Parfor sliced variable with nonuniform sized slices

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Andy Hsu
Andy Hsu 2022-4-5
回答： Vatsal 2023-11-17
Hi all,
I am having some trouble with implementing a parfor loop into my code -- my challenge can be summed up with the following code:
matrix = zeros(20, 2);
firstIndex = [1 4 10 19];
maximumLength = [3 6 9 2];
parfor i = 1:length(maximumLength)
randLength = randi(maximumLength(i));
firstIndex_ = firstIndex(i);
matrix(firstIndex_:firstIndex_+randLength-1,:) = zeros(randLength,2);
end
MATLAB informs me that due to the way that matrix is indexed, this is an invalid parfor loop. I understand that the issue boils down to the fact that the "slices" of matrix are of nonuniform sizes and can be easily solved using a cell array. However, is there a way to resolve this issue without using a cell array and keeping matrix as, well, a matrix?

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回答（1 个）

Vatsal
Vatsal 2023-11-17
Hi,
I understand that you are facing a problem due to the non-uniform slicing of the matrix in the parfor loop. There is no straightforward way to resolve this issue while keeping the matrix as a matrix. You can create a temporary matrix, perform the operations, and then assign the results back to the main matrix after the loop.
I am also attaching the modified code below: -
matrix = zeros(20, 2);
firstIndex = [1 4 10 19];
maximumLength = [3 6 9 2];
tempMatrix = cell(1, length(maximumLength));
parfor i = 1:length(maximumLength)
randLength = randi(maximumLength(i));
firstIndex_ = firstIndex(i);
tempMatrix{i} = zeros(randLength,2);
end
for i = 1:length(maximumLength)
firstIndex_ = firstIndex(i);
matrix(firstIndex_:firstIndex_+size(tempMatrix{i},1)-1,:) = tempMatrix{i};
end
I hope this helps!

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