Set element i,j in matrix to i*j in Matlab

Question

I want to generate a matrix in which the i,j element equals i*j, where i != j.

e.g.

0 2 3
2 0 6
3 6 0

So far I've figured out that I can access the non-diagonal elements with this index matrix

idx = 1 - eye(3)

but I haven't figured out how to incorporate the indices of the matrix cells into the computation.

Answer 1

I'm considering the general case (matrix not necessarily square). Let

m = 4; %// number of rows
n = 3; %// number of columns

There are quite a few approaches:

1. Using ndgrid:

   [ii jj] = ndgrid(1:m,1:n);
   result = (ii.*jj).*(ii~=jj);
   

2. Using bsxfun:

   result = bsxfun(@times, (1:m).',1:n) .* bsxfun(@ne, (1:m).',1:n);
   

3. Using repmat and cumsum:

   result = cumsum(repmat(1:n,m,1));
   result(1:m+1:m^2) = 0;
   

4. Using matrix multiplication (added by @GastónBengolea):

   result = (1:m).'*(1:n).*~eye(m,n);
   

Answer 2

How about

N=3;  %size of matrix
A=[1:N]'*[1:N]-diag([1:N].^2)