Question

What would experienced R developers consider the most efficient (yet still readable) way to construct a matrix with a given number of rows and columns from a given function, such that e.g. A_ij = someFun(i,j) with 1 <= i <= rows, 1 <= j <= cols?

Since I couldn’t find something in the documentation I came up with

initMatrix <- function(rows, cols, fn) {
    A <- matrix(nrow=rows, ncol=cols)
    for (i in 1:rows)
        for (j in 1:cols)
            A[i,j] <- fn(i,j)
    return(A)
}

which seems silly and slow to me. Any improvements (particularly one-liners) welcome! 🙂

Answer 1

I think you’re looking for outer(seq(rows),seq(cols),fn) (or as suggested below, outer(seq_len(rows),seq_len(cols),fn): would need some examples to see how much difference that made).

You can gain a lot in readability (at least if you don’t have to go look up ?outer to find out what’s going on) this way, but I don’t actually think you save much time. Something cleverer and more efficient might be possible if your fn is vectorized to begin with: is it?