I need to create a diagonal matrix where the diagonal elements are a vector

The vector should look something like,

[1 2 3 0 0 0
0 1 2 3 0 0
0 0 1 2 3 0
0 0 0 1 2 3];


I know the vector ([1 2 3]) that I wish to 'paste' along the diagonal but I do not know the size of the array so the number of rows would need to be determined by a variable N.

3 Answers
3

You can use spdiags to set the diagonals and have the desired shape:

spdiags

n = 4;
A = full(spdiags(ones(n,1)*[1,2,3],[0,1,2],n,n+2));


This returns:

A =

1 2 3 0 0 0
0 1 2 3 0 0
0 0 1 2 3 0
0 0 0 1 2 3


It's a bit crude, but possible to construct the desired matrix as toeplitz:

a = [1 2 3];
toeplitz([a(1); zeros(length(a),1)],[a(:); zeros(length(a),1)])


with answer:

ans =

1 2 3 0 0 0
0 1 2 3 0 0
0 0 1 2 3 0
0 0 0 1 2 3


You can also use 2D-convolution:

v = [1 2 3];
N = 4;
result = conv2(v, eye(N))


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