From: Robin Hankin <rksh_at_soc.soton.ac.uk>

Date: Thu 15 Jul 2004 - 18:14:48 EST

Date: Thu 15 Jul 2004 - 18:14:48 EST

hi again

what a stimulating R discussion! This is R-help at its very best!

I think I understand why you don't want pure inter-block permutations.

My solution would be to realize that weeding out "forbidden" permutations is quite difficult and time-consuming (also as several people have pointed out forbidden permutations are very rare, accounting for only a proportion of (3!)^4/12! ...about one in 400000).

The extra expense of this weeding process is likely to outweigh the slight loss of efficiency caused by generating a forbidden permutation. My solution would therefore be:

x <- c(1,2,3,4,5,6,7,8,9,10,11,12)

x.new <- sample(x)

(note the not-inconsiderable advantage of code simplicity!)

I think my algorithm generated repeats because in
it there are only (4!)^3=13284 distinct

permutations; see help(birthday). The system
above has 12! ~= 4x10^8, much higher.

Hope this helps

Robin

*>
**>}
**>7.- Robin Hankin
**>a<-matrix(1,200,12)
**>for (i in 1:200)
**>{
*

>x <- c(1,2,3,4,5,6,7,8,9,10,11,12)

*>dim(x) <- c(3,4)
**> jj <- t(apply(x,1,sample))
**>a[i,]<-as.vector(jj)
**>}
**>##In 200 permutations, there are 5 repetitions.
**>
**>Thanks to all and sorryfor the confusion that
**>have generated the "intra-block" permutation.
**>
**>Jordi Altirriba
**>PhD student
**>Hospital Clinic - Barcelona - Spain
**>
**>P.S. I think that I don't have forgot to anybody...(sorry if I have done it)
**>
**>_________________________________________________________________
**>¿Cuánto vale tu auto? Tips para mantener tu
**>carro. ¡De todo en MSN Latino Autos!
**>http://latino.msn.com/autos/
*

-- Robin Hankin Uncertainty Analyst Southampton Oceanography Centre SO14 3ZH tel +44(0)23-8059-7743 initialDOTsurname@soc.soton.ac.uk (edit in obvious way; spam precaution) ______________________________________________ R-help@stat.math.ethz.ch mailing list https://www.stat.math.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.htmlReceived on Thu Jul 15 18:21:18 2004

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