Matrix of symbols?

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 4x4 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.

[madhan ravi]

T,n,joint,q values? upload full code

[Matteo Sodano]

You're right. I've answered below with the values of interest.

Answer 1

[Walter Roberson]

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.

[Walter Roberson]

The data type for a variable is determined by what is first assigned to it. When you happen to assign numeric [0; 0; 0] first (because you did not pre-allocate the matrix) then the datatype becomes double. Numeric arrays cannot store symbolic expressions, and try to convert them to double precision numbers, but that will fail if the symbolic expressions contain unresolved symbolic variables.

So you work around the problem by initializing to symbolic in the first place.

Answer 2

[Matteo Sodano]

Values of interest:

n = 5;
q = [q1 q2 q3 q4 q5]; %we know that those are symbols from the code i posted before
joint = ['r' 'p' 'p' 'r' 'r'];

I attach an image of the Ts.