Correlation funciton to find eigenvalues

Hi I'm trying to do a correlation matrix between 12 matrices in MatLab. I'm using this command:

for j=1:12;
    for i=1:12;
        for k=1:181; 
            A(i,j) = A(i,j)+z(k,i)*z(k,j); %which A= matrix elements
        end
    end
end

but i keep getting this error: Undefined function 'A' for input arguments of type 'double'.

how can I make this work? Thanks

Answer 1

[the cyclist]

In the very first line inside all the for loops, you have A(i,j) on the right-hand side of the equation, before you have defined it. MATLAB doesn't know what to do there.

You perhaps need to initialize A:

A = zeros(12,12);

before this code.

Answer 2

[Cedric]

Why not using CORR2? If your 12 matrices were stored in a cell array M, you would do something like

 n = numel(M) ; 
 C = zeros(n) ;
 for ii = 2 : n
    for jj = 1 : ii-1
       C(ii,jj) = corr2(M{ii}, M{jj}) ;
    end
 end
 C = C + C.' + eye(n) ;

[Nicole]

Because I have to use this other equation for my coastal profile work. (: