r - How to find offset diagonal of a matrix? -

is there easy way extract vector of 'offset' , 'reverse' "diagonal" (the x's) of matrix in r?

      [,1] [,2] [,3] [,4] [,5] [1,]    x    0    0    0    0 [2,]    0    0    0    0    x [3,]    0    0    0    x    0 [4,]    0    0    x    0    0 [5,]    0    x    0    0    0 

i tried diag() does'nt seem take options..

1) i-th offset reverse diagonal of square matrix m:

off.rev.diag <- function(m, = 0) m[ (row(m) + col(m) - 1) %% ncol(m) == ] 

for example:

> m <- matrix(1:25, 5); m      [,1] [,2] [,3] [,4] [,5] [1,]    1    6   11   16   21 [2,]    2    7   12   17   22 [3,]    3    8   13   18   23 [4,]    4    9   14   19   24 [5,]    5   10   15   20   25 > off.rev.diag(m, 1) [1]  1 10 14 18 22 > off.rev.diag(m, 2) [1]  2  6 15 19 23 

2) can write replacement function:

"off.rev.diag<-" <- function(m, = 0, value) {      m.ix <- matrix(seq_along(m), nrow(m))     replace(m, off.rev.diag(m.ix, i), value) } 

for example,

> off.rev.diag(m, 1) <- -(1:5) > m      [,1] [,2] [,3] [,4] [,5] [1,]   -1    6   11   16   21 [2,]    2    7   12   17   -5 [3,]    3    8   13   -4   23 [4,]    4    9   -3   19   24 [5,]    5   -2   15   20   25