MATLAB: Matrix of symbols

Question

I have an init script with some symbols declared:

syms q1 q2 q3 q4 q5 q6

This script calls a function, where I try to build a matrix that depends on those symbols:

function angular_jacobian(T,n,joint,q)
      JA(:,1) = [0; 0; 1];
      for i=2:n
         if joint(i) == 'p'
             JA(:,i) = [0; 0; 0];
         else
             disp(T(1:3,3,i-1));
             JA(:,i) = simplify(T(1:3,3,i-1));
         end
      end
      disp(JA);
end

where T is a vector of 4×4 matrices. I'm just trying to build a 3xN matrix, called JA, that has first column [0;0;1], and then all the others equal to a column of zeros or to T(1:3,3,i-1). The error I get is the following:

The following error occurred converting from sym to double:
Unable to convert expression into double array.

This happens only if T(1:3,3,i-1) has symbols in it (I tried with scalar T(1:3,3,i-1) and it works). Of course every matrix T(1:3,3,i-1) has only declared symbols (I built T with other functions).

Does anybody know what I am ignoring? I have actually no idea.

Answer 1

JA = sym(zeros(3,n));

simplify always returns a sym even if given numeric input, so we can conclude that you always want a sym output.