Re: List Count

Message-ID	<51755C38.4000204@nowhere.org> (permalink)
Date	2013-04-22 16:50 +0100
From	Blind Anagram <blindanagram@nowhere.org>
Newsgroups	comp.lang.python
Subject	Re: List Count
References	<sridnffI6YhxuOjMnZ2dnUVZ7tSdnZ2d@brightview.co.uk> <5175377f$0$29977$c3e8da3$5496439d@news.astraweb.com> <517545F7.5090209@nowhere.org> <mailman.927.1366644147.3114.python-list@python.org>

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On 22/04/2013 16:14, Oscar Benjamin wrote:

> On 22 April 2013 15:15, Blind Anagram <blindanagram@nowhere.org> wrote:
>> On 22/04/2013 14:13, Steven D'Aprano wrote:
>>> On Mon, 22 Apr 2013 12:58:20 +0100, 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 am currently using:
>>>>
>>>>    sieve[:hi].count(True)
>>>>
>>>> but I believe this may be costly because it copies a possibly large part
>>>> of the sieve.
> [snip]
>>
>> But when using a sub-sequence, I do suffer a significant reduction in
>> speed for a count when compared with count on the full list.  When the
>> list is small enough not to cause memory allocation issues this is about
>> 30% on 100,000,000 items.  But when the list is 1,000,000,000 items, OS
>> memory allocation becomes an issue and the cost on my system rises to
>> over 600%.
> 
> Have you tried using numpy? I find that it reduces the memory required
> to store a list of bools by a factor of 4 on my 32 bit system. I would
> expect that to be a factor of 8 on a 64 bit system:
> 
>>>> import sys
>>>> a = [True] * 1000000
>>>> sys.getsizeof(a)
> 4000036
>>>> import numpy
>>>> a = numpy.ndarray(1000000, bool)
>>>> sys.getsizeof(a)  # This does not include the data buffer
> 40
>>>> a.nbytes
> 1000000
> 
> The numpy array also has the advantage that slicing does not actually
> copy the data (as has already been mentioned). On this system slicing
> a numpy array has a 40 byte overhead regardless of the size of the
> slice.
> 
>> I agree that this is not a big issue but it seems to me a high price to
>> pay for the lack of a sieve.count(value, limit), which I feel is a
>> useful function (given that memoryview operations are not available for
>> lists).
> 
> It would be very easy to subclass list and add this functionality in
> cython if you decide that you do need a builtin method.

Thanks Oscar, I'll take a look at this.

But I was really wondering if there was a simple solution that worked
without people having to add libraries to their basic Python installations.

As I have never tried building an extension with cython, I am inclined
to try this as a learning exercise if nothing else.

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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