Need help porting Perl function

eatrnr at gmail.com eatrnr at gmail.com
Sun Jun 8 00:17:52 CEST 2008


On Jun 7, 5:56 pm, John Machin <sjmac... at lexicon.net> wrote:
> On Jun 8, 6:05 am, eat... at gmail.com wrote:
>
>
>
> > On Jun 7, 2:42 pm, "Daniel Fetchinson" <fetchin... at googlemail.com>
> > wrote:
>
> > > > Hi.  I'd like to port a Perl function that does something I don't
> > > > know how to do in Python.  (In fact, it may even be something that
> > > > is distinctly un-Pythonic!)
>
> > > > The original Perl function takes a reference to an array, removes
> > > > from this array all the elements that satisfy a particular criterion,
> > > > and returns the list consisting of the removed elements.  Hence
> > > > this function returns a value *and* has a major side effect, namely
> > > > the target array of the original argument will be modified (this
> > > > is the part I suspect may be un-Pythonic).
>
> > > > Can a Python function achieve the same effect?  If not, how would
> > > > one code a similar functionality in Python?  Basically the problem
> > > > is to split one list into two according to some criterion.
>
> > > This function will take a list of integers and modify it in place such
> > > that it removes even integers. The removed integers are returned as a
> > > new list (disclaimer: I'm 100% sure it can be done better, more
> > > optimized, etc, etc):
>
> > > def mod( alist ):
> > >     old = alist[:]
> > >     ret = [ ]
> > >     for i in old:
> > >         if i % 2 == 0:
> > >             ret.append( alist.pop( alist.index( i ) ) )
>
> > >     return ret
>
> > > x = range(10,20)
>
> > > print x
> > > r = mod( x )
> > > print r
> > > print x
>
> > > HTH,
> > > Daniel
> > > --
> > > Psss, psss, put it down! -http://www.cafepress.com/putitdown
>
> > def mod( alist ):
> >     return [ alist.pop( alist.index( x ) ) for x in alist if x % 2 ==
> > 0 ]
>
> > alist = range(10,20)
> > blist = mod( alist )
>
> > print alist
> > print blist
>
> > The same thing with list comprehensions.
>
> Not the same. The original responder was careful not to iterate over
> the list which he was mutating.
>
> >>> def mod(alist):
>
> ...    return [alist.pop(alist.index(x)) for x in alist if x % 2 == 0]
> ...>>> a = range(10)
> >>> print mod(a), a
>
> [0, 2, 4, 6, 8] [1, 3, 5, 7, 9]>>> a = [2,2,2,2,2,2,2,2]
> >>> print mod(a), a
>
> [2, 2, 2, 2] [2, 2, 2, 2]
> # should be [2, 2, 2, 2, 2, 2, 2, 2] []

Alas, it appears my understanding of list comprehensions is
significantly less comprehensive than I thought =)



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