Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.python > #50389

Re: Prime number generator

Show all headers | View raw

On 10 July 2013 19:56, Ian Kelly <ian.g.kelly@gmail.com> wrote:
> On Wed, Jul 10, 2013 at 11:47 AM, Joshua Landau <joshua@landau.ws> wrote:
>>>> If you care about speed, you might want to check the heapq module. Removing the smallest item and inserting a new item in a heap both cost O(log(N)) time, while finding the minimum in a dictionary requires iterating over the whole dictionary, which cost O(N) time.
>>
>> Actually, because it's a list under the hood I'd imagine push and pop
>> still take O(n) time :/.
>
> It shouldn't.  You can implement push by appending the new item and
> then getting it into the right place by performing O(log n) swaps.
> Likewise for pop, you can update the heap with O(log n) swaps and then
> removing the tail element.  Appending or removing the tail element of
> a list is amortized O(1).

Genius. Bas replied off-list that it won't matter too much anyway as
they're over-allocated, but this makes tons of sense.

>> PS: It's faster to use heapreplace(...) than
>> heappop(...);heappush(...) but it only saves a few %.
>
> The problem with heapreplace here is that the value to be pushed
> is dependent on the value returned by pop.

That's fine because indexing is much faster than pop. The "few %" was
a real statistic from working, timed code.

Back to comp.lang.python | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Prime number generator Chris Angelico <rosuav@gmail.com> - 2013-07-11 00:00 +1000
  Re: Prime number generator Bas <wegwerp@gmail.com> - 2013-07-10 07:35 -0700
    Re: Prime number generator Chris Angelico <rosuav@gmail.com> - 2013-07-11 01:12 +1000
      Re: Prime number generator bas <blswinkels@gmail.com> - 2013-07-10 08:47 -0700
        Re: Prime number generator Chris Angelico <rosuav@gmail.com> - 2013-07-11 02:15 +1000
        Re: Prime number generator Joshua Landau <joshua@landau.ws> - 2013-07-10 18:47 +0100
        Re: Prime number generator Ian Kelly <ian.g.kelly@gmail.com> - 2013-07-10 12:56 -0600
        Re: Prime number generator Joshua Landau <joshua@landau.ws> - 2013-07-10 20:06 +0100
      Re: Prime number generator bryanjugglercryptographer@yahoo.com - 2013-07-30 21:57 -0700
  Re: Prime number generator Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-07-10 16:01 +0000
    Re: Prime number generator Chris Angelico <rosuav@gmail.com> - 2013-07-11 02:52 +1000
  Re: Prime number generator albert@spenarnc.xs4all.nl (Albert van der Horst) - 2013-07-30 10:58 +0000
    Re: Prime number generator Ian Kelly <ian.g.kelly@gmail.com> - 2013-07-30 11:33 -0600

csiph-web