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Parfor loop variable cannot be classified

Asked by Quentin on 10 Aug 2014
Latest activity Edited by Walter Roberson
on 7 Apr 2016
Hi all,
I'm just not quite clear why this won't work, or what I can do to fix it.
I get the error
Error: File: LabTracker_Serial_Parralel.m Line: 105 Column: 5
The variable microMovie in a parfor cannot be classified.
See Parallel for Loops in MATLAB, "Overview".
But it would seem to me like each loop is independent of the others. So shouldn't parfor be suitable here?
Any thoughts? Thank you
Here is the code,
microMovie = zeros(NumberOfTargets * pointsToSave, 3, numFrames);
parfor i = 1: numFrames
[sortedValues,sortIndex] = sort(ghostFrame(:),'descend');
maxIndex = sortIndex(1:NumberOfFlies * pointsToSave);
[rowsub, colsub] = ind2sub([size(ghostFrame,1) size(ghostFrame,2)], maxIndex);
microMovie(:,1,i) = rowsub;
microMovie(:,2,i) = colsub;
microMovie(:,3,i) = movieFrame(maxIndex);
end
save('microMovie');

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2 Answers

Answer by Jill Reese
on 11 Aug 2014
 Accepted Answer

The parfor variable classificiation is getting confused by the type of indexing you are performing with the microMovie variable. You can work around this by creating a temporary variable.
parfor i = 1: numFrames
[sortedValues,sortIndex] = sort(ghostFrame(:),'descend');
maxIndex = sortIndex(1:NumberOfFlies * pointsToSave);
[rowsub, colsub] = ind2sub([size(ghostFrame,1) size(ghostFrame,2)], maxIndex);
% start of modified code
temp = zeros(NumberOfTargets*pointsToSave,3);
temp(:,1) = rowsub;
temp(:,2) = colsub;
temp(:,3) = movieFrame(maxIndex);
microMovie(:,:,i) = temp;
end

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Answer by Quentin on 11 Aug 2014
Edited by Walter Roberson
on 7 Apr 2016

Thanks so much Jill. After much tinkering that's actually the same conclusion that I came to. I find it surprising that such a simple work around works. Can I ask you for one more.
I am also trying to place some of these variables into a matrix but I get the same error.
For example,
parfor i = 1: numFrames
[sortedValues,sortIndex] = sort(ghostFrame(:),'descend');
maxIndex = sortIndex(1:NumberOfFlies * pointsToSave);
[rowsub, colsub] = ind2sub([size(ghostFrame,1) size(ghostFrame,2)], maxIndex);
% start of modified code
temp = zeros(NumberOfTargets*pointsToSave,3);
temp(:,1) = rowsub;
temp(:,2) = colsub;
temp(:,3) = movieFrame(maxIndex);
microMovie(:,:,i) = temp;
for j = 1:2
some_variable(i,j) = some_math
end
end
Here, I get the error for some_variable. Again this seems to me like it "should" work, since each look is independent. Is there a work around for this? Is there something that I am missing?
Thank you again Jill,
Quentin

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Old thread, I know, but thought I'd do what I could to answer your second question, as I found Jill's answer to the first question very helpful.
I can't reproduce your second problem; please see your code below with some random initialization of the variables... it runs without error in 8.6.0.267246 (R2015b):
ghostFrame = rand(1000,1);
numFrames = 3;
NumberOfFlies = 10;
pointsToSave = 5;
NumberOfTargets = 10;
microMovie = zeros(NumberOfTargets * pointsToSave, 3, numFrames);
movieFrame = rand(10*NumberOfTargets * pointsToSave);
parfor i = 1:numFrames
[sortedValues,sortIndex] = sort(ghostFrame(:),'descend');
maxIndex = sortIndex(1:NumberOfFlies * pointsToSave);
[rowsub, colsub] = ind2sub([size(ghostFrame,1)...
size(ghostFrame,2)], maxIndex);
% start of modified code
temp = zeros(NumberOfTargets*pointsToSave,3);
temp(:,1) = rowsub;
temp(:,2) = colsub;
temp(:,3) = movieFrame(maxIndex);
microMovie(:,:,i) = temp;
for j = 1:2
some_variable(i,j) = 2*pi;
end
end

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