From: Uwe Schmitt <schmitt@...>
Date: 2005-12-07T01:03:29+09:00
Subject: Re: Shuffling an array, sort_by{rand}'s bias (was Re: need some Ruby


|| > || arr.sort_by{rand}
|| > that is not equivalent to my solution.
|| > the python equivalent to yours is
|| >   arr.sort(key = lambda x: random())

|| > || > In Python it is quite easy by using list comprehensions...
|| > || What does that look like in Python? 
|| > 
|| > from random import random
|| > 
|| > def shuffle(a):
|| >     b = [ (random(), i) for i in a]
|| >     b.sort()
|| >     return [ x[1] for x in b ]
|| > 
|| > print shuffle(range(10))

|| I would have thought those two functions do the same thing
|| with the exception that shuffle does not sort in place.
|| I'm probably missing something obvious. Could you explain
|| the difference between the two versions?

If you use random as sorting key, the result of a<=>b
will differ each time you evaluate a<=>b. 
In my case I attach a random number to each item,
then I sort my list according to this number and remove
this 'decoration' afterwards.

Pythonprogrammers call this pattern "decorate-sort-undecorate".

As I do not now anything about the ruby interanls of sort,
I prefer my version.

Greetings, Uwe.