List Count

[88888 Dihedral]

Blind Anagram於 2013年4月22日星期一UTC+8下午7時58分20秒寫道：
> 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?

For those problems related to a homogeneous list of numbers
, please check whether  the arrays in numpy can fit your needs practically or not.


Sometimes I work on numbers  in varied ranges, 
then the list and the long integers in Python is really handy.