How to creat a four dimentional (from a vector) matrix and reset it's 'lower triangle'

Hi,

I am trying to get a 4 dimentional matrix out of a vector and then reset it's 'lower triangel'. for example, if my original vector is two dimentional: A = [1 2]' then I would like my initial matrix to be:

    C(:,:,1,1) = [1*1*1*1 1*1*1*2 ; 1*1*2*1 1*1*2*2] = [ 1 2 ; 2  4]
    C(:,:,2,1) = [2*1*1*1 2*1*1*2 ; 2*1*2*1 2*1*2*2] = [ 2 4 ; 4  8]
    C(:,:,1,2) = [1*2*1*1 1*2*1*2 ; 1*2*2*1 1*2*2*2] = [ 2 4 ; 4  8]
    C(:,:,2,2) = [2*2*1*1 2*2*1*2 ; 2*2*2*1 2*2*2*2] = [ 4 8 ; 8 16]

So C is:

    C(:,:,1,1) = [ 1 2 ; 2  4]  C(:,:,2,1) = [ 2 4 ; 4  8]
    C(:,:,1,2) = [ 2 4 ; 4  8]  C(:,:,2,2) = [ 4 8 ; 8 16]

and after reset I would like it to be:

    C(:,:,1,1) = [ 1 2 ; 2  4]  C(:,:,2,1) = [ 0 0 ; 0  0]
    C(:,:,1,2) = [ 0 0 ; 4  8]  C(:,:,2,2) = [ 0 0 ; 8 16] 

 shotrly, I want no rows repetitions.

I tried the following code:

    A = [1 2]';
    C = bsxfun(@times, permute(C, [4 3 2 1]), C*C');
    disp('C before reset is:');
    disp(C);

    for k = 2:size(C, 4)
      C(1:k-1,:,k) = 0;
    end

    disp('C after reset is:');
    disp(C);
    disp('The size of C is:');
    disp(size(C));

But the output is:

    BB before reset is:
    (:,:,1,1) =
         1     2
         2     4
    (:,:,1,2) =
         2     4
         4     8
    C after reset is:
    (:,:,1,1) =
         1     2
         2     4
    (:,:,1,2) =
         0     0
         4     8
    The size of BB is:
         2     2     1     2

What did I miss? I think I don't understand what is behind the line:

    C = bsxfun(@times, permute(C, [4 3 2 1]), C*C');

what is the meaning of each number in the row [4 3 2 1]?

Thanks!

Answer 1

[Alexandra Harkai]

permute

[JazzMusic]

I (obviosley) looked there before asking this question... but they only talk about 2 dimentional permutations. of course, [2 1] is like doinge transpose, but this doesn't help mh in higher dimentions like: permute([1 2]',[4 3 2 1]) I can't figure what does the permute function is doing in such situations...

thanks anyway

Answer 2

[Guillaume]

I've not tried to understand exactly what you're doing but this produces the right result:

A = [1 2];
k1 = kron(A.', A);
B = reshape(kron(k1, k1), [], 2);  %to be reshape later
%duplicate only 1st instance of each row
C = zeros(size(B));
[~, irows] = unique(B, 'rows');
C(irows, :) = B(irows, :);
%reshape into final matrix
C = permute(reshape(C.', [2 2 2 2]), [2 1 3 4]);