Parallel for loop parfor clasiffying variable for a structure input how to?

Hi,

I have a structure that I fill in by loading a data in a loop, where each loop opens a particular file and fills the structure.

What would be the correct way of doing it in a parfor?

Currently, the code works if I use for loop instead of parfor.

When I use parfor, I get this error:

Error: Unable to classify the variable 'indvoutputagg' in the body of the parfor-loop. For more information, see Parallel for Loops in

MATLAB, "Solve Variable Classification Issues in parfor-Loops".

form=1;
scenarios = 1:4;
N = numel(scenarios);

filedir = 'Outputs';
indvoutputagg = cell(N,1); % preallocate
indvoutputcomp={};
tic
parfor k = 1:N
    filename = sprintf('Form1_Scenario%d_indvoutputagg.mat',scenarios(k));
    indvoutputagg{k,:} = importdata(fullfile(filedir,filename)); % allocate using indexing
    % Make it into one big table
    indvoutputcomp(1+size(indvoutputcomp,1):size(indvoutputcomp,1)+size(indvoutputagg{k,1},1),:)=indvoutputagg{k};
end

Answer 1

[Sunand Agarwal]

Hello,

This usually happens because the variable 'indvoutputagg' exists in the function workspace on the worker while the loop runs in the worker's base workspace.

Use either evalin or assignin inside of the parfor body to create or move the variable to the base workspace of the worker. For this example, you could use

assignin('base', 'indvoutputagg{k,:}', importdata(fullfile(filedir,filename))); 

to assign a value 'indvoutputagg' in the worker's base workspace.

Hope this helps.

[In-chan Kim]

Thanks very much.

If I understand correctly, the problem i that I have 'indvoutputagg' within the parfor?

I've tried putting in the suggested line, but get the same error

parfor k = 1:N
    filename = sprintf('Form1_Scenario%d_indvoutputagg.mat',scenarios(k));
    indvoutputagg{k,:} = importdata(fullfile(filedir,filename)); % allocate using indexing
    assignin('base', 'indvoutputagg{k,:}', importdata(fullfile(filedir,filename))); 
    
    % Make it into one big table
    indvoutputcomp(1+size(indvoutputcomp,1):size(indvoutputcomp,1)+size(indvoutputagg{k,1},1),:)=indvoutputagg{k};
end

[Sunand Agarwal]

Hello

The code within the parfor loop is treated as a function for each worker and hence the variables 'indvoutputagg' as well as 'indvoutputcomp' are in the worker's function workspace. As I mentioned before, these variables need to be in the base workspace of the worker and we use the 'assignin' function to do the same.

Now, you don't need to write the following line in your code if you're using the 'assignin' function:

indvoutputagg{k,:} = importdata(fullfile(filedir,filename)); % allocate using indexing

Therefore, please remove the above line and the error should not arise.

Please repeat the same workflow for the 'indvoutputcomp' variable.

Hope this helps.

[In-chan Kim]

Thank you.

Just so I understand correctly, is this what you had in mind?

form=1;
scenarios = 1:9;
N = numel(scenarios);

filedir = 'Outputs';
indvoutputagg = cell(N,1); % preallocate
indvoutputcomp={};
tic
for k = 1:N
    filename = sprintf('Form1_Scenario%d_indvoutputagg.mat',scenarios(k));
    assignin('base', 'indvoutputagg{k,:}', importdata(fullfile(filedir,filename))); 
end

[Sunand Agarwal]

Hello

Yes, your code should work now if you change the loop to parfor as well.

Usually, whenever we assign a variable in the parfor loop using the assignment operator, it is assigned to the worker's function workspace as mentioned before.

For example,

b = 1;

The above statement assigns the variable 'b' to value '1' in the function workspace of the worker. To assign it to the base workspace of the worker for the parfor loop to work successfully, we use the 'assignin' function as below:

assignin('base', 'b', 1);

Hope this helps.