Hello,
The question is a bit confusing.
If you nedd to compute B = X'X, all you have to do is
B <- t(X) %*% X
If you want to compute the other formula, the following avoids loops.
n <- nrow(d)
B <- -d^2/2 - rowSums(d^2)/n - colSums(d^2)/n + sum(d^2)/n^2
Hope this helps,
Rui Barradas
Em 28-01-2013 22:35, Eleonora Schiano escreveu:
I have a matrix d representing distances.
I need to find B=X'X
X=matrix that generated these distances.+
B[i,j]=-(1/2)*{[d[i,j]^2-[(1/n)*sum for i=1...n(d[i,j])^2]-[(1/n)sum for
j=1...n(d[i,j])^2]+[(1/n^2)sum for i=1...n sum for j=1...n (d[i,j])^2]}
it's right if i do
B=matrix(,nrow=dim(d)[1],ncol=dim(d)[2])
for (i in 1:dim(d)[1])
for (j in 1:dim(d)[2]){
B=(-(1/2))*(d[i,j]^2-d[i,]^2-d[,j]^2+d[,]^2)
}
or i have to put
B[i,j]=(-(1/2))*(d[i,j]^2-d[i,]^2-d[,j]^2+d[,]^2)
??
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