looping versus comprehension
Steven D'Aprano
steve+comp.lang.python at pearwood.info
Thu Jan 31 00:17:32 EST 2013
On Thu, 31 Jan 2013 02:49:31 +1100, Chris Angelico wrote:
> it's worth
> noting that list appending is not going to be O(N*N), because it's going
> to allow room for expansion.
This is true for list.append, which is amortized constant time. But it is
not true for list addition, alist + blist, which is O(N**2) and hence
gets really, really slow:
steve at runes:~$ python -m timeit "L = []" "for i in xrange(1000): L = L + [1]"
100 loops, best of 3: 3.08 msec per loop
steve at runes:~$ python -m timeit "L = []" "for i in xrange(5000): L = L + [1]"
10 loops, best of 3: 71 msec per loop
steve at runes:~$ python -m timeit "L = []" "for i in xrange(25000): L = L + [1]"
10 loops, best of 3: 2.06 sec per loop
Notice that as the number of list additions goes up by a factor of 5,
the time taken goes up by a factor of 25.
--
Steven
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