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?
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
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