How to convert a 1D array to a 2D array (indexed by 1)?

A very similar question has already been asked here. However, in the answer, the author assumed that the indexes started from 0, which is not the case of the problem I am currently working on.

So let's say you have an array of 9 elements:

a = [1,2,3,4,5,6,7,8,9]

and you want to flatten this vector by calculating the corresponding indices (i, j) of the table

A = [[1,2,3], [4,5,6], [7,8,9]]

Then, I know that, given (i, j), if we want to find the corresponding index of the vector, we have
a[N(i-1)+j] = One[i][j]

where i and j go from 1 to N. I want to know how to go in the other direction, that is to say the given index of the table a, how to find the corresponding index on the matrix A.

If we are given a[k] and we want to find I and j then we could try

j = k% N


i = int ((k-j) / N + 1)

The problem here is that j is the rest and can therefore take values ​​of 0 at N-1. I guess it would be nice to see a technique to change the clues.