Repeating entries of blkdiag

I created a block tridiagonal matrix sysytem using the code below.

n = 4;
A = [ 6 2 0; 3 6 2; 0 3 6];
B = 7*eye(3);
D =  4*eye(3)
C = kron(eye(3),A);
C(1:end-(n-1),(n-1)+1:end) = C(1:end-(n-1),(n-1)+1:end) + blkdiag(B,B);
C((n-1)+1:end,1:end-(n-1)) = C(1:end-(n-1),(n-1)+1:end) + blkdiag(D,D);

How do i avoid typing B or D twice in blkdiag? Is there a function that can repeat B or D twice or more without having to type B or D twice or more in case my A is a huge matrix?

Thanks

Answer 1

[John D'Errico]

B = ones(2);
BB = repmat({B},1,5);

BDiag = blkdiag(BB{:});
spy(BDiag)

Create a cell array. Then use blkdiak properly, by turning the cell array into a comma separated list. BB{:} does that. Thus...

BB{:}

ans = 2×2

1 1 1 1

ans = 2×2

1 1 1 1

ans = 2×2

1 1 1 1

ans = 2×2

1 1 1 1

ans = 2×2

1 1 1 1

●

[SA]

It worked thanks a lot.

This my new code.

n = 4;

A = [ 6 2 0; 3 6 2; 0 3 6];

B = 7*eye(3);

BB = repmat({B},1,2);

BDiag = blkdiag(BB{:});

D = 4*eye(3);

DD = repmat({D},1,2);

DDiag = blkdiag(DD{:});

C = kron(eye(3),A);

C(1:end-(n-1),(n-1)+1:end) = C(1:end-(n-1),(n-1)+1:end) + BDiag;

C((n-1)+1:end,1:end-(n-1)) = C((n-1)+1:end,1:end-(n-1)) + DDiag;