On 24/05/13 10:11, Chris Angelico wrote:
On Fri, May 24, 2013 at 6:47 PM, Fábio Santos <fabiosantos...@gmail.com> wrote:
On 24 May 2013 09:41, "Chris Angelico" <ros...@gmail.com> wrote:
On Fri, May 24, 2013 at 6:14 PM, Peter Brooks
<peter.h.m.bro...@gmail.com> wrote:
What is the easiest way to reorder a sequence pseudo-randomly?
That is, for a sequence 1,2,3,4 to produce an arbitrary ordering (eg
2,1,4,3) that is different each time.
...
It works, it produces a unique list for any given index provided, but
it's not the cleanest or most efficient. But I know someone'll improve
on it... or tell me I'm an idiot for not taking a more obvious
approach :)
ChrisA
I think that is pretty much itertools.permutations from the standard
library. The OP should check it out.
That works if all the permutations are wanted at once. Is there a way,
short of iterating over it N times, to request permutation #N? Or
maybe I'm misreading the OP and that's not a requirement.
ChrisA
A long time ago I wrote some code to do that.
import gmpy
def LexPermFromIndex(items, index):
n = len(items)
inds = range(n)
perm = []
for i in range(1, n+1):
r, index = divmod(index, gmpy.fac(n-i))
r = int(r)
perm.append(inds[r])
inds = inds[:r] + inds[r+1:]
return [items[i] for i in perm]
>>> LexPermFromIndex([1,2,3,4], 0)
[1, 2, 3, 4]
>>> LexPermFromIndex([1,2,3,4], 1)
[1, 2, 4, 3]
>>> LexPermFromIndex([1,2,3,4], 10)
[2, 4, 1, 3]
>>>
I can't remember exactly why I wrote it. But I also have something for
generating a permutation's index and similar functions for combinations.
Duncan
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