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Re: list comprehension return a list and sum over in loop

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KK Sasa wrote:

> Mark Lawrence於 2014年12月12日星期五UTC+8下午3時17分43秒寫道：
>> On 12/12/2014 06:22, KK Sasa wrote:
>> > Hi there,
>> >
>> > The list comprehension is results = [d2(t[k]) for k in xrange(1000)],
>> > where d2 is a function returning a list, say [x1,x2,x3,x4] for one
>> > example. So "results" is a list consisting of 1000 lists, each of
>> > length four. Here, what I want to get is the sum of 1000 lists, and
>> > then the result is a list of length four. Is there any efficient way to
>> > do this? Because I found it is slow in my case. I tried sum(d2(t[k])
>> > for k in xrange(1000)), but it returned error: TypeError: unsupported
>> > operand type(s) for +: 'int' and 'list'. Thanks.
>> >
>> 
>> I think you need something like this
>> http://stackoverflow.com/questions/19339/a-transpose-unzip-function-in-python-inverse-of-zip
>> 
>> I'll let you add the finishing touches if I'm correct :)
>> 
>> --
>> My fellow Pythonistas, ask not what our language can do for you, ask
>> what you can do for our language.
>> 
>> Mark Lawrence
> 
> Hi Mark and Yotam,
>   Thanks for kind reply. I think I didn't make my problem clear enough.
>   The slow part is "[d2(t[k]) for k in xrange(1000)]". In addition, I
>   don't need to construct a list of 1000 lists inside, but my aim is to
>   get the sum of all "d2(t[k])". I wonder if there is any method to sum up
>   efficiently.

If that is slow the culprit is probably the d2() function. If so 

results = [0] * 4
for k in xrange(1000):
    for i, v in enumerate(d2(t[k])):
        results[i] += v

won't help. Can you tell us what's inside d2()?

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list comprehension return a list and sum over in loop KK Sasa <genwei007@gmail.com> - 2014-12-11 22:22 -0800
  Re: list comprehension return a list and sum over in loop Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-12-12 07:17 +0000
    Re: list comprehension return a list and sum over in loop KK Sasa <genwei007@gmail.com> - 2014-12-12 00:30 -0800
      Re: list comprehension return a list and sum over in loop Christian Gollwitzer <auriocus@gmx.de> - 2014-12-12 10:09 +0100
      Re: list comprehension return a list and sum over in loop Peter Otten <__peter__@web.de> - 2014-12-12 10:13 +0100
        Re: list comprehension return a list and sum over in loop KK Sasa <genwei007@gmail.com> - 2014-12-12 01:45 -0800
          Re: list comprehension return a list and sum over in loop Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2014-12-12 13:12 +0200
            Re: list comprehension return a list and sum over in loop Peter Otten <__peter__@web.de> - 2014-12-12 13:32 +0100
              Re: list comprehension return a list and sum over in loop KK Sasa <genwei007@gmail.com> - 2014-12-12 05:09 -0800
                Re: list comprehension return a list and sum over in loop Peter Otten <__peter__@web.de> - 2014-12-12 17:08 +0100
                Re: list comprehension return a list and sum over in loop KK Sasa <genwei007@gmail.com> - 2014-12-12 19:04 -0800
                Re: list comprehension return a list and sum over in loop Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-12-13 11:27 +0000
            Re: list comprehension return a list and sum over in loop KK Sasa <genwei007@gmail.com> - 2014-12-12 05:03 -0800
  Re: list comprehension return a list and sum over in loop Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-12-12 20:46 +1100
  Re: list comprehension return a list and sum over in loop Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2014-12-12 16:47 +0000
  Re: list comprehension return a list and sum over in loop pecore@pascolo.net - 2014-12-13 13:38 +0100

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