# For what it's worth (was Re: [R] Permutations).

Rolf Turner rolf at math.unb.ca
Wed Jul 14 00:37:05 CEST 2004

```For what it's worth, here is a mild revision of my restr.perm()
function, which seems NOT to fall over.  I.e. it appears to
``reliably'' generate restricted permutations.

Whether these are genuinely ***random*** restricted permutations
(i.e. does each restricted permutation of 1:12 have the same
probability of being generated?) is not clear to me.  So ***DON'T
TRUST IT***!!!

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restr.perm <- function ()
{
S <- 4:12
G <- NULL
A <- list(1:3,4:6,7:9,10:12)
for(k in 1:4) {
for(i in A[[k]]) {
tmp <- union(i,S)
tmp <- setdiff(tmp,G)
if(length(tmp)==0) return(Recall())
x <- if(length(tmp)==1) tmp else sample(tmp,1)
G <- c(G,x)
S <- setdiff(S,G)
}
S <- union(S,A[[k]])
R <- if(k < 4) A[[k+1]] else NULL
R <- union(R,G)
S <- setdiff(S,R)
}
G
}
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cheers,

Rolf Turner
rolf at math.unb.ca

```