Unrecognized function or variable error code help required

I have created code on low rank assumption using nuclear norm using CVX. I am getting the following error code -

Unrecognized function or variable 'A_i'.

Error in Untitled3 (line 21)

y(i_m) == trace(A_i)*Xe;

here is my code any help will be appricated

r = 2; % the rank;
n = 4; % the dimension

a = randn(n,r);
b = randn(n,r);
X = a*b'; % low rank Matrix;

m=fix(n^2-1);
A = randn(n, n);
y = trace(A*X);
% low rank approximation using nuclear norm

cvx_begin
variable Xe(n,m)
minimize norm_nuc(Xe)
subject to
A*Xe == y
cvx_end

     for i_m = 1 : m
   y(i_m) == trace(A_i*Xe);
end

error = norm(X-Xe)

for m = 1 : 10 : n^2
   for mc = 1 : MONTE_CARLO
     % Generate measurements
     y = zeros(m, 1);
for i_m = 1 : m
   A_i = randn(n, n);
   y(i_m) = trace(A_i*X);
end
     % Solve problem

     % Store results
   end
end

Answer 1

[Tayyab Khalil]

This is because A_i is undefined in the code but is being used. What value do you want it to have?

[Karim Wahdan]

I am definding A_i to have random n x n. So im not sure if it has a proper value

[Tayyab Khalil]

That's A, but you are trying to use A_i. If you want to use the ith entry of A, use A(i_m) since you are looping with i_m.

[Jan]

@Karim: You are defining A_i after it is used. Of course it must be defined before you can use it.

[Karim Wahdan]

@Tayyab Khalil A(i_m) wont work because the matrix must be squared, when I implemented A(i_m) it now comes up with an error message saying Matrix must be squared on line 12 (minimize norm_nuc(Xe)) . So in this case would we assign a value to A_i

[Karim Wahdan]

@Jan I see, how would I define it? What code would I have to add?

[Jan]

I cannot guess, what you want A_i to be.

[Karim Wahdan]

@Jan Ok I want A_i to be the value 10. How would I put this into my code and fix my "matrix needs to be squared" error

[Jan]

Of course you should do this by:

A_i = 10;

Please post the complete error message and the corresponding code. I cannot guess where this error occurs.