Variable classification error while storing structure within parfor

Question

I have a function which runs a parfor loop. Within the loop, I call another function which generates a structure as a result. I need to store all the structures.

function myFunction(arguments)
    % do some preliminary calcultions
    parfor i = 1:num_sim % number of simulations
        name = sprintf('result_%i',i)
        % do some calculations and generate a structure as a result called "struct_result"
        total_results.(name) = struct_result
    end
end

This gives me an error message:

The variable total_results in a parfor cannot be classified.

How can I store the structure "struct_result" from all the simulations? It is a nested structure.

Edric Edric · Accepted Answer · 2019-10-08T08:18:07

The problem here is that you are assigning to part of total_results during the loop, but not in a "sliced" manner. It's probably simpler to collect up the names and values separately, and then use cell2struct after the parfor loop, like this:

N = 10;
names = cell(1, N);
results = cell(1, N);
parfor i = 1:10
    name = sprintf('result_%i',i)
    names{i} = name;
    results{i} = struct('a', rand(i), 'b', rand(i));
end
total_results = cell2struct(results, names, 2);

EDIT another way of slicing the outputs (as suggested in the comments) is to use

parfor i = 1:10
    total_results(i).result = struct('a', rand(i), 'b', rand(i));
end

In this case, this works because the first-level indexing expression is a "sliced" expression.

Variable classification error while storing structure within parfor

1 Answers