From: Dan Zwell <dzwell@...>
Date: 2007-08-28T06:06:07+09:00
Subject: Re: sort_by { rand } not working

Lloyd Linklater wrote:
> Dan Zwell wrote:
>> Lloyd Linklater wrote:
>>> dang it!  I *knew* that I should have been thinking more ruby and less 
>>> pascal.  This may look more rubyish:
>>>
>>> def arr_rand(ar)
>>>   arr = []
>>>   while ar.length > 0 {arr << ar.delete_at(rand(ar.length))}
>>>   arr
>>> end
>> Yes, that is fairly simple to understand. But it is equivalent to
>> sort_by {rand} in that it does not actually change the array that is
>> passed in. 
> 
> Actually, it was different.  The sort_by {rand} gives the same sequence 
> of shuffling every time you run it.  Mine is different every time.  I 
> get a bang out of the declaration you have.  That is a nice touch.  I 
> also like your use of until temp.empty?  That is very Ruby-ish to me. 
> :)

Thanks. Wanna see a better version of this algorithm? Check out a sample 
chapter from Hal Fulton's book--he does it a little more elegantly ( 
http://www.informit.com/articles/article.aspx?p=26943&rl=1 ). And I was 
very much mistaken in what I said--your code /does/ change the array 
that is passed in, but it zeros it (not randomizes). You could add a 
line ar.replace(arr) to mitigate that, or make a copy (ar.dup) and 
operate on that instead, depending on whether you expect the method to 
perturb the array that is passed in.

And by the way, sort_by {rand} does return a different result every 
time. If it doesn't, I would assume you're installation of ruby is old 
(or broken?) and doesn't initialize a new random seed (srand) each time 
it is started. Or maybe this is how ruby behaves on Windows? I'm not 
quite sure.

Dan