Nested parfor and for-Loops gives error

Hi,

My code is as follows:

result1 = nan(totaal, 7);
tx_range=tx_min:stepsize_t:tx_max;
totaal_temp=1e6;
parfor t_x_index = 1:numel(tx_range)
    result1_temp = zeros(totaal_temp, 7);
    t_x = tx_range(t_x_index);
    disp(['t_x = ', num2str(t_x)]);
    % multiple for loops which does not interact with "t_x_index" or "t_x". So, this part is basically independent of the parfor variable. 
    % the results of the for loops are saved into a variable named "result1_temp". 
    % now, i want to store the "result1_temp" in "result1":
    indexing = (1:size(result1_temp, 1)) + (t_x_index - 1) * size(result1_temp, 1);
    result1(indexing, :) = result1_temp;
end

Matlab gives me an error and does not allow me to run this. The error says:

Matlab PARFOR loop cannot run due to the way variable "result1" is used. Could someone help with what could be wrong here?

Thanks.

Answer 1

[Shivam]

Hi Moein,

I understand that you are facing an error while indexing into "result1" inside PARFOR loop.

To address this, you can store the results in a temporary variable inside the PARFOR loop and then assign them to the appropriate section of "result1" outside the PARFOR loop.

You can refer the following workaround to work on changes:

% ...

parfor t_x_index = 1:numel(tx_range)
    % ...
    % ...
    
    % Store the "result1_temp" in the temporary variable "result1_temp_combined"
    result1_temp_combined{t_x_index} = result1_temp;
end

% Assign the results from "result1_temp_combined" to the appropriate section of "result1"
for t_x_index = 1:numel(tx_range)
    indexing = (1:size(result1_temp_combined{t_x_index}, 1)) + (t_x_index - 1) * size(result1_temp_combined{t_x_index}, 1);
    result1(indexing, :) = result1_temp_combined{t_x_index};
end

You can refer to the following documentation: https://in.mathworks.com/help/parallel-computing/troubleshoot-variables-in-parfor-loops.html

I hope it helps.