Anagram <blindanagram@nowhere.org> - 2013-04-23 17:57 +0100
              Re: List Count Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-04-23 18:45 +0100
                Re: List Count Blind Anagram <blindanagram@nowhere.org> - 2013-04-23 19:30 +0100
                Re: List Count Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-04-23 20:16 +0100
              Re: List Count Terry Jan Reedy <tjreedy@udel.edu> - 2013-04-23 16:00 -0400
                Re: List Count Blind Anagram <blindanagram@nowhere.org> - 2013-04-23 21:41 +0100
              Re: List Count Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-04-23 21:38 +0100
              Re: List Count Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-04-24 01:59 +0000
                Re: List Count Blind Anagram <blindanagram@nowhere.org> - 2013-04-24 10:01 +0100
  Re: List Count Peter Otten <__peter__@web.de> - 2013-04-22 15:22 +0200
  Re: List Count 88888 Dihedral <dihedral88888@googlemail.com> - 2013-04-22 06:36 -0700

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