Changing multiple elements (of known coordinates) of a matrix without a for loop

Question

I have a matrix say

Z = [1 2 3;
     4 5 6;
     7 8 9]

I have to change its values, say at positions (2,2) and (3,1), to some specified v

Answer 1

You would like to do this

z(rowNos, colNos)

but you can not - MATLAB does a Cartesian product of the indices. You can do this trick

idx=(colNos-1)*size(z, 1)+rowNos;
z(idx)=0

Flatten the z-matrix and access it through a linear index, which is a combination of rowNos and colNos. Remember that MATLAB flattens the matrix by columns (column-based matrix storage).