On Nov 16, 2007, at 6:42 PM, Andrew Park wrote:
> Hi there,
>
> I would like to find a more efficient way of permuting the rows and
> columns of a symmetrical matrix that represents ecological or
> actual distances between objects in space. The permutation is of
> the type used in a Mantel test.
>
> Specifically, the permutation has to accomplish something like this:
>
>
> Original matrix addresses:
>
> a11 a12 a13
>
> a21 a22 a23
>
> a31 a32 a33
>
>
> Example permutation
>
> a22 a23 a21
>
> a32 a33 a31
>
> a12 a13 a11
>
> that is relative positions of rows and columns are conserved in the
> permutation.
>
> The problem is, R-users will know, is that using "for" loops like
> this is slow, and gets slower the further into the loop you get.
>
> However, I am not a sophisticated programmer, and cannot think of a
> more efficient way to do this.
Would this work do what you want? (Main point: You can index a vector
by an array):
n <- 3
x <- apply(expand.grid(1:n,1:n),1,paste,collapse="")
x <- paste("a",x,sep="")
dim(x) <- c(n,n)
prm <- sample(1:n)
h<-apply(expand.grid(prm,prm),1,function(x) x[1]+n*x[2]-n)
matrix(x[h],c(n,n))
> Thanks in advance,
>
> Andy Park (University of Winnipeg).
>
Haris Skiadas
Department of Mathematics and Computer Science
Hanover College
______________________________________________
R-help@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.
