(j3.2006) RANDOM_INIT question

[John Reid]

Bill Long wrote:
>
> On Sep 20, 2015, at 4:39 AM, John Reid <John.Reid at stfc.ac.uk> wrote:
>
>>
>>
>> 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.
>>
>
> That is fine with me. I?m just trying to clarify what to answer when we get a request to use RDRAND for the repeatable = .false. case.  The question is whether it is just the initial seed that is not repeatable between runs, but thereafter random_number produces a repeatable (deterministic) sequence, or whether the entire sequence is not repeatable between runs.   Since this is new territory, it might be useful to include a Note to say which case we intend by the word ?repeatable?.  I?m assuming you intend that the sequence itself is repeatable for a given seed, which is fine, and simplifies usage and implementation.

I think J3/15-007r2 is already unambiguous. In the description of 
RANDOM_INIT, we have
"Initialize the pseudorandom number generator."
and under REAPEATABLE, we have
"If it has the value true, the seed accessed by the pseudorandom number 
generator is set to a processor-dependent value that is the same each 
time RANDOM_INIT is called from the same image. If it has the value 
false, the seed is set to a processor-dependent, unpredictably different 
value on each call."

Would it help to add "of RANDOM_INIT" at the end of this text?

Cheers,

John.


>
>
>> 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
>>>> Cray Inc./ Cray Plaza, Suite 210/ 380 Jackson St./ St. Paul, MN 55101
>>>>
>>>>
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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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>
> 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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