How to reset the 'lower triangle' of a 3 dimentional matrix

Hi,

I need to reset the 'lower triangle' of a 3 dimentional matrix. This means, that if the original matrix is:

    C(:,:,1) = [1 2 3 ;  2 4 6  ;  3  6  9]

    C(:,:,2) = [2 4 6 ;  4 8 12 ;  6 12 18]

    C(:,:,3) = [3 6 9 ;  6 12 18 ; 9 18 27]

Then the resulting matrix should be:

    C(:,:,1) = [1 2 3 ;  2 4 6  ;  3  6  9]

    C(:,:,2) = [0 0 0 ;  4 8 12 ;  6 12 18]

    C(:,:,3) = [0 0 0 ;  0 0 0  ;  9 18 27]

Any idea how such a thing csn be done? (My original 3 dim matrix is large)

Thanks!

Answer 1

[Ryan Smith]

Brute force method:

D3 = length(C(1,1,:));
D2 = length(C(1,:,1));
D1 = length(C(:,1,1));
for i = 2:D1
  for j = 1:i-1
    for k = 1:i-1
       C(i,j,k) = 0;
    end
  end
end

Above provides [1 2 3; 0 4 6; 0 0 9] ; [2 4 6; 4 8 12; 0 0 18]; [3 6 9; 6 12 18; 9 18 27], which I believe would be the 'true' lower triangle. Don't quote me on that. To get what you requested, via 'brute force':

b = zeros([1 length(C(1,:,1))]);
for k = 2:D3
  for i = 1:k-1
     C(i,:,k) = b;
  end
end

[JazzMusic]

Thanks!!