Why does MATLAB throw a “too many output arguments” error when I overload subsref (subscripted reference)?

Question

As a toy example, I have a class that simply wraps a vector or matrix in an object and includes a timestamp of when it was created. I\'m trying to overload

Answer 1

I am working on a class to handle polynomials and polynomial matrices. I was having the same dificulty because I want different behaviors for the '.' indexing in the cases of scalar polynomials and polynomial matrices.

In my case I want P.coef to return a vector of coefficients if P is a scalar polynomial. If P is a polynomial matrix, P.coef must return a cell array of the same size of P, in which the cell {i,j} contains the coefficient vector of the polynomial P(i,j).

The problem appeared when P.coef was used with a matrix. My desired behavior returns only one object as an answer, but Matlab is expecting the function to return numel(P) objects.

I found a very simple solution. When declaring subsref, I used one mandatory output and a varargout:

function [R,varargout] = subsref(P,S) 

The body of the function defines R as needed, according to my design. And at the very end of the function I added:

varargout(1:nargout-1) = cell(1,nargout-1);

To just return empty matrices as the extra outputs that Matlab wants.

This should create no problem if the function is always called with a single output argument, e.g., as in R = P.coef. If the function is called without assigning, the user will see numel(P)-1 empty matrices, which is really not a big deal. Anyway, the user is warned about this in the function help.