SCOUG-Programming Mailing List Archives

Date:	Thu, 1 May 2003 19:28:56 PDT7
From:	"Gregory W. Smith" <gsmith@well.com >
Reply-To:	scoug-programming@scoug.com
To:	scoug-programming@scoug.com
Subject:	SCOUG-Programming: permutation

Content Type: text/plain

Here is a Python routine that uses a recursive algorithmn
to permute a list L:

def changering ( N ):
global L
if N==0: print L
for c in range(N):
L[c], L[N-1] = L[N-1], L[c]
changering ( N-1 )
L[c], L[N-1] = L[N-1], L[c]

Sedgewick attributes this algorithm to seventeenth century change
ringings (http://www.cs.princeton.edu/~rs/talks/perms.pdf). To
run the algorithm, define the list and call changering with the
length of the list.

L=['A','B','C']
changering(len(L))

I ran it with a list of eight items--it takes a while to scroll by.
-- Greg Smith

> Zdenek Jizba wrote:
> >
> > I wrote a quick (15 minutes) and dirty (tested only up
> > to 4 elements) permutation function using APL2. I will
> > bring it Saturday and run it if anyone is interested.
>
> Hi Jizba,
>
> Thanks. Unfortunately I won't be at the Saturday meeting.
>
> - Peter
--
Gregory W. Smith (WD9GAY) gsmith@well.com

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