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

Answers (3)

Walter Roberson
Walter Roberson on 17 Aug 2022
BR = Binarizing_Rowsum(:,idx);
BR(LEDdurIdx) = Light;
Binarizing_Rowsum(:, idx) = BR;
  5 Comments
Lina Koronfel
Lina Koronfel on 22 Aug 2022
Edited: Lina Koronfel on 22 Aug 2022
@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 on 19 Aug 2022
Edited: Bruno Luong on 19 Aug 2022
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 Comment
Lina Koronfel
Lina Koronfel on 22 Aug 2022
Edited: Lina Koronfel on 22 Aug 2022
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 on 22 Aug 2022
Edited: Bruno Luong on 22 Aug 2022
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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