How to assign symbolic values to a 3D array in a for loop?

I want to assign symbolic vallues of a 2D array to a 3D array in a for loop.

 % % C = Coriolis(M,qdot,n)
% Returns Christoffels symbols build from
% - n = degrees of freedom (numeric. In this case n=6)
% - nxn matrix M (6x6 very complex symbolic matrix)
% - qdot (0x6 symbolic column vector)
function C = Coriolis(M,qdot,n)
C = sym(zeros(n,n));
c = sym(zeros(n,n,n));
dM = simplify(diff(M));
for i=1:n
    for j=1:n
        for k=1:n
            c(i,j,k) = 0.5*(dM(i,j)+dM(i,k)-dM(k,j));            
        end
    end
end
for i=1:n
   C(:,i) = c(:,:,i)*qdot;
end
end

As you can see I want to increase the numeric variables i, j and k from 0 to n to create all possible combinations. I am sure this can be done in a more efficient way, but this is not the problem here. I was able to run this function correctly for the numeric version. The symbolic version triggers the following set of errors:

 Error using sub2ind (line 52)
Out of range subscript.
Error in sym/subsref (line 766)
                R_tilde = sub2ind(size(L), Idx.subs{:});
Error in Coriolis (line 12)
            c(i,j,k) = 0.5*(dM(i,j)+dM(i,k)-dM(k,j));

Answer 1

[Walter Roberson]

You assume that your dM is n by n, but we do not at present have evidence that it is really that size. It would be more robust to extract the limits of the matrices by size(dM)

We also do not know for sure that M is symbolic; if it is numeric then diff(M) is not going to be the expected size. It is more robust to use the two-argument form of diff(), passing in the symbolic variable to be differentiated with respect to, so that you at least get an error message if M is not symbolic.

[Luc Ahrens]

I changed the code by adding q, delete dM and chance c(i,j,k):

 % C = Coriolis(M,q,qdot,n)
% Returns christoffels symbol build from
% - n = degrees of freedom
% - nxn matrix M
% - q = joint configurations (0x6 symbolic column vector)
% - qdot (0x6 symbolic column vector)
function C = Coriolis(M,q,qdot,n)
C = sym(zeros(n,n));
c = sym(zeros(n,n,n));
for i=1:n
    for j=1:n
        for k=1:n
            c(i,j,k) = 0.5*(simplify(diff(M(i,j),q(k)))+simplify(diff(M(i,k),q(j)))-simplify(diff(M(k,j),q(i))));            
        end
    end
end
for i=1:n
   C(:,i) = c(:,:,i)*qdot;
end
end

Similar errors occur:

Error using sub2ind (line 52)
Out of range subscript.
Error in sym/subsref (line 766)
              R_tilde = sub2ind(size(L), Idx.subs{:});
Error in Coriolis (line 12)
          c(i,j,k) =
          0.5*(simplify(diff(M(i,j),q(k)))+simplify(diff(M(i,k),q(j)))-simplify(diff(M(k,j),q(i))));

[Walter Roberson]

At the MATLAB command prompt command

dbstop if error

and then run the program. When it stops, give the command

dbup

repeatedly until you are in the workspace of your Coriolis function. Then show

size(M)
size(q)
n, i, j, k

I notice in your comments that q is a 0 x 6 column vector. If the 0 is accurate then q would be empty and q(k) would be out of range. If the 6 is accurate then q(k) would be out of range if n was 7 or more.

[Luc Ahrens]

sorry, q is 6x1

[Luc Ahrens]

I was able to debug everything. Some dimensions did indeed not match. Thank you.