looping versus comprehension

Robin Becker robin at reportlab.com
Wed Jan 30 18:58:27 CET 2013


On 30/01/2013 15:49, Chris Angelico wrote:
> On Thu, Jan 31, 2013 at 1:58 AM, Robin Becker <robin at reportlab.com> wrote:
>> however, when I tried an experiment in python 2.7 using the script below I
>> find that the looping algorithms perform better. A naive loop using list +=
>> list would appear to be an O(n**2) operation, but python seems to be doing
>> better than that. Also why does the append version fail so dismally. Is my
>> test coded wrongly or is pre-allocation of the list making this better than
>> expected?
>
> First off, are you aware that your first three blocks of code and your
> last four produce different results? The first ones flatten the list,
> the others simply convert tuples to lists. With n = 3:
>
>>>> points = []
>>>> for xy in row:
>      points += [xy[0],xy[1]]
>>>> points
> [0, 1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6]
>>>> map(list,row)
> [[0, 1], [1, 2], [2, 3], [3, 4], [4, 5], [5, 6]]
>
> Once that's sorted out, then timings can be played with. But it's
> worth noting that list appending is not going to be O(N*N), because
> it's going to allow room for expansion.
>
> ChrisA
>

No I missed that :( the list is a flattened one.

That'll teach me not to copy the code from the users without checking. Now you 
point it out it's clear that his code is doing something different. Presumably 
it's not drawing the same thing at all :) no wonder it got much faster.
-- 
Robin Becker




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