Parfor sliced variable with nonuniform sized slices

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?

Answer 1

[Vatsal]

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!