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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