List Count

[Dave Angel]

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.


-- 
DaveA