Jeroen Ooms <jeroenooms <at> gmail.com> writes:
>
> Is there an easy way to turn a vector of length n into an n by n matrix, in
> which the diagonal equals the vector, the first off diagonal equals the
> first order differences, the second... etc. I.e. to do this more
> efficiently:
>
> diffmatrix <- function(x){
> n <- length(x);
> M <- diag(x);
> for(i in 1:(n-1)){
> differences <- diff(x, dif=i);
> for(j in 1:length(differences)){
> M[j,i+j] <- differences[j]
> }
> }
> M[lower.tri(M)] <- t(M)[lower.tri(M)];
> return(M);
> }
>
> x <- c(1,2,3,5,7,11,13,17,19);
> diffmatrix(x);
>
I do not know whether you will call the appended version more elegant,
but at least it is much faster -- up to ten times for length(x) = 1000,
i.e. less than 2 secs for generating and filling a 1000-by-1000 matrix.
I also considered col(), row() indexing:
M[col(M) == row(M) + k] <- x
Surprisingly (for me), this makes it even slower than your version with
a double 'for' loop.
-- Hans Werner
# ----
diffmatrix <- function(x){
n <- length(x)
if (n == 1) return(x)
M <- diag(x)
for(i in 1:(n-1)){
x <- diff(x) # use 'diff' in a loop
for(j in 1:(n-i)){ # length is known
M[j, i+j] <- x[j] # and reuse x
}
}
M[lower.tri(M)] <- t(M)[lower.tri(M)]
return(M)
}
# ----
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