GMM estimation -> nested for loop in while loop

Hello everyone!

I want to estimate the variable "b" using GMM estimation. I already did the first 2 steps manually:

%d)
%step 1
V1=eye(size(Z,2));


%step 2
%I wrote the function GMM_crit for the function that we have to minimize

%estimate beta(1)
beta0=repmat(-0.28,18,1);


options = optimoptions( ...
    @fminunc, ...
    'Algorithm', 'quasi-newton', ...
    'HessUpdate', 'bfgs', ...           
    'MaxIter', 100000, ...
    'MaxFunEvals', 100000, ...
    'TolX', 10^-12, ...
    'TolFun', 10^-12,...
    'OptimalityTolerance',1e-6);


% Estimate beta(1)
[be1] = fminunc(@GMM_crit, beta0,options, N, Y, X, Z, V1 );

% Estimate V(2)
g_tilde1=Z.*Y-Z.*exp(X*be1);
g_bar1=(1/N)*(g_tilde1);
V2=(1/N)*(g_tilde1-g_bar1)'*(g_tilde1-g_bar1);

%step 3
% Estimate beta(2)
[be2] = fminunc(@GMM_crit, be1,options, N, Y, X, Z, V2 );

with the following function GMM crit:

function crit=GMM_crit(be, N, Y, X, Z, V)
crit=(1/N)*(Z'*(Y-exp(X*be)))'*inv(V)*(1/N)*(Z'*(Y-exp(X*be)));
end

To repeat this procedure until the euclidean norm of beta(j+1)-b(j) is smaller than 0.001, I scripted the following loop:

epsilon=0.002;
j=1;



while epsilon>0.001 && j<100
 
      for j=1; 
      
        b(j)=be2  %use my calculated be2 as a starting value
      end 
      for j=j+1;
      %calculate new weighting matrix
        g_tilde(j)=Z.*Y-Z.*exp(X*b(j))
        g_bar(j)=(1/N)*(g_tilde(j))
        V(j+1)=(1/N)*(g_tilde(j)-g_bar(j))'*(g_tilde(j)-g_bar(j))
    % estimate next step beta
       [b(j+1)] = fminunc(@GMM_crit, b(j),options, N, Y, X, Z, V(j+1) )
       
     epsilon=norm(b(j+1)-b(j))
      end 
end

But I always get the following error:

Unable to perform assignment because the indices on the left side are not compatible with the size of the

right side.

I never scripted a loop so I would be really glad if someone could help me.

Thank you guys.