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Re: Simple algorithm question - how to reorder a sequence economically

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On 5/24/2013 6:52 AM, Steven D'Aprano wrote:
> 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
>
>

Also, heed the note in the docs:  "Note that for even rather small 
len(x), the total number of permutations of /x/ is larger than the 
period of most random number generators; this implies that most 
permutations of a long sequence can never be generated."  The default 
random number generator has a period of 2**19937-1, which means that 
lists longer than 2080 have more permutations than the period of the 
generator (because factorial(2081) > 2**19937). Most shuffles involve 
lists far shorter than 2080, but it's still good to keep it in mind.

--Ned.

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Simple algorithm question - how to reorder a sequence economically Peter Brooks <peter.h.m.brooks@gmail.com> - 2013-05-24 01:14 -0700
  Re: Simple algorithm question - how to reorder a sequence economically Chris Angelico <rosuav@gmail.com> - 2013-05-24 18:37 +1000
  Re: Simple algorithm question - how to reorder a sequence economically Fábio Santos <fabiosantosart@gmail.com> - 2013-05-24 09:47 +0100
  Re: Simple algorithm question - how to reorder a sequence economically Chris Angelico <rosuav@gmail.com> - 2013-05-24 19:11 +1000
    Re: Simple algorithm question - how to reorder a sequence economically duncan smith <buzzard@invalid.invalid> - 2013-05-24 15:33 +0100
  Re: Simple algorithm question - how to reorder a sequence economically Terry Jan Reedy <tjreedy@udel.edu> - 2013-05-24 06:10 -0400
  Re: Simple algorithm question - how to reorder a sequence economically Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-05-24 10:52 +0000
    Re: Simple algorithm question - how to reorder a sequence economically Ned Batchelder <ned@nedbatchelder.com> - 2013-05-24 07:26 -0400
      Re: Simple algorithm question - how to reorder a sequence economically Peter Brooks <peter.h.m.brooks@gmail.com> - 2013-05-24 06:23 -0700
        Re: Simple algorithm question - how to reorder a sequence economically Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-05-24 13:57 +0000
        Re: Simple algorithm question - how to reorder a sequence economically Robert Kern <robert.kern@gmail.com> - 2013-06-12 14:14 +0100
    Re: Simple algorithm question - how to reorder a sequence economically John Ladasky <john_ladasky@sbcglobal.net> - 2013-05-24 10:33 -0700
      Re: Simple algorithm question - how to reorder a sequence economically John Ladasky <john_ladasky@sbcglobal.net> - 2013-05-25 18:25 -0700
        Re: Simple algorithm question - how to reorder a sequence economically Roy Smith <roy@panix.com> - 2013-05-25 21:49 -0400
  RE: Simple algorithm question - how to reorder a sequence economically Carlos Nepomuceno <carlosnepomuceno@outlook.com> - 2013-05-24 18:00 +0300
    Re: Simple algorithm question - how to reorder a sequence economically Peter Brooks <peter.h.m.brooks@gmail.com> - 2013-05-24 12:01 -0700
      RE: Simple algorithm question - how to reorder a sequence economically Carlos Nepomuceno <carlosnepomuceno@outlook.com> - 2013-05-25 00:33 +0300
        Re: Simple algorithm question - how to reorder a sequence economically Peter Brooks <peter.h.m.brooks@gmail.com> - 2013-05-24 17:28 -0700

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