# question on list comprehensions

Frohnhofer, James james.frohnhofer at csfb.com
Thu Oct 14 18:01:12 CEST 2004

```I have to admit, I'm still not sure I understand the problem, but is this what
you're trying to do?

>>>
res=[[0,1,2,3,4,5,6,7,8,9],[2,2,2,2,2,2,2,2,2,2],[0,-1,-2,-3,-4,-5,-6,-7,-8,-9]]

where in reality rather than a list of 3 lists of 10 elements, res would be a
list of 2500 lists of 2000 elements each.

>>> map(lambda x: x*x,
map(lambda tup: reduce(lambda x,y:x+y,tup),
[
[res[i][j] for i in range(0,len(res))]
for j in range(0,len(res[0]))
]
)
)
[4, 4, 4, 4, 4, 4, 4, 4, 4, 4]

If that's what you're trying to do.  I have no idea about whether this is
faster than other methods, although it certainly seems to me to be less clear.
If you stored your data cross-sectionally, it would be much simpler.  (Again,
if this it what you're trying to do.)

> -----Original Message-----
> From: python-list-bounces+james.frohnhofer=csfb.com at python.org
> [mailto:python-list-bounces+james.frohnhofer=csfb.com at python.org]On
> Behalf Of Darren Dale
> Sent: Thursday, October 14, 2004 11:16 AM
> To: python-list at python.org
> Subject: Re: question on list comprehensions
>
>
> Roberto Antonio Ferreira De Almeida wrote:
>
> > Darren Dale wrote:
> >> Hi,
> >>
> >> I need to replace the following loop with a list comprehension:
> >>
> >> res=[0]
> >> for i in arange(10000):
> >>  res[0]=res[0]+i
> >
> > res[0] = (10000 * (10000-1))/2.0   ;-)
> >
> >> In practice, res is a complex 2D numarray. For this
> reason, the regular
> >> output of a list comprehension will not work: constructing
> a list of
> >> every intermediate result will result in huge hits in
> speed and memory.
> >
> > Why do you *need* to replace the for loop with a listcomp? Could you
> > give more details about what you're doing with the complex array?
> >
> > Roberto
>
>
> OK. As usual, I am having trouble clearly expressing myself.
> that. Prepare for some physics:
>
> I am simulating diffraction from an array of particles. I
> have to calculate
> a complex array for each particle, add up all these arrays,
> and square the
> magnitude of the result. If I do a for loop, it takes about
> 6-8 seconds for
> a 2000 element array added up over 250 particles. In reality,
> I will have
> 2500 particles, or even 2500x2500 particles.
>
> The list comprehension takes only 1.5 seconds for 250
> that means the time has decreased from 40 hours to 10, and
> that time can be
> reduced further if python is not constantly requesting
> to grow the resulting list.
>
> I guess that means that I would like to avoid growing the
> list and popping
> the previous result if possible, and just over-write the
> previous result.
> --
> http://mail.python.org/mailman/listinfo/python-list
>

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