From: Florian Gross <flgr@...>
Date: 2004-07-12T10:02:26+09:00
Subject: Re: Ruby quickies and useful idioms

Mikael Brockman wrote:

> gabriele renzi <surrender_it@rc1.vip.ukl.yahoo.com> writes:
>>> [some shuffling stuff]
>> the best shuffle I can think of is the Grossian shuffle (quoting
>> Flroain Gross (actually written with a different character than ss):

It's actually "Florian Gro�", but I've seen the last character come out 
as an y with two dots above it which is why I usually use "Gross" where 
I can't be sure about it coming out right. ;)

Oh, and thanks for naming this after me. :)

>> class Array
>>   def shuffle
>>     sort_by {rand}
>>   end
>> end
>> You may even found extensions.rubyforge.org interesting
> It's very nifty indeed.  I heard it was biased in some obscure way,
> though.

It's biased when rand returns the same float for two or more elements of 
the Array.

This means that it will be ~0.012% biased on an Array that has one 
million entries and ~0.0001% biased on an Array with 100000 entries when 
you're on a 32-bit machine.

On 64-bit machines you will have a 50% chance of one element being 
biased for an Array with 5,000,000,000 elements.

If that's still to likely for you you can use sort_by { Array.new(100) { 
rand }} instead.

The problem with all this is that people frequently confuse sort_by { 
rand } with sort { rand } and variants of it.

Oh, and it might even make sense to add it to Enumerable instead of 
Arrays. After all #sort_by is defined for all Enumerables.

Regards,
Florian Gross