Re: random generator and seed

From: Marc Escalier <escalier_at_lal.in2p3.fr>
Date: Sat, 5 Mar 2011 12:41:42 +0100

all right

thank you Lorenzo

---

On Sat, 5 Mar 2011, Lorenzo Moneta wrote:

> Hi ,
>
> If you in the constructor or in TRandom3::SetSeed a value of seed=0, a UUID is automatically generated inside and will give you
> a different generator event by event.
> In the example provided by Jason, a UUID is used only for a single number of the 624 numbers which makes the state of TRandom3.
> When using SetSeed(0), 8 independent seeds are generated which gives a much lower probability of reproducing the sequence.
>
> Best Regards
>
> Lorenzo
> On Mar 4, 2011, at 10:57 PM, Jason Detwiler wrote:
>
>> This should usually be fine but it depends on the generator. I have
>> used TUUID to get a quick semi-random seed like this:
>>
>> TUUID uuid;
>> UInt_t buffer[4];
>> uuid.GetUUID((UChar_t*) buffer);
>> UInt_t seed = (buffer[0] + buffer[1] + buffer[2] + buffer[3]);
>> // Negative seeds give nasty sequences for some engines. Might
>> // as well make all seeds positive; randomness is not really affected
>> // (one bit goes unused).
>> if (seed < 0) seed = -seed;
>>
>> You throw out 3/4 of the info, but it should be very different from
>> event to event. If you really need a good, different random seed from
>> event-to-event, try reading from /dev/random (assuming you are on
>> *nix):
>>
>> ifstream devrandom("/dev/random");
>> UInt_t seed;
>> devrandom.read((char*)(&seed), sizeof(UInt_t));
>>
>> The read can take some time but you will get a high quality random
>> number every time.
>>
>> Thanks,
>> Jason
>>
>> On Fri, Mar 4, 2011 at 12:21 PM, Marc Escalier <escalier_at_lal.in2p3.fr> wrote:
>>> Hello,
>>>
>>> when one is using a random generator as, for example TRandom3,
>>> one can choose a given seed, with myrandom3.SetSeed(MyValue)
>>>
>>> one could do the following to make a smearing of a given variable inside a
>>> file, according to a gaussian centred to 0) :
>>>
>>> (option A) :
>>> myrandom3.SetSeed(a given seed);
>>> for (...) {
>>> ...
>>> myrandom3.Gaus(0,a given value);
>>> make the correction to the variable
>>> ...
>>> }
>>>
>>> (option B)
>>> for (...) {
>>> ...
>>> myrandom3.SetSeed(TheEventNumberOfAFile); //for example one needs to smear
>>> a variable inside a file
>>> // for which each event has a different event number
>>> myrandom3.Gaus(0,a given value);
>>> make the correction to the variable
>>> ...
>>> }
>>>
>>> Is there a "pathology" in the option B in terms of distribution of the
>>> random numbers ?
>>>
>>> (there are no problems, just asking a naive question)
>>>
>>> thanks
>>>
>>>
>>
>
>
Received on Sat Mar 05 2011 - 12:41:49 CET