Why do I receive an error message saying: “??? Index exceeds matrix dimensions. Error in ==> spdiags at 114” when using the SPDIAGS function to create a 2-by-1 sparse matrix from a null matrix in MATLAB?

When I execute the following command to create a 2-by-1 sparse matrix from the columns of a null matrix ([]):

spdiags([], 0, 2,1)

I expect MATLAB to return an empty matrix, but I receive the following error message:

 ??? Index exceeds matrix dimensions.

 Error in ==> spdiags at 114

         a((len(k)+1):len(k+1),:) = [i i+d(k) B(i+(m>=n)*d(k),k)];

Answer 1

[MathWorks Support Team]

This is expected behavior in MATLAB. The SPDIAGS function results in an error message saying: "??? Index exceeds matrix dimensions." if the number of rows in the matrix B [ in the syntax "A = spdiags(B,d,m,n)" ] is less than the length of the diagonal of the sparse matrix to be created. Consider the following example:

A = spdiags([1 2], [0 1], 1, 2)  % This is correct syntax as number_of_rows(B) = length_of_diagonal(A)

However, the following code would result in the error message:

A = spdiags([1 2], [0 1], 2, 2)  % This is incorrect syntax as number_of_rows(B) < length_of_diagonal(A) which is 2