Index in position 1 is invalid. Array indices must be positive integers or logical values.

function findalph = impGradDes(M, P)
  [n, m, d] = size(M);
  %% Initializing the optimal weight vector
  findalph = ones(d,1);
  %% Optimization process
  for i=1:d
    for j=1:d
      A(i,j) = sum(sum(M(:,:,i).*M(:,:,j)));
    end
    B(i,1) = sum(sum(P.*M(:,:,i)));
  end
  tau = 5;
  iter = 150000;
  gamma1 = 1/200000;
  gamma2 = 1;
  inv = (eye(d) + 2*tau*gamma1*A)^(-1);
  for i = 1:iter
    findalph = inv * (findalph+2*tau*max(-findalph,0)+2*tau*gamma1*B);
  end
end

[dpb]

Include all the error message in context of the code that generated it.

How are we supposed to guess even which code is generating the error -- altho doesn't look on quick perusal this code would.

The last for loop is pointless--there's nothing in the loop dependent upon the loop variable i

[Stephen23]

"The last for loop is pointless--there's nothing in the loop dependent upon the loop variable i"

Not necessarily pointless: each iteration's findalph value uses the previous iteration's findalph value, which is a perfectly valid, non-pointless construct (whether it is correctly used in this situation is another question entirely). It is not required to use the loop variable in order for a for loop to be useful, a for loop simply repeats some code a set number of times... whether it is pointless or not depends on what the code does, not only on the usage of the loop variable.

A simple demonstration of this, that calculates the exponent of matrix values:

>> N = 5;
>> M = randi(9,5,3)
M =
   9   5   8
   6   8   5
   6   4   7
   8   1   8
   2   2   2
>> Z = M;
>> for k = 2:N, Z = Z.*M; end, Z % k is NOT used!
Z =
   59049    3125   32768
    7776   32768    3125
    7776    1024   16807
   32768       1   32768
      32      32      32
>> M.^N % for comparison
ans =
   59049    3125   32768
    7776   32768    3125
    7776    1024   16807
   32768       1   32768
      32      32      32

[ACHALA SHAKYA]

A(i,j) = sum(sum(M(:,:,i).*M(:,:,j)))

B(i,1) = sum(sum(P.*M(:,:,i)));

The above error is in these lines.

[dpb]

Yeah, I missed seeing the LHS variable also being on RHS, Stephen...

Answer 1

[Bjorn Gustavsson]

OK, when you have a problem that simple (in terms of figuring out what is going on) you do this:

dbstop if error

Then rerun the code, matlab will then stop at the offending line and you get a prompt at that line making it possible for you to inspect the variables to see what's going on. Presumably something strange is happening with either i or j, so you

can just check those variable and the M and P variables too.

HTH