R help - Aug 2010 - How to obtain seed after generating random number?

Dear all, I was doing an experiment to disprove some theory therefore
performing lot of random simulation. Goal is to show the audience that
although something has very rare chance to occur but it doesn't mean that
event would be impossible.

 

In this case after getting that rare event I need to show that same scenario
for multiple times to explain other audience. Hence I need to somehow save
that seed which generates that random numbers after doing the experiment.
However as it is very rare event it is not very practical to start with a
fixed seed and then generate random numbers. Hence I am looking for some way
which will tell me about that corresponding seed which was responsible to
generate that particular series of random numbers responsible for occurrence
of that rare event.

 

In short, I need to know the seed ***after*** generating the random numbers.

 

Is there any possibility to know this?

 

Thanks and regards,


	[[alternative HTML version deleted]]

I have wondered this in the past too so thanks for the question.

On Aug 24, 2010, at 10:11 AM, Bogaso Christofer wrote:

Dear all, I was doing an experiment to disprove some theory therefore
performing lot of random simulation. Goal is to show the audience that
although something has very rare chance to occur but it doesn't mean that
event would be impossible.



In this case after getting that rare event I need to show that same scenario
for multiple times to explain other audience. Hence I need to somehow save
that seed which generates that random numbers after doing the experiment.
However as it is very rare event it is not very practical to start with a
fixed seed and then generate random numbers. Hence I am looking for some way
which will tell me about that corresponding seed which was responsible to
generate that particular series of random numbers responsible for occurrence
of that rare event.



In short, I need to know the seed ***after*** generating the random numbers.



Is there any possibility to know this?



Thanks and regards,


	[[alternative HTML version deleted]]

______________________________________________
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.

If you find yourself doing things like this often, but don't want to
explicitly set the seed, or save seeds before simulating, then you can run the
following code (or put it into .Rprofile or similar):

.Last.Random.seed <- .Random.seed

addTaskCallback( function(expr, val, ok, visible){
	if(!isTRUE( all.equal(.Last.Random.seed, .Random.seed)) ) {
		.Last.Random.seed <- .Random.seed
	}
	TRUE
	})

Then the previous seed will be stored in .Last.Random.seed and you can restore
the seed to be able to rerun the same values, e.g.:
> rnorm(10)
 [1] -0.28361138  0.86951931 -0.54435528  0.62880324 -1.42233446 -1.22751263
 [7] -1.67410552  0.08439848 -0.20612566  1.44187164
> .Random.seed <- .Last.Random.seed
> rnorm(10)
 [1] -0.28361138  0.86951931 -0.54435528  0.62880324 -1.42233446 -1.22751263
 [7] -1.67410552  0.08439848 -0.20612566  1.44187164
> rnorm(10)
 [1] -0.0417821  1.3537545  1.9452253 -0.4909382  0.3884391 -0.8448933
 [7]  0.7379904 -1.0797603 -1.0264739  0.2887934

The above code only keeps the most recent seed, but the code could be modified
to store a longer history if that were desired.


If you want to set your own seeds (a little easier to save, pass to others), but
find integers to unimaginative, then look at the char2seed function in the
TeachingDemos package.

Hope this helps,

-- 
Gregory (Greg) L. Snow Ph.D.
Statistical Data Center
Intermountain Healthcare
greg.snow at imail.org
801.408.8111

> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of Bogaso Christofer
> Sent: Tuesday, August 24, 2010 11:12 AM
> To: r-help at r-project.org
> Subject: [R] How to obtain seed after generating random number?
> 
> Dear all, I was doing an experiment to disprove some theory therefore
> performing lot of random simulation. Goal is to show the audience that
> although something has very rare chance to occur but it doesn't mean
> that
> event would be impossible.
> 
> 
> 
> In this case after getting that rare event I need to show that same
> scenario
> for multiple times to explain other audience. Hence I need to somehow
> save
> that seed which generates that random numbers after doing the
> experiment.
> However as it is very rare event it is not very practical to start with
> a
> fixed seed and then generate random numbers. Hence I am looking for
> some way
> which will tell me about that corresponding seed which was responsible
> to
> generate that particular series of random numbers responsible for
> occurrence
> of that rare event.
> 
> 
> 
> In short, I need to know the seed ***after*** generating the random
> numbers.
> 
> 
> 
> Is there any possibility to know this?
> 
> 
> 
> Thanks and regards,
> 
> 
> 	[[alternative HTML version deleted]]
> 
> ______________________________________________
> 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.

It is nearly impossible to go back after the fact and figure out the
seed you started with. So, you need to be careful to record the seed
first. If you are doing your simulations with a function then it is a
good idea to always start in the function by saving a record of the seed
and returning it with the result. Then you always have the information
you need.

Beware that the seed is not the only thing you need to save. You also
need to record the uniform generator you are using, and the generator
for the distribution you are using. Of course, if you always only use
the default then you won't have a problem here, until the default
changes.

You might want to look at the utilities and examples in the package
setRNG.

Paul
>-----Original Message-----
>From: r-help-bounces at r-project.org
[mailto:r-help-bounces at r-project.org]
>On Behalf Of Bogaso Christofer
>Sent: August 24, 2010 1:12 PM
>To: r-help at r-project.org
>Subject: [R] How to obtain seed after generating random number?
>
>Dear all, I was doing an experiment to disprove some theory therefore
>performing lot of random simulation. Goal is to show the audience that
>although something has very rare chance to occur but it doesn't mean
>that
>event would be impossible.
>
>
>
>In this case after getting that rare event I need to show that same
>scenario
>for multiple times to explain other audience. Hence I need to somehow
>save
>that seed which generates that random numbers after doing the
>experiment.
>However as it is very rare event it is not very practical to start with
>a
>fixed seed and then generate random numbers. Hence I am looking for
some
>way
>which will tell me about that corresponding seed which was responsible
>to
>generate that particular series of random numbers responsible for
>occurrence
>of that rare event.
>
>
>
>In short, I need to know the seed ***after*** generating the random
>numbers.
>
>
>
>Is there any possibility to know this?
>
>
>
>Thanks and regards,
>
>
>	[[alternative HTML version deleted]]
>
>______________________________________________
>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.
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