On Wed, 12 Sep 2007, Robin Hankin wrote:
> Hello
>
>
> I have X, an n-by-n matrix and want to convert it to Y, an
> n(n-1)/2 -by- n matrix such that each row of Y
> corresponds to an element of the upper diagonal
> of X. Say row k of Y corresponds to [i,j] with i\neq j.
>
> Then Y[i,k] = X[i,j] and Y[j,k] = X[j,i].
> and Y[-c(i,j),k] = NA.
>
> How to do this vectorizedly?
>
If I follow you:
upper.indexes <- which( lower.tri( X ), arr.ind=TRUE )
from.mat <- rbind( upper.indexes, upper.indexes[ , 2:1 ] )
to.mat <- cbind( rep( 1:nrow(upper.indexes), 2 ), as.vector( upper.indexes[,
2:1] ) )
Y[ to.mat ] <- X[ from.mat ]
HTH,
Chuck
> Example follows:
>
>
>
> > X
> [,1] [,2] [,3] [,4]
> [1,] NA 10 8 7
> [2,] 10 NA 7 12
> [3,] 12 13 NA 8
> [4,] 13 8 12 NA
> > Y
> [,1] [,2] [,3] [,4]
> [1,] 10 10 NA NA
> [2,] 12 NA 8 NA
> [3,] 13 NA NA 7
> [4,] NA 13 7 NA
> [5,] NA 8 NA 12
> [6,] NA NA 12 8
> >
>
> [matrix X corresponds to an all-play-all competition amongst 4
> individuals,
> entry [i,j] corresponding to the number of times individual "i" won
> when competing against individual "j". Thus individual 2 beat
> individual
> 3 seven times and individual 3 beat individual 2 thirteen times.
> Note X[i,j] + X[j,i]=20 as there were 20 trials for each pair]
>
>
> Pitiful nonvectorized code follows.
>
> n <- nrow(X)
> Y <- matrix(NA,n*(n-1)/2,n)
> k <- 1
> for(i in 1:(n-1)){
> for(j in (i+1):n){
> if( !(i==j)){
> print(c(i,j,k))
> Y[k,i] <- X[i,j]
> Y[k,j] <- X[j,i]
> }
> k <- k+1
> }
> }
>
>
>
>
>
> --
> Robin Hankin
> Uncertainty Analyst
> National Oceanography Centre, Southampton
> European Way, Southampton SO14 3ZH, UK
> tel 023-8059-7743
>
> ______________________________________________
> 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.
>
Charles C. Berry (858) 534-2098
Dept of Family/Preventive Medicine
E mailto:[EMAIL PROTECTED] UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/ La Jolla, San Diego 92093-0901
______________________________________________
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.
