Parfor loop classification "fix usage of indicated variable" error

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I'm running a parfor loop and I'm trying to output an array with the columns representing each parfor iteration, and the row elements are converted to a non-zero value at specific locations for each column index. The non-zero element row index is different for every iteration.
The problem: Unable to classify the variable 'Binarizing_Rowsum' in the body of the parfor-loop. For more information, see Parallel for Loops in MATLAB, "Solve Variable Classification Issues in parfor-Loops".
I tried to understand how to fix it by readig the documentation and other answers on the forum but I'm lost. I understand it is considered a sliced variable, however, I don't know how to fix the representation of the variable so that it matchs parfor requirements to get the output I want.
P.S. the correspoding for-loop worked perfectly
Here is the code:
jValues=1:2:length(baseFileNames)-1;
Binarizing_Rowsum=zeros(6100,length(jValues));
LEDdurIndx=NaN(1000,length(jValues));
Light=3000;
parfor idx=1:numel(jValues)
j=jValues(idx);
%Read files
fullFileName=fullfile(thisFolder, baseFileNames{j});
C_data_Array(:,:,idx)= imread(fullFileName,2);
C_data_ArrayMean(:,:,idx)=mean(C_data_Array(:,:,idx),2);
C_data_ArrayStd(:,:,idx)=std(C_data_ArrayMean(:,:,idx));
LEDdetectThresh=(mean(C_data_ArrayMean(:,:,idx),1))+C_data_ArrayStd(:,:,idx);
LEDdurIndx=find(C_data_ArrayMean(:,:,idx)>LEDdetectThresh);
LED_duration_=LEDdurIndx([1,end]); %Updated
LEDinit(idx)=LED_duration(1);%Updated
LEDend(idx)=LED_duration(2);%Updated
Binarizing_Rowsum([LEDdurIndx],idx)=Light; %OUTPUT ERROR: cannot be classified
end

回答(3 个)

Walter Roberson
Walter Roberson 2022-8-17
BR = Binarizing_Rowsum(:,idx);
BR(LEDdurIdx) = Light;
Binarizing_Rowsum(:, idx) = BR;
  5 个评论
Edric Ellis
Edric Ellis 2022-8-19
Inside your loop, you are assigning a whole new value to LEDdurIndx. This makes the variable a parfor "temporary", and so any value assigned inside the loop is discarded. I don't see any such problem for Binarizing_Rowsum though...
Lina Koronfel
Lina Koronfel 2022-8-22
编辑:Lina Koronfel 2022-8-22
@Edric EllisThank you for the comment. Can you help me understand then how do I assign this temporary value of LEDduration to an array?
In this case I'm assigning the value from the temporary LEDdurIndx to LED_duration, and then I'm allocating that to both LEDinit(idx) and LEDend(idx) which I want to output outside the loop. But I don't get the output I expect. What is the most proper way to allocate the value LED_duration and LEDdurIdx outside of the parfor loop?

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Bruno Luong
Bruno Luong 2022-8-19
编辑:Bruno Luong 2022-8-19
LEDdetectThresh=(mean(C_data_ArrayMean(:,:,idx),1))+C_data_ArrayStd(:,:,idx);
LEDdurIndx=find(C_data_ArrayMean(:,:,idx)>LEDdetectThresh)
I suspect the code is buggy, the mean can never be bigger than mean + std
LEDdurIndx is therefore empty.
  1 个评论
Lina Koronfel
Lina Koronfel 2022-8-22
编辑:Lina Koronfel 2022-8-22
Thanks for the comment. The mean in the second line is in the second dimension, however the one in the first line is in the first dimension. It shouldn't be a problem, the code is working perfectly in regular for-loop, but now I'm trying to convert it to parfor loop and that s why I'm facing these problems.

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Bruno Luong
Bruno Luong 2022-8-22
编辑:Bruno Luong 2022-8-22
Your code is good to be called "spagetti". If all the image have the same size, you must tell parfor what they are:
jValues=1:2:length(baseFileNames)-1;
n = numel(jValues);
Binarizing_Rowsum=zeros(6100,n);
LEDdurIndx=NaN(1000,n);
Light=3000;
j=jValues(1);
%Read first file to figure out the array sizes
fullFileName=fullfile(thisFolder, baseFileNames{j});
I1= imread(fullFileName,2);
[p,q]=size(I1);
C_data_Array = zeros(p,q,n);
C_data_ArrayMean = zeros(p,1,n);
C_data_ArrayStd = zeros(1,q,n);
LEDdetectThresh = zeros(1,q,n);
Binarizing_Rowsum = zeros(p*q,n);
parfor idx=1:n
j=jValues(idx);
%Read files
fullFileName=fullfile(thisFolder, baseFileNames{j});
C_data_Array(:,:,idx)= imread(fullFileName,2);
C_data_ArrayMean(:,:,idx)=mean(C_data_Array(:,:,idx),2);
C_data_ArrayStd(:,:,idx)=std(C_data_ArrayMean(:,:,idx));
LEDdetectThresh=(mean(C_data_ArrayMean(:,:,idx),1))+C_data_ArrayStd(:,:,idx);
LEDdurIndx = find(C_data_ArrayMean(:,:,idx)>LEDdetectThresh);
LED_duration_=LEDdurIndx([1,end]); %Updated
LEDinit(idx)=LED_duration(1);%Updated
LEDend(idx)=LED_duration(2);%Updated
Temp = zeros(p*q,1);
Temp(LEDdurIndx) = Light;
Binarizing_Rowsum(:,idx)=Temp;
end

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