[R] matrix diagonal calculations follow up

[Griffith Feeney]

Thanks to Alex Stephenson and Martin Henry H. Stevens for replies. The code
below uses a loop but is quick enough for my purposes, which involve small
numbers of calculations with matrices with numbers of cells in the low 5
figures.

On Sat, 19 May 2001, Griffith Feeney wrote:
> Given an nxm matrix A I want to compute the nxm matrix B whose ij-th 
> element is the sum of the elements of A lying on the diagonal that ends 
> with element ij, i.e.,
> 
>      b_ij = a_ij + a_(i-1)(j-1) + a_(i-2)(j-2) + ...
> 
> In APL (which I no longer use), I would use the 'rotate' operator to derive 
> an array whose columns are diagonals of the given array and then cumulate 
> down columns. Is there a similar operator in R, or is there
another/better way?

> cumprod.diagonals
function (matrix.in) 
{
    dump("cumprod.diagonals", "cumprod.diagonals.R")
    n <- dim(matrix.in)[1]
    m <- dim(matrix.in)[2]
    matrix.out <- matrix(0, nrow=n, ncol=m)
    for (j in 1:m) {
        cols <- j:min(m, n + j - 1)
        rows <- 1:length(cols)
        index.matrix <- matrix(c(rows, cols), ncol = 2)
        matrix.out[index.matrix] <- cumprod(matrix.in[index.matrix])
    }
    for (i in 1:n) {
        rows <- i:min(n, m + i - 1)
        cols <- 1:length(rows)
        index.matrix <- matrix(c(rows, cols), ncol = 2)
        matrix.out[index.matrix] <- cumprod(matrix.in[index.matrix])
    }
    matrix.out
}

(I've switched from sum to product. Repeated calculation for the diagonal
is intentional, as I may want to comment out one or the other loops.)


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