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MATLAB parfor index exceeds the number of array elements

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Preetham Manjunatha
Preetham Manjunatha 2020 年 7 月 26 日
回答済み: Edric Ellis 2020 年 7 月 27 日
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
Image Analyst 2020 年 7 月 26 日
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
Preetham Manjunatha 2020 年 7 月 26 日
Even with the a regular array, it throws the same error.

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Edric Ellis
Edric Ellis 2020 年 7 月 27 日
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.

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