MATLAB parfor index exceeds the number of array elements

Question

Preetham Manjunatha 2020-7-26

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/570619-matlab-parfor-index-exceeds-the-number-of-array-elements

评论： Jungmin kim 2021-6-20

Why MATLAB throws an error even if it shouldn't go to the first case of the switch statement? Below is the minimal example:

mycase = 2;
non_crack_bytes = num2cell(1:6000);
syn_crack_bytes = num2cell(1:10000);
imgCount = 10000;
parfor j = 1 : imgCount
    switch mycase
        case 1
            if ~(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

The error I am getting is:

Error using ScratchPaperFile>(parfor supply)
Index exceeds the number of array elements (6000).
Error in ScratchPaperFile (line 10)
parfor j = 1 : imgCount

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Image Analyst 2020-7-26

Why are you using cell arrays? That's not efficient. Don't do that when you have no reason to.

For me your code just hangs. However if I use "for" instead of "parfor" it finished in a split second.

Preetham Manjunatha 2020-7-26

Even with the a regular array, it throws the same error.

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Answer 1

Edric Ellis 2020-7-27

1
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https://ww2.mathworks.cn/matlabcentral/answers/570619-matlab-parfor-index-exceeds-the-number-of-array-elements#answer_471295

在 MATLAB Online 中打开

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).

To fix this, one way is to introduce 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. One way to achieve that is 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.