(j3.2006) RANDOM_INIT question

[John Reid]

Bill Long wrote:
>
> 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.

If you look at the definitions of RANDOM_NUMBER and RANDOM_SEED 
together, I do not see how a generation scheme that does not use a seed 
could possibly conform to the standard.

Cheers,

John.


>>
>> 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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>>
>>
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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
>
>
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