Dear R-users,
is there any way to sum up the elements of the "diagonals" of a matrix
without using a for-loop? While there is a simple way over rows and
columns, I don't see a straightforward multiplication for the diagonals,
am I too demanding? Or, more likely, I'm lack some algebra trick? Is
there any R-function that can deal with this problem w/o loop?
Actually I would need to sum up just the upper diagonals.
Here a simple, but general, example for presenting the problem.
Thanks in advance for any help,
Carlo Giovanni Camarda
m <- 7
n <- 5
mat <- matrix(1:35, n, m)
ones.r <- rep(1,n)
ones.c <- rep(1,m)
# sum over the rows
sum.r <- mat%*%ones.c
# sum over the cols
sum.c <- t(mat)%*%ones.r
# sum over the diags
sum.d <- numeric(m+n-1)
sum.d[1] <- mat[n,1]
sum.d[m+n-1] <- mat[1,m]
for(i in 2:n){
sum.d[i] <- sum(diag(mat[(n+1-i):n,1:i]))
}
for(i in 2:(m-1)){
sum.d[i+n-1] <- sum(diag(mat[,i:m]))
}
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