Unable to perform assignment because the size of the left side is 1-by-1 and the size of the right side is 3-by-3.

My goal is to have a 9x9 matrix which looks like

T I 0 0 0 0 0 0 0

I T I 0 0 0 0 0 0

0 I T I 0 0 0 0 0

0 0 I T I 0 0 0 0

0 0 0 I T I 0 0 0

0 0 0 0 I T I 0 0

0 0 0 0 0 I T I 0

0 0 0 0 0 0 I T I

0 0 0 0 0 0 0 I T

0 0 0 0 0 0 0 0 T

Its a 2D matrix and not a multideminsional matrix however, the diagonal elements are another matrix. Below is my code

Can someone tell me what is wrong? Logically?

T = [-4 1 0;1 -4 1;0 1 -4]
I = [1 0 0;0 1 0;0 0 1]

A = zeros(9,9)

for i=1:9
    for j=1:9
        if i==j
            A(i,j)=T
        elseif j==(i+1)
            A(i,j)=I
        elseif j==(i-1)
            A(i,j)=I
        end
        
    end
end

[darova]

Trying to assign a matrix to (1,1) element?

T = [-4 1 0;1 -4 1;0 1 -4]
%% ...
A(i,j)=T

Answer 1

[Stephan]

You could check:

https://de.mathworks.com/matlabcentral/fileexchange/69896-linalgsubstitute

I used this function to build the matrix you expect - as far as i understood you correctly:

T = [-4 1 0;1 -4 1;0 1 -4];
I = [1 0 0;0 1 0;0 0 1];
A = zeros(9,9);

for k = 1:size(T,1):size(A,1)
    A = linalg_substitute(A,T,k,k);
end

for k = 1:size(I,1):size(A,1)
    A = linalg_substitute(A,I,k,k+size(T,2));
end

for k = size(I,1):size(I,1):size(A,1)
    A = linalg_substitute(A,I,k+1,k-size(T,2)+1);
end

which produces:

A =

    -4     1     0     1     0     0     0     0     0
     1    -4     1     0     1     0     0     0     0
     0     1    -4     0     0     1     0     0     0
     1     0     0    -4     1     0     1     0     0
     0     1     0     1    -4     1     0     1     0
     0     0     1     0     1    -4     0     0     1
     0     0     0     1     0     0    -4     1     0
     0     0     0     0     1     0     1    -4     1
     0     0     0     0     0     1     0     1    -4

Is this the desired result?

Answer 2

[Andrei Bobrov]

A = kron(diag([1 1],1),I) + kron(diag([1 1],-1),I) + kron(eye(3),T);