URGENT!!! Matlab Symbolic Variable Error???

Im tring to substract the diagonal values with eigval and store the new value in the matrix Diagonal :

CovarianceMatrix=[8 -3 1;2 1 0;3 4 5];
 Col=3;
 Row=3;
 store=1;
  syms eigval;

   for loop1= Col:-1:1
    Rw=1;
    syms eigval;
    for loop2= 1:Row
        if Rw==loop1
            Diagonal= (CovarianceMatrix(Rw,loop1)-eigval);
             Fix_Diagonal_2(loop2,store)=Diagonal;
             %Fix_Diagonal_2(loop2,store)=sym(Diagonal); % I already test it
        else
            Diagonal= CovarianceMatrix(Rw,loop1);
            Fix_Diagonal_2(loop2,store)=Diagonal;
        end

        Rw=Rw+1;
        loop1=loop1-1;

        if loop1==0
             loop1=3;
        end

    end
    store=store+1;

   end

That is a part of my program to calculate the eigen value from the matrix. And that code using to keep the second diagonal (det=1st diag- 2nd diag) and then multiply and sum all of the value. So it will be use to be substracted with First Diag Value to get the eigen Value.

But because I'm using a symbolic variable it gives an error:

      The following error occurred converting from sym to double:
      Error using mupadmex
      Error in MuPAD command: DOUBLE cannot convert the input expression into a double
      array.

      If the input expression contains a symbolic variable, use the VPA function      
      instead.

How can I solve this? I would like to copy the new substracted value into the diagonal matrix.

[Walter Roberson]

http://www.mathworks.com/matlabcentral/answers/29922-why-your-question-is-not-urgent-or-an-emergency

Answer 1

[Matt Tearle]

The reason you're getting the error is because one branch of the if construct defines an element of Diagonal directly from CovarianceMatrix, which is a double, but the other branch defines an element of Diagonal from the difference of CovarianceMatrix and eigval. Because eigval is a symbolic variable, the result of this calculation is a symbolic variable as well. Consequently, you're trying to assign a symbolic variable as an element of a double array. That's a type mismatch.

I don't know if this is how you want to proceed, but a simple fix is to make CovarianceMatrix a symbolic variable from the outset:

CovarianceMatrix=sym([8 -3 1;2 1 0;3 4 5]);