From: Duncan Murdoch <murdoch_at_stats.uwo.ca>

Date: Wed 08 Jun 2005 - 23:53:10 EST

+ tmpSeed <- .Random.seed

+ }

R-help@stat.math.ethz.ch mailing list

https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Thu Jun 09 00:03:26 2005

Date: Wed 08 Jun 2005 - 23:53:10 EST

On 6/8/2005 9:27 AM, Gorjanc Gregor wrote:

> Hello!

*>
**> I am performing coupling of chains in MCMC and I need the same value
**> of seed for two chains. I will show demo of what I want:
**>
**> R code, which might show my example is:
**> niter <- 3
**> nchain <- 2
**> tmpSeed <- 123
**> for (i in 1:niter) { # iterations
**> for (j in 1:nchain) { # chains
**> set.seed(tmpSeed)
**> a <- runif(1)
**> cat("iter:", i, "chain:", j, "runif:", a, "\n")
**> tmpSeed <- .Random.seed
**> }
**> }
**>
**> I get this:
**>
**> iter: 1 chain: 1 runif: 0.43588
**> iter: 1 chain: 2 runif: 0.43588
**> iter: 2 chain: 1 runif: 0.43588
**> iter: 2 chain: 2 runif: 0.43588
**> iter: 3 chain: 1 runif: 0.43588
**> iter: 3 chain: 2 runif: 0.43588
**>
**> but I would like to get:
**>
**> iter: 1 chain: 1 runif: 0.43588
**> iter: 1 chain: 2 runif: 0.43588
**> iter: 2 chain: 1 runif: 0.67676
**> iter: 2 chain: 2 runif: 0.67676
**> iter: 3 chain: 1 runif: 0.12368
**> iter: 3 chain: 2 runif: 0.12368
**>
**> Note that seed value is of course changing, but it is parallel
**> between chains.
*

set.seed takes an integer, but .Random.seed is a complicated vector. You need to play with .Random.seed directly, and move your setting of tmpSeed out of the inner loop, i.e.

> niter <- 3 > nchain <- 2 > set.seed(123) > tmpSeed <- .Random.seed > for (i in 1:niter) { # iterations + for (j in 1:nchain) { # chains + .Random.seed <- tmpSeed + a <- runif(1) + cat("iter:", i, "chain:", j, "runif:", a, "\n") + }

+ tmpSeed <- .Random.seed

+ }

iter: 1 chain: 1 runif: 0.2875775 iter: 1 chain: 2 runif: 0.2875775 iter: 2 chain: 1 runif: 0.7883051 iter: 2 chain: 2 runif: 0.7883051 iter: 3 chain: 1 runif: 0.4089769 iter: 3 chain: 2 runif: 0.4089769

However, heed the warnings in ?set.seed: with some generators .Random.seed *does not* contain the full state of the RNG. As far as I know there is no way to obtain the full state.

Duncan Murdoch

R-help@stat.math.ethz.ch mailing list

https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Thu Jun 09 00:03:26 2005

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