# [R] calculations on diagonals of a matrix

Alec Stephenson a.stephenson at lancaster.ac.uk
Sun May 20 01:06:30 CEST 2001

```oops, that should've been

>
> you could index the elements explicitly via
>
> index.matrix <-
> matrix(c(seq(to=i,len=min(i,j)),seq(to=j,len=min(i,j))),ncol=2)
>
> and so b_ij = sum(a.matrix[index.matrix]), or you could use the function
> diag on the appropriate sub matrices of a.matrix.
>
>
> Alec Stephenson                            tel +44 (0) 1524 593950
> Department of Mathematics and Statistics   fax +44 (0) 1524 592681
> Lancaster University            email a.stephenson at lancaster.ac.uk
> Lancaster LA1 4YF
>
> 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?
> >
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