Re: [FWP] Factorial Elegance

[Tim Ayers <tayers@bridge.com>]

>>>>> "a" == abigail  <abigail@foad.org> writes:

>> On Thu, Sep 14, 2000 at 10:19:38PM -0500, tayers@bridge.com wrote:
>> {
>> my @cache = (1);
>> sub factorial {
>> my $x = shift;
>> return undef if $x < 0;
>> unless (defined $cache[$x]) {
>> my $p = $cache[$#cache];
>> $cache[$_] = $p *= $_ for ($#cache+1..$x);
>> }
>> return $cache[$x];
>> }
>> }

a> Given the pace of growth of factorial, using a cache isn't that helpful.
a> If you need to do many factorials, you might as well pre calculate all
a> of them and save all the conditionals in your sub.

Okay. And then I could use Math::BigInt to calculate exact results and
not be limited to 170! where my 32-bit machine stops.

a> However, if you want a cute little function, that does factorials
a> giving exact answers, no matter how large the argument is:
a>   sub factorial {local $" = "*"; my @a = (1, 1 .. shift); `echo '@a' | bc`}

Same method as Rick's. Neat.

a> (Granted, it returns the wrong results for arguments < 0, but so does the
a>  above code.)

You lost me here. I am not a mathematician. I learned enough math so
they would give me my diploma and got the hell out of there. But I
thought factorials are not defined for N < 0. So I thought that
returning 'undef' if $x < 0 would be okay. I'm definitely curious.

Hope you have a very nice day, :-)
Tim Ayers (tayers@bridge.com)


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