Need help porting Perl function

John Machin sjmachin at lexicon.net
Sat Jun 7 23:56:37 CEST 2008


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] []



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