MATLAB: All combinations of matrix elements

Question

Hello,

I'd like to find all combinations of 2 elements in a matrix.

For example, a = [1:4; 5:8; 9:12]', the result should be [1*1 1*2 1*3 1*4 1*5 … 1*12 2*2 2*3 … 2*12 3*3 … 12*11 12*12]';

Thanks for your help!

Answer 1

Method One: matrix multiply and tril:

You can use a simple matrix multiply to do this:

 >> z = a(:) * a(:)'

and use tril to get the triangular lower portion:

 >> z(tril(true(size(z))))

Method Two: FEX submission combinator:

>> prod(a(combinator(numel(a),2,'c','r')),2)