problem with parfor loop

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freebil 2013-8-24

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https://ww2.mathworks.cn/matlabcentral/answers/85523-problem-with-parfor-loop

I have the following code

 parfor edge = 1:1000
    var_node = edge_2_var_node(edge, num_of_variable_nodes)  
    check_node = ceil(edge_perm(edge)/dc);                                              
    H(check_node,var_node) = 1;    
 end

but when i try to run it, i have this "The variable H in a parfor cannot be classified." Could anyone help me? Thanks

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Walter Roberson 2013-8-24

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You will have difficulty with that set-up. parfor has to assume the possibility that two different edge_2_var_node lookups might give back the same value, leading to the possibility that different parfor cycles might be trying to write into the same H() location.

Is edge_2_var_node a function or an array? Are the edge_perm() values certain to be different by more than dc so that check_node values are certain to be unique? What can you give us that would allow it to be proven that no two H(check_node,var_node) references will be to the same location ?

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freebil 2013-8-27

Thanks for the answer. The maximum var_node value is 500000 and the maximum check_node value is 250000. Before this loop run H is sparse with zero elements and its size is 250000x500000. I appreciate your help.

Walter Roberson 2013-8-27

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sparse with zero elements -- so nothing in it, right? In that case you need not create it ahead of time, and can instead use

H = mod( accumarray([check_node(:), var_node(:)], 1, size(H), [], [], true), 2);

You might also want to experiment with

H = accumarray([check_node(:), var_node(:)], 1, size(H), @(X) mod(sum(X),2), [], true);

I would expect this to turn out to be slower because of the many calls to the anonymous function, but at the same time it might end up using less temporary memory.

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freebil 2013-8-26

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Thanks for your answer. Actually, the code is

 for edge = 1:sum(num_of_edges)            
    var_node = edge_2_var_node(edge, num_of_variable_nodes, num_of_edges, degrees);    
    check_node = ceil(edge_perm(edge)/dc);                                             
    if (H(check_node,var_node) == 0)      
        H(check_node,var_node) = 1;       
    else                                   
        H(check_node,var_node) = 0;        
    end
 end