On 22 April 2013 15:15, Blind Anagram <blindanag...@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. Oscar -- http://mail.python.org/mailman/listinfo/python-list
