[Tutor] list comprehension problem

[Chris Fuller]

On Friday 03 July 2009 15:37, Emile van Sebille wrote:
> On 7/3/2009 1:21 PM Kent Johnson said...
>
> > On Fri, Jul 3, 2009 at 3:49 PM, Dinesh B
> >
> > Vadhia<dineshbvadhia at hotmail.com> wrote:
> >> d = [0, 8, 4, 4, 4, 7, 2, 5, 1, 1, 5, 11, 11, 1, 6, 3, 5, 6, 11, 1]
> >>
> >> and we want:
> >>
> >> [0, 8, 12, 16, 20, 27, 29, 34, 35, 36, 41, 52, 63, 64, 70, 73, 78, 84,
> >> 95, 96]
> >> dd = [ sum(d[:j]) for j in range(len(d)) ][1:]
> >>
> >> gives:
> >>
> >> [0, 8, 12, 16, 20, 27, 29, 34, 35, 36, 41, 52, 63, 64, 70, 73, 78, 84,
> >> 95]
> >
> > In [9]: [ sum(d[:j+1]) for j in range(len(d)) ]
> > Out[9]: [0, 8, 12, 16, 20, 27, 29, 34, 35, 36, 41, 52, 63, 64, 70, 73,
> > 78, 84, 95, 96]
>
> So, did we get an 'A'...
>
> Emile
>
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The problem with these list comprehensions is that they have O(n**2) 
complexity.  Whether they are faster or not depends on the speedup from the 
list comprehension and the length of the input.  I'd be inclined to favor the 
linear for loop.

Cheers