[Chicago-talk] Random first numbers

[Lee Aylward]

Hi,

I typically use List::Util's shuffle function. I can't really say if
it will give you better results, though.

-- 
Lee

On Mon, 17 May 2010 16:25:10 -0500
Alan Mead <amead2 at alanmead.org> wrote:

> I have a CGI-based study coded in Perl that needs to randomly assign 
> participants to conditions.  The code below sucks:
> 
>   my @conditions = qw/cond1 cond2 cond3 cond4a/;
>   shuffle(@conditions);
>   $$state{'condition'} = $conditions[0];
> 
> I've just run through a dozen tests and cond3 appeared 90% of the
> time and cond2 10% of the time (cond1 and cond4a never appearing).  I
> haven't run the numbers, but I think this is "statistically
> significant". I think my shuffle is a "good" one from the on-line
> Perl lore, but maybe I'm wrong:
> 
> #----------------------------------------------------------
> # Durstenfeld's implementation of Fisher and Yates
> #----------------------------------------------------------
> sub shuffle {
>   my $n = scalar @_;
>   while ( $n > 0 ) {
>     $n--;
>     my $j = int rand $n;
>     ($_[$j], $_[$n]) = ($_[$n], $_[$j]); # swap
>   }
> }
> 
> Assuming that shuffle() is not the culprit, I think it's just that
> when I run this code quickly/repeatedly, rand() is getting seeded
> with substantially the same value and is generating essentially the
> same initial value.  If so, my question is whether there is a better
> way to generate random numbers (so that the first values are more
> random). 
> 
> I was wondering if anyone agreed/disagreed and whether anyone had a 
> "lazy" solution... if not, I can enforce better randomness by keeping 
> some sort of state in a database.
> 
> -Alan
>