Use structures in a parfor loop

Hello everyone.

I have a problem using structures in a parfor loop. I would like to save my output of the parfor loop in a structure called "Output".

if strcmpi(String.Frequency_Filter,'Yes')
    WaitMessage = parfor_wait(NN,'Waitbar',true);
    if strcmpi(String.Output_type, 'displacement') || strcmpi(String.Differentiation_Variable, 'Frequency')
        parfor ii=1:NN
            if strcmpi(String.DOF,'X')
                Output.u_G_Interp(ii,:)=bandpass(Output.u_G_Interp(ii,:),B,fs);
            elseif strcmpi(String.DOF,'Y')
                Output.v_G_Interp(ii,:)=bandpass(Output.v_G_Interp(ii,:),B,fs);
            elseif strcmpi(String.DOF,'Z')
                Output.w_G_Interp(ii,:)=bandpass(Output.w_G_Interp(ii,:),B,fs);
            elseif strcmpi(String.DOF,'XYZ')
                Output.u_G_Interp(ii,:)=bandpass(Output.u_G_Interp(ii,:),B,fs); Output.v_G_Interp(ii,:)=bandpass(Output.v_G_Interp(ii,:),B,fs); w_G_Interp(ii,:)=bandpass(Output.w_G_Interp(ii,:),B,fs);
            end
            WaitMessage.Send;
        end
    end
end

Unfortunately (like in the figure below) it is not possible to use directly structures in a parfor loop and I cannot use a temporary variable inside the parfor loop and after writing it inside my structure because the if that i will use in the parfor loop depends on a defind String in the main of my code and not all the variables exists. For example if i have String.DOF='Z' just the w_G_Interp variable exists and I get an error in writing the u_G_Interp variable beacause it doesn't exist.

if strcmpi(String.Frequency_Filter,'Yes')
    WaitMessage = parfor_wait(NN,'Waitbar',true);
    if strcmpi(String.Output_type, 'displacement') || strcmpi(String.Differentiation_Variable, 'Frequency')
        parfor ii=1:NN
            if strcmpi(String.DOF,'X')
                u_G_Interp(ii,:)=bandpass(Output.u_G_Interp(ii,:),B,fs);
            elseif strcmpi(String.DOF,'Y')
                v_G_Interp(ii,:)=bandpass(Output.v_G_Interp(ii,:),B,fs);
            elseif strcmpi(String.DOF,'Z')
                w_G_Interp(ii,:)=bandpass(Output.w_G_Interp(ii,:),B,fs);
            elseif strcmpi(String.DOF,'XYZ')
                u_G_Interp(ii,:)=bandpass(u_G_Interp(ii,:),B,fs); v_G_Interp(ii,:)=bandpass(Output.v_G_Interp(ii,:),B,fs); w_G_Interp(ii,:)=bandpass(Output.w_G_Interp(ii,:),B,fs);
            end
            WaitMessage.Send;
        end
    end
end
Output.u_G_Interp=u_G_Interp;
Output.v_G_Interp=v_G_Interp;
Output.w_G_Interp=w_G_Interp;

I hope the question is clear and thank you in advance for the reply.

Answer 1

[Deepak]

Hi @Davide Mastrodicasa,

We can resolve this by using temporary variables to handle parallel processing within a “parfor” loop. Initialize these variables outside the loop and use a “switch-case” or conditional statements inside the loop to process only the relevant data based on conditions (e.g., “String.DOF”). After the loop, assign the results back to the structure for the fields that were processed, avoiding errors from non-existent fields.

This approach avoids attempting to access or modify non-existent fields during parallel execution, thereby preventing runtime errors and ensuring that the structure is updated correctly.

Below is the MATLAB code to achieve the same:

if strcmpi(String.Frequency_Filter, 'Yes')
    WaitMessage = parfor_wait(NN, 'Waitbar', true);
    
    % Initialize temporary variables
    u_G_Interp = [];
    v_G_Interp = [];
    w_G_Interp = [];
    
    if strcmpi(String.Output_type, 'displacement') || strcmpi(String.Differentiation_Variable, 'Frequency')
        parfor ii = 1:NN
            switch String.DOF
                case 'X'
                    u_G_Interp(ii, :) = bandpass(Output.u_G_Interp(ii, :), B, fs);
                case 'Y'
                    v_G_Interp(ii, :) = bandpass(Output.v_G_Interp(ii, :), B, fs);
                case 'Z'
                    w_G_Interp(ii, :) = bandpass(Output.w_G_Interp(ii, :), B, fs);
                case 'XYZ'
                    u_G_Interp(ii, :) = bandpass(Output.u_G_Interp(ii, :), B, fs);
                    v_G_Interp(ii, :) = bandpass(Output.v_G_Interp(ii, :), B, fs);
                    w_G_Interp(ii, :) = bandpass(Output.w_G_Interp(ii, :), B, fs);
            end
            WaitMessage.Send;
        end
    end
    
    % Assign results back to the structure
    if any(strcmpi(String.DOF, {'X', 'XYZ'}))
        Output.u_G_Interp = u_G_Interp;
    end
    if any(strcmpi(String.DOF, {'Y', 'XYZ'}))
        Output.v_G_Interp = v_G_Interp;
    end
    if any(strcmpi(String.DOF, {'Z', 'XYZ'}))
        Output.w_G_Interp = w_G_Interp;
    end
end

Please find attached the documentation of functions used for reference:

switch block: www.mathworks.com/help/matlab/ref/switch.html

bandpass: www.mathworks.com/help/signal/ref/bandpass.html

I hope this assists in resolving the issue.