---------------------------------------- > Date: Fri, 24 May 2013 01:14:45 -0700 > Subject: Simple algorithm question - how to reorder a sequence economically > From: peter.h.m.bro...@gmail.com > To: python-list@python.org > > 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. > > I'm writing a simulation and would like to visit all the nodes in a > different order at each iteration of the simulation to remove the risk > of a fixed order introducing spurious evidence of correlation. > -- > http://mail.python.org/mailman/listinfo/python-list
I don't know what "spurious evidence of correlation" is. Can you give a mathematical definition? Here's a snippet for creating a random shuffle by Fisher–Yates algorithm: def FY_shuffle(l): from random import randint for i in range(len(l)-1,0,-1): j = randint(0,i) l[j],l[i] = l[i],l[j] It looks just like random.shuffle() mentioned before, but you can change it as you see fit. If you can afford to test all permutations you can iterate over it by doing: >>> from itertools import permutations >>> l=range(4) >>> l [0, 1, 2, 3] >>> for i in permutations(l): print i ... (0, 1, 2, 3) (0, 1, 3, 2) (0, 2, 1, 3) [...] (3, 1, 2, 0) (3, 2, 0, 1) (3, 2, 1, 0) >>> Note that 'i' is a tuple. If you need a list of all permutations to make a selection: >>> l=range(4) >>> l [0, 1, 2, 3] >>> [list(i) for i in permutations(l)] [[0, 1, 2, 3], [0, 1, 3, 2], [0, 2, 1, 3], [0, 2, 3, 1], [0, 3, 1, 2], [0, 3, 2, 1], [1, 0, 2, 3], [1, 0, 3, 2], [1, 2, 0, 3], [1, 2, 3, 0], [1, 3, 0, 2], [1, 3, 2, 0], [2, 0, 1, 3], [2, 0, 3, 1], [2, 1, 0, 3], [2, 1, 3, 0], [2, 3, 0, 1], [2, 3, 1, 0], [3, 0, 1, 2], [3, 0, 2, 1], [3, 1, 0, 2], [3, 1, 2, 0], [3, 2, 0, 1], [3, 2, 1, 0]] This will produce big lists: -for 10 elements (l=range(10)) the size of the list is about 30MB (on Windows). -for 11 elements (l=range(11)) the size of the list is about 335MB (on Windows). It took more than 7GB for CPython 2.7.5 to create that list. Didn't try after that. -- http://mail.python.org/mailman/listinfo/python-list
