parfor loop including cell array

In a section of my code, I intend to do parallel computing for cells: This section is as followed

SpaceNeighbInfo= cell(length(StatesMat),4);
parfor ii=1:length(StatesMat)
    [NeigbStates,NeigbStatesIdx,MLNeigb,MRNeigb]=NeigborStates(NJ,StatesMat,StatesMat(ii,:));% a function that has 4 matrix outputs
    SpaceNeighbInfo{ii,1}=NeigbStates;
    SpaceNeighbInfo{ii,2}=NeigbStatesIdx;
    SpaceNeighbInfo{ii,3}=MLNeigb;
    SpaceNeighbInfo{ii,4}=MRNeigb;
end

I recieve the following error:

The variable SpaceNeighbInfo in a parfor cannot be

classified.

See Parallel for Loops in MATLAB, "Solve Variable

Classification Issues in parfor-Loops".

But If I commentize the following lines the code can be compiled.

SpaceNeighbInfo{ii,2}=NeigbStatesIdx;
SpaceNeighbInfo{ii,3}=MLNeigb;
SpaceNeighbInfo{ii,4}=MRNeigb;

I am confused, would anyone help me with this?

I thank you very much in advance.

Best regards,

Omid

Answer 1

[Matt J]

The way to work around this is to write the loop as,

parfor ii=1:length(StatesMat)
    
    [SpaceNeighbInfo{ii,:}]=NeigborStates(NJ,StatesMat,StatesMat(ii,:));% a function that has 4 matrix outputs

end

The reason for the problem is that you are trying to make SpaceNighbInfo a slived variable, but the Fixed-Index Listing restriction on sliced variables is violated. From the documentation:

[Omid Sharifi Sedeh]

Thank you very much for your kind answer. I tried what you mentioned but It didn't work for me.

But, the way I could finally resolve it is as the following:

SpaceNeighbInfo = cell(length(StatesMat),4);

SpaceNeighbInfo1= cell(length(StatesMat),1);

SpaceNeighbInfo2= cell(length(StatesMat),1);

SpaceNeighbInfo3= cell(length(StatesMat),1);

SpaceNeighbInfo4= cell(length(StatesMat),1);

parfor ii=1:length(StatesMat)

[NeigbStates,NeigbStatesIdx,MLNeigb,MRNeigb]=NeigborStates(NJ,StatesMat,StatesMat(ii,:));

SpaceNeighbInfo1{ii}=NeigbStates;

SpaceNeighbInfo2{ii}=NeigbStatesIdx;

SpaceNeighbInfo3{ii}=MLNeigb;

SpaceNeighbInfo4{ii}=MRNeigb;

end

SpaceNeighbInfo(:,1)=SpaceNeighbInfo1;

SpaceNeighbInfo(:,2)=SpaceNeighbInfo2;

SpaceNeighbInfo(:,3)=SpaceNeighbInfo3;

SpaceNeighbInfo(:,4)=SpaceNeighbInfo4;

[Matt J]

That would indeed fix it, but it's terribly unnecessary, as the following simplified example should demonstrate,

SpaceNeighbInfo=cell(6,4);

parfor ii=1:size(SpaceNeighbInfo,1)
    [SpaceNeighbInfo{ii,:}]=NeighborStates(ii)  % a function that has 4 matrix outputs
end

SpaceNeighbInfo,

SpaceNeighbInfo =

6×4 cell array

{[1]} {[11]} {[21]} {[31]}

{[2]} {[12]} {[22]} {[32]}

{[3]} {[13]} {[23]} {[33]}

{[4]} {[14]} {[24]} {[34]}

{[5]} {[15]} {[25]} {[35]}

{[6]} {[16]} {[26]} {[36]}

function [varargout]=NeighborStates(ii)
 varargout={ii,ii+10,ii+20,ii+30};
end

[Omid Sharifi Sedeh]

Thank you very much. Indeed this time it worked nicely! The problem was with my Matlab!

After a restart, your command worked like a charm!

best regards,

Omid