question on list comprehensions

Mustafa Demirhan mustafademirhan at gmail.com
Fri Oct 15 19:36:30 CEST 2004


Why not just use while loops instead of for loops? You dont have to
create a new array each time you want a loop - you can simply use an
index integer.

    i = 0
    while i < 5000000:
        res [0] = res [0] + i
        i = i + 1

Takes less than 2 seconds on my laptop.


Darren Dale <dd55 at cornell.edu> wrote in message news:<ckm2up$cjv$1 at news01.cit.cornell.edu>...
> Hi,
> 
> I need to replace the following loop with a list comprehension:
> 
> res=[0]
> for i in arange(10000):
>  res[0]=res[0]+i
> 
> 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.
> 
> I saw this article at ASPN:
> http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/204297
> 
> def thislist():
>     """Return a reference to the list object being constructed by the
>     list comprehension from which this function is called. Raises an
>     exception if called from anywhere else.
>     """
>     import sys
>     d = sys._getframe(1).f_locals
>     nestlevel = 1
>     while '_[%d]' % nestlevel in d:
>         nestlevel += 1
>     return d['_[%d]' % (nestlevel - 1)].__self__
> 
> Could the list comprehension include something like thislist().pop(0), to be
> called when len(thislist)>1? (I think this could work, but am having
> trouble with the syntax.) Or is there a better way to approach the problem?
> 
> Thank you,
> 
> Darren



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