Subscripted assignment dimension mismatch

I am receiving this error in my code and I cannot figure out why. Any suggestions would be greatly appreciated.

[M,N,K] = size(X);

for j=1:K,
  mu(j) = mean(X(:,:,j));
  X(:,:,j) = X(:,:,j) - mu(j);
end

This goal is simply to take the mean of the third component and subtract it out from itself. There's a lot more going on in the code but this point is giving me the trouble.

Answer 1

[Geoff Hayes]

Benjamin - put a breakpoint at the line

 mu(j) = mean(X(:,:,j));

and run your code. When the debugger pauses at this line, check to see what is

 mean(X(:,:,j))

Is it a scalar or a 1xN array which has the mean of each column? What are you expecting it to be?

I suspect, given your statement, that you are assuming that it is a scalar that you can then subtract out from every other element, but that isn't the case. If this is true, then what you can do is either

 mu(j) = mean(mean(X(:,:,j)));

or

 T = X(:,:,j);
 mu(j) = mean(T(:));

See mean for details on this function.

[Benjamin Anderson-Sackaney]

I see what you're saying and looking at this with a fresh mind, the problem makes sense to me now. Thanks!