Home Original page

All permutations of a list

June Kim junaftnoon at nospamplzyahoo.com
Mon Oct 23 02:23:08 EDT 2000 
"Rainer Deyke" <root at rainerdeyke.com> wrote in message
news:ycQI5.111500$g6.50529775 at news2.rdc2.tx.home.com...
> "Emile van Sebille" <emile at fenx.com> wrote in message
> news:8t08bb$lnvu5$1 at ID-11957.news.cis.dfn.de...
> > "Rainer Deyke" <root at rainerdeyke.com> wrote in message
> > news:9nNI5.110763$g6.50159527 at news2.rdc2.tx.home.com...
> > > Here's my short Pythonic permutation function:
> > >
> > > def perms(list):
> > >   if list == []:
> > >     return [[]]
> > >   return [[list[i]] + p for i in range(len(list))\
> > >     for p in perms(list[:i] + list[i+1:])]
> > >
> >
> > ... which permutes into...
> >
> > def perms3(list):
> >   return not [list] or [[list[i]] + p for i in
> > range(len(list))\
> >     for p in perms(list[:i] + list[i+1:])]
>
> No. If 'list' is '[]', '[list]' evalutes to '[[]]'.   'not [[]]' evalutes
to
> 0, effectively removing all special handling for the empty list.
>
> If the list of permutations of the empty list was redefined as the empty
> list, the following would work:
>
> def perms(list):
>   return list and [[list[i]] + p for i in range(len(list))\
>     for p in (perms(list[:i] + list[i+1:]) or [[]])]
>
> If you're really desparate, you can save an additional line by using
lambda:
>
> perms = lambda list: list and [[list[i]] + p for i in\
>   range(len(list)) for p in (perms(list[:i] + list[i+1:]) or [[]])]
>
>

Cool! Those are all braintwisters to me. I think I'd feel more
clever after studying some of you guys' snippets. It seems,
the more I try to understand the codes, the more I get to
respect you.
Surely this Python thing must go for the Programming for Everyone
even in elementary schools, which will make our students
squeeze their brains and let them be more pythonically clever.

> --
> Rainer Deyke (root at rainerdeyke.com)
> Shareware computer games           -           http://rainerdeyke.com
> "In ihren Reihen zu stehen heisst unter Feinden zu kaempfen" - Abigor
>
>

Best Regards,
June
More information about the Python-list mailing list