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Date: Mon, 22 Apr 2013 14:03:25 +0100
From: Blind Anagram <blindanagram@nowhere.org>
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To: Dave Angel <davea@davea.name>
Subject: Re: List Count
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On 22/04/2013 13:51, Dave Angel wrote:
> On 04/22/2013 07:58 AM, 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.
>>
>> Ideally I would like to be able to use:
>>
>>     sieve.count(True, hi)
>>
>> where 'hi' sets the end of the count but this function is, sadly, not
>> available for lists.
>>
>> The use of a bytearray with a memoryview object instead of a list solves
>> this particular problem but it is not a solution for me as it creates
>> more problems than it solves in other aspects of the program.
>>
>> Can I assume that one possible solution would be to sub-class list and
>> create a C based extension to provide list.count(value, limit)?
>>
>> Are there any other solutions that will avoid copying a large part of
>> the list?
>>
> 
> Instead of using the default slice notation, why not use
> itertools.islice() ?
> 
> Something like  (untested):
> 
> import itertools
> 
> it = itertools.islice(sieve, 0, hi)
> sum(itertools.imap(bool, it))
> 
> I only broke it into two lines for clarity.  It could also be:
> 
> sum(itertools.imap(bool, itertools.islice(sieve, 0, hi)))
> 
> If you're using Python 3.x, say so, and I'm sure somebody can simplify
> these, since in Python 3, many functions already produce iterators
> instead of lists.

Thanks, I'll look at these ideas.  And, yes, my interest is mainly in
Python 3.