MATLAB parfor index exceeds the number of array elements

@David's answer shows a valid workaround, but the reason it works is not quite all that it seems.

To run a parfor loop, MATLAB analyses each variable used within the loop and classifies them. In the original code, non_crack_bytes is classified as a "sliced input" variable - in other words, MATLAB considers that each iteration of the loop needs a single value from non_crack_bytes corresponding to the loop index j. The error occurs long before the worker tries to read from non_crack_bytes - the error occurs on the client trying to send the elements of non_crack_bytes that it thinks the worker will need. (This is why the error mentions "parfor supply", a bit of internal jargon relating to sending sliced loop inputs).

@David's fix introduces an additional access of non_crack_bytes inside the loop which changes it from a "sliced input" variable to a "broadcast" variable. That means that the client sends the whole of non_crack_bytes to each worker. This is why the error doesn't occur, not the careful indexing. In fact, you can just as well fix the loop like this:

parfor j = 1:imgCount
    size(non_crack_bytes); % access whole of non_crack_bytes
    switch mycase
        case 1
            if non_crack_bytes(j) ~= 0
        ... % etc.

It's the access of the whole of non_crack_bytes that forces the analysis to consider it to be a "broadcast" variable, and it's that that prevents the error.

@Jay-Pi identified the issue in their comment. Matlab is careful with parfor loops, so here you need to make it more explicit that nothing bad can happen.

mycase = 2;
non_crack_bytes = 1:6000;
syn_crack_bytes = 1:10000;
imgCount = 10000;
parfor j = 1 : imgCount
    switch mycase
        case 1
            if j<=numel(non_crack_bytes) & ~(non_crack_bytes(j) == 0)
                % Do something
            else
                continue;
            end
        case 2
            if ~(syn_crack_bytes(j) == 0)
                % Do something
            else
                continue;
            end
    end
end