How to make a matrix from trace of the original matrix ?

When I have a square matrix A like the one in the picture, I want to create a matrix B which is the sum of the kth diagonal component to the right below itself, is there any easy way to write this?

Currently, I am calculating in the form of creating reduced matrices and getting the jth diagonal component of each reduced matrix, but I feel it is quite slow and inefficient.

A = reshape(1:9,3,3);
B = zeros(3,3);
for i = 1:3
    A_partial = A(i:end,i:end);
  
    for j = 1:3-i+1
        B(i,i+j-1) = sum(diag(A_partial,j-1));
        B(i+j-1,i) = sum(diag(A_partial,-1*(j-1)));
    end
end
A
B

Answer 1

[Dyuman Joshi]

Building on your approach - Breaking the matrix into smaller matrices and assigning the diagonal sums to the corressponding elements.

y=spiral(3)

y = 3×3

7 8 9 6 1 2 5 4 3

mat=y;
for i=1:size(y,1)-1
    for j=1:size(y,2)-1
        mat(i,j)=sum(diag(y(i:end,j:end)));
    end
end
mat

mat = 3×3

11 10 9 10 4 2 5 4 3

[Yamada Taro]

Thank you!

It seems to work well and more sofisticated than my code.