(j3.2006) RANDOM_INIT question

[Bill Long]

On Sep 17, 2015, at 7:22 PM, Cohen Malcolm <malcolm at nag-j.co.jp> wrote:

> No.
> 
> The description (I'm not going to copy it) says that RANDOM_INIT sets the 
> seed.  That is all it does!  Saying "the concept of seed [has no] meaning" 
> w.r.t. RANDOM_INIT flies directly in the face of the specification in the 
> standard.
> 
> Calling RANDOM_INIT once and then RANDOM_SEED (PUT=) is not any different in 
> action from calling RANDOM_SEED (PUT=) once, and RANDOM_SEED (PUT=) a second 
> time.
> 
> More to the point, you can call RANDOM_INIT (REPEATABLE=.FALSE.) once, then 
> RANDOM_SEED(GET=), and thus save away the seed that RANDOM_INIT set for 
> later use!

The actual question reduces to whether we can allow, for the non-repeatable case, for random_number to use any generation scheme that does not use a seed.  While random_init does set a seed, it only implies that the corresponding random_number actually uses that seed. 


> 
> There is no contradiction or any kind of conflict in the descriptions as far 
> as I can see...
> 
> As for RDRAND, it does not return a pseudo-random number sequence so is 
> completely unsuitable for implementing RANDOM_NUMBER(*), especially when 
> many values are required in a short space of time.  However, it is Very 
> Suitable for providing entropy to go into the seed value.  (Though one might 
> wish to use other entropy sources as well.)


Intel separately provides that with RDSEED. 

Cheers,
Bill

> 
> (*) Completely unsuitable in that it literally cannot provide the actual 
> semantics of the RANDOM_* family as specified in Fortran standard.
> 
> Cheers,
> 
> -----Original Message----- 
> From: Bill Long
> Sent: Thursday, September 17, 2015 10:44 PM
> To: fortran standards email list for J3
> Subject: (j3.2006) RANDOM_INIT question
> 
> 
> Suppose the program calls RANDON_INIT with the REPEATABLE argument having 
> the value false.   In this case, does the concept of a ?seed? have any 
> meaning?  Would a call to RANDOM_SEED have any effect for a non-repeatable 
> generator?  More particularly, would it be conforming to use the RDRAND 
> instruction on recent Intel chips to create the random number returned by 
> RANDOM_NUMBER,  and just ignore the seed value set by RANDOM_SEED?
> 
> Cheers,
> Bill
> 
> Bill Long 
> longb at cray.com
> Fortran Technical Support  &                                  voice: 
> 651-605-9024
> Bioinformatics Software Development                     fax:  651-605-9142
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> -- 
> ........................Malcolm Cohen, Nihon NAG, Tokyo. 
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Bill Long                                                                       longb at cray.com
Fortran Technical Support  &                                  voice:  651-605-9024
Bioinformatics Software Development                     fax:  651-605-9142
Cray Inc./ Cray Plaza, Suite 210/ 380 Jackson St./ St. Paul, MN 55101