Question

I would like to get (and store) the corresponding row and column number in a matrix, eg. to element number 12.

adj.m <- diag(c(3,2,1),nrow = 3, ncol = 3)

now i want to find the row and corresponding to element 4 or 5 or any.
adj.m[4] gives back the value(0), but I want to get and store in a variable the corresponding row and column numbers (1 and 2 in this case).

I assume its a simple command, but I could not formulate the question, that Rseek or Google returns an answer.

Answer 1

If i is your index and m is your matrix, then the row and column of i can be obtained with

get_row <- function(i, m) (i-1) %% ncol(m) + 1
get_col <- function(i, m) ((i-1) %/% ncol(m)) + 1

Usage is, e.g.,

get_row(4, adj.m)
get_col(4, adj.m)

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Actually, Iain’s answer is possibly more natural. You can wrap it into a function as

get_row_col <- function(i, m)
{
  x <- array(seq_along(m), dim = dim(m))
  which(x == i, arr.ind = TRUE)
}

This also works in more than two dimensions, e.g.,

m <- array(1:24, dim = 2:4)
get_row_col(1:24, m)