problem with parfor loop

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

Accepted Answer

Walter Roberson
Walter Roberson on 24 Aug 2013
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 ?
Walter Roberson
Walter Roberson on 27 Aug 2013
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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More Answers (1)

freebil on 26 Aug 2013
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;
H(check_node,var_node) = 0;
And yes, there is possibility that (check_node,var_node) is the same more than 1 time. Is there any possibility to parfor this loop?

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