[R] set seed for random draws

[Prof Brian Ripley]

On Sun, 6 Nov 2011, R. Michael Weylandt wrote:

> I think it's easier than you are making it: the random seed is created
> in a "pretty-random" way when you first use it and then it is updated

Ah: It is unless you then save the workspace.  If you do, then evey 
subsequent session starts with the same seed until you save the 
workspace again.  So never saving and always saving works fine, but 
occasional saving can lead to puzzlement.

That "pretty-random" way is as unpredictable as pseado-random numbers 
(It uses a PRNG internally.)

> with each call to rDIST().
>
> For example,
>
> set.seed(1)
> x1 <- .Random.seed
> rnorm(1)
> x2 <- .Random.seed
> rnorm(1)
> x3 <- .Random.seed
>
> identical(x1, x2)
> FALSE
>
> identical(x1, x3)
> FALSE
>
> identical(x2, x3)
> FALSE
>
> set.seed(1)
> identical(x1, .Random.seed)
> TRUE
>
> rnorm(2)
> identical(x3, .Random.seed)
> TRUE
>
> But the period for the random seed to repeat is very, very long so you
> don't have to think about it unless you really need to (or for
> reproducible simulations)
>
> Michael
>
> On Sat, Nov 5, 2011 at 7:22 PM, Md Desa, Zairul Nor Deana Binti
> <zndeana at ku.edu> wrote:
>> Thank you everybody for the helpful advices.
>> Basically, I try to figure out why I get different numbers as there are more than one seed for a loop within a loop. Well, I guest I got it now. Because every time random seed is called or specified it'll output different random numbers, as it's requested.
>>
>> Thanks!
>>
>> D
>> ________________________________________
>> From: r-help-bounces at r-project.org [r-help-bounces at r-project.org] on behalf of Rolf Turner [rolf.turner at xtra.co.nz]
>> Sent: Saturday, November 05, 2011 3:22 PM
>> To: Patrick Burns
>> Cc: r-help at r-project.org; Achim.Zeileis at uibk.ac.at
>> Subject: Re: [R] set seed for random draws
>>
>> On 05/11/11 22:00, Patrick Burns wrote:
>>
>> <SNIP>
>>> I'd suggest two rules of thumb when coming
>>> up against something in R that you aren't
>>> sure about:
>>>
>>> 1. If it is a mundane task, R probably
>>> takes care of it.
>>>
>>> 2. Experiment to see what happens.
>>>
>>>
>>> Of course you could read documentation, but
>>> no one does that.
>>
>> <SNIP>
>>
>> Fortune nomination!
>>
>>     cheers,
>>
>>         Rolf
>>
>> ______________________________________________
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>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>
>>
>> ______________________________________________
>> R-help at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595