Simple algorithm question - how to reorder a sequence economically

[Steven D'Aprano]

On Fri, 24 May 2013 01:14:45 -0700, Peter Brooks wrote:

> What is the easiest way to reorder a sequence pseudo-randomly?

import random
random.shuffle(sequence)


The sequence is modified in place, so it must be mutable. Lists are okay, 
tuples are not.


> That is, for a sequence 1,2,3,4 to produce an arbitrary ordering (eg
> 2,1,4,3) that is different each time.

You can't *guarantee* that it will be different each time. With a four-
item list, there are only 4! = 24 combinations, so on average you'll get 
the original order one time in 24. For a ten-item list, that is once 
every 3628800 times, and for a twenty-item list, once in 
2432902008176640000 times. But of course these are *random*, and there's 
always a chance of this:

http://dilbert.com/strips/comic/2001-10-25


-- 
Steven