Add rows to a table in a parfor loop

Hi,

I'm trying to add a row to a Table inside each iteration of a parfor loop, simplified would be something like this:

T=table;
base='/Volumes/Cortegada/AUDIOS'; %Base directory of the audios to read an process

for m=1:length(months)
    
    days=dir(strcat(base,months{m}));
    days=days(3:end-1);% 
    for d=1:length(days)
        files=dir(strcat(base,months{m},'/',days(d).name,'/*.wav'));
        
        parfor f=1:length(files)
            %Each month and day folder has a different number of files
                [x,fs]=audioread(strcat(base,months{m},'/',days(d).name,'/',files(f).name));
              %% Some processing with some OUTPUTVARIABLES
                temp=table(OUTPUTVARIABLES);
                T=[T;temp];  
        end
    end
end
save('T.mat','T');

But it gives me the error:

{ Error using table (line 245)
Transparency violation error.
 See Parallel Computing Toolbox documentation about Transparency
}

Any clues on how to solve it? I can accessto the exact table position because there each month/day folder has a different number of audio files

All the best

Answer 1

[Edric Ellis]

This problem should have been fixed in R2019b (please let me know if you're using a later version and things are not working). It might work to explicitly specify the 'VariableNames' for your table, like this:

T = table();
parfor i = 1:4
    p = rand(); q = datetime();
    temp = table(p,q,'VariableNames', {'p', 'q'});
    T = [T; temp];
end
disp(T)

       p                q          
    _______    ____________________

    0.95376    16-Apr-2021 08:13:32
    0.62206    16-Apr-2021 08:13:32
    0.71322    16-Apr-2021 08:13:32
    0.69861    16-Apr-2021 08:13:32

[David Santos]

I'm using 2017a and finally solve the problem initializing the table to the correct size and then accessing to an exact position in the for loop, something like:

T=table(0,0,'VariableNames', {'p', 'q'});
T=repmat(T,totalRows,1);
...
parfor i=1:totalRows
..
    T(i,:)=(Var1, Var2)
..
end

[Edric Ellis]

It might be better still to preallocate the table to the correct size, i.e.

T = table(zeros(totalRows,1), zeros(totalRows,1), 'VariableNames', {'p', 'q'})

[David Santos]

Thanks