How to resolve "The variable in a parfor cannot be classified" error?

When I try to run the following code:

parfor n = 1:size(chan_combos_idx,1)
    for pre_same_z = 1:size(pre_same_snip_all,2)
        pre_same_cpsd(:,pre_same_z,n) = cpsd(pre_same_snip_all(:,pre_same_z,chan_combos_idx(n,1)),pre_same_snip_all(:,pre_same_z,chan_combos_idx(n,2)));
    end
    for pre_diff_z = 1:size(pre_diff_snip_all,2)
        pre_diff_cpsd(:,pre_diff_z,n) = cpsd(pre_diff_snip_all(:,pre_diff_z,chan_combos_idx(n,1)),pre_diff_snip_all(:,pre_diff_z,chan_combos_idx(n,2)));
    end
    for post_same_z = 1:size(post_same_snip_all,2)
        post_same_cpsd(:,post_same_z,n) = cpsd(post_same_snip_all(:,post_same_z,chan_combos_idx(n,1)),post_same_snip_all(:,post_same_z,chan_combos_idx(n,2)));
    end
    for post_diff_z = 1:size(post_diff_snip_all,2)
        post_diff_cpsd(:,post_diff_z,n) = cpsd(post_diff_snip_all(:,post_diff_z,chan_combos_idx(n,1)),post_diff_snip_all(:,post_diff_z,chan_combos_idx(n,2)));
    end
end

I get the error "The variable pre_same_cpsd in a parfor cannot be classified. I think this has something to do with this being a sliced variable, but I'm not sure how to resolve it.

Thanks in advance!

Answer 1

[Walter Roberson]

Inside the parfor loops, you should write to temporary variables like local_same_pre_cpsd(:,pre_samez) with no third subscript. Once you have completed the local variable for loop, write the entire variable into same_pre_cpsd(:,:,n) as one assignment statement. Use the same kind of technique for the other output variables.

You might also need to extract pre_same_snip_all(:,pre_same_z,:) into a local variable, and index that local variable at chan_combos_idx(n,2) and pass that to your function.

In summary, if you have an input variable being accessed with a parfor variable, then you should copy out the entire slice into a local variable and access the slice, and if you have an output variable being accessed with a parfor variable then create a local variable which is a slice, and then afterwards copy the slice to the output variable all at one time.