chandra sekaran on Wed, 23 May 2018 07:53:33 +0200

 Re: Fisher-Yates reverse shuffle is possible?

Hi,

By setting seed number of the random generator, is it possible to write
reverse the shuffle?

Regards
Chandrasekaran B

On Wed, May 23, 2018 at 11:09 AM, Remy José Cano wrote:
Hi again,

Not much time ago I was intrigued by the same kind of questions
involving randomness,

I must say that a true FY implementation cannot, *must not* have reverse.

Please take a look to https://www.random.org/

Cheers,

Remy

On 23/05/2018, Remy José Cano <cano.remy.jose@gmail.com> wrote:
>  Hi,
>
>  With respect, if something like that were reversible, the undelying
> process has not to do with randomness (a way of defining random is:
> The lack of pattern at all).
>
> Sorry, as far I know, the answer definitely is NO.
>
> In other hand, please notice the code you pasted from Rosetta is NOT
> optimal, and likely wrong due 1) to place a my() inside a loop, 2) By
> desing, for using a forstep() where a for() suffices. 2.999999....) a
> "};" instead of "}" followed by a line break.
>
> Cheers,
>
> Remy
>
> P.S.: Always ensure anywhere wether a function named "random" uses a
> provably random source of "entropy".
>
>
> On 23/05/2018, chandra sekaran <sekar.bc@gmail.com> wrote:
>> Fisher-Yates reverse shuffle is possible?
>>
>> Following PARI-GP code. from www.rosettacode.org
>>
>> FY(v)={
>>   forstep(n=#v,2,-1,
>>     my(i=random(n)+1,t=v[i]);
>>     v[i]=v[n];
>>     v[n]=t
>>   );
>>   v
>> };
>>
>> (09:56) gp > FY(vector(10,i,i))
>> %14 = [2, 10, 7, 4, 5, 1, 9, 8, 3, 6]
>> (09:56) gp > FY(vector(10,i,i))
>> %15 = [2, 8, 1, 5, 9, 3, 7, 4, 6, 10]
>> (09:56) gp > FY(vector(10,i,i))
>> %16 = [8, 7, 2, 5, 4, 10, 6, 9, 3, 1]
>> (09:56) gp > FY(vector(10,i,i))
>> %17 = [10, 4, 2, 5, 7, 3, 6, 9, 1, 8]
>>
>> Regards,
>> Chandrasekaran B
>>
>
>
> --
> _____________________________
>
> Remy José Cano Ramírez
> 1-43 Entrada principal a Santa Juana. Mérida, ME 5101, Venezuela
> Phone: +58 412 077 6 077;
> Page: https://www.oeis.org/wiki/User:R._J._Cano
> Email: cano.remy.jose@gmail.com
>

--
_____________________________

Remy José Cano Ramírez
1-43 Entrada principal a Santa Juana. Mérida, ME 5101, Venezuela
Phone: +58 412 077 6 077;
Page: https://www.oeis.org/wiki/User:R._J._Cano
Email: cano.remy.jose@gmail.com