On 24/04/2013 02:59, Steven D'Aprano wrote: > On Tue, 23 Apr 2013 17:57:17 +0100, Blind Anagram wrote:
[snip] > In my opinion, it is more important to be efficient for large sieves, not > small. As they say, for small N, everything is fast. Nobody is going to > care about the difference between small-N taking 1ms or 10ms, but they > will care about the difference between big-N taking 1 minute or 1 hour. > So, as a general rule, optimize the expensive cases, and if the cheap > cases are a little less cheap than they otherwise would have been, nobody > will care or notice. I agree in general but it is often the case that more sophisticated algorithms only gain traction over simpler ones at much higher points than might be expected from a simple analysis. In my experience a naive sieve is an efficient solution for a general purpose sieve class primarily intended for situations where the sieve length can be large but not huge. As I think you have pointed out, memory is cheap and the memory operations needed to do copying and counting operations are fast. So using up memory is not normally an issue. I have just found a situation where a copy can have a serious impact on performance in an admittedly limiting, minority use case. It the fact that this copy is not, in principle, necessary, that I find disappointing. [snip] > In this case, I would say that adding a limit argument to list.count is > *absolutely not* worthwhile, because it is insufficiently general. To be > general enough to be worthwhile, you would have to add three arguments: > > list.count(value, start=0, end=None, step=1) > > But even there I don't think it's general enough to cover the costs. I > believe that a better solution would be to create list views to offer a > subset of the list without actually copying. I don't know a great deal about Python internals but I suspect that these solutions would offer more but also cost more. So where the balance of advantages would lie is unclear. But I am not pushing for a particular (or, indeed, any) solution. -- http://mail.python.org/mailman/listinfo/python-list
