cannot assign values to a cell during a parfor loop

I'm trying to evaluate the output of a high number of url in my domain, but I cannot deal with the "parfor" loop rules.

I get the following error:

Error: Unable to classify the variable 'checklist' in the body of the parfor-loop. For more information, see Parallel for Loops in MATLAB, "Solve Variable Classification Issues in parfor-Loops".

fold_list = cellstr(strcat('Folder_',num2str((100:125).','%#4.d')));
user_lists = cellstr(strcat('User_',num2str((1:100).','%#6.d')));
base_url = 'https://mydomain.com';
n_folds = length(fold_list);
n_users = length(user_lists);
checklist = cell(n_users+1,n_folds+1);
checklist(2:end,1) = user_lists;
checklist(1,2:end) = fold_list;
parfor j=1:n_users
    for k=1:n_folds
        try
            checklist{j+1,k+1} = webread([base_url '/' fold_list{k} '/' user_lists{j} '.txt']);
        catch
            checklist{j+1,k+1} = 'N/A';
        end
    end
end

Error: Unable to classify the variable 'checklist' in the body of the parfor-loop. For more information, see Parallel for Loops in MATLAB, "Solve Variable Classification Issues in parfor-Loops".

Answer 1

[Jan]

Try this:

fold_list  = cellstr(strcat('Folder_',num2str((100:125).','%#4.d')));
% By the way: Simpler:
% fold_list  = sprintfc('folder_%03d', (100:125).'):
user_lists = cellstr(strcat('User_',num2str((1:100).','%#6.d')));
base_url   = 'https://mydomain.com';
n_folds    = length(fold_list);
n_users    = length(user_lists);
checklist  = cell(n_users+1,n_folds+1);
checklist(2:end,1) = user_lists;
checklist(1,2:end) = fold_list;
C = cell(n_users,n_folds)
parfor j=1:n_users
    for k=1:n_folds
        try
            C{j,k} = webread([base_url '/' fold_list{k} '/' user_lists{j} '.txt']);
        catch
            C{j,k} = 'N/A';
        end
    end
end
cecklist(2:end, 2:end) = C;

[endystrike]

Thanks Jan!

I cannot just work with sprintfc function as Matlab cannot find it...

[Jan]

@endystrike: What a pity. This function was undocumented, but working since at leastr Matlab 5.3 . which Matlab version do you use?

version

ans = '9.12.0.2006483 (R2022a) Update 3'

sprintfc('%d', magic(3))

ans = 3×3 cell array
    {'8'}    {'1'}    {'6'}
    {'3'}    {'5'}    {'7'}
    {'4'}    {'9'}    {'2'}

[endystrike]

@JanI'm running R2022a, but there's no function called "sprintfc" unfortunately... :(

[Bruno Luong]

Humm it exists on mine

>> ver
-----------------------------------------------------------------------------------------------------
MATLAB Version: 9.12.0.1975300 (R2022a) Update 3
MATLAB License Number: xxxxxx
Operating System: Microsoft Windows 11 Home Version 10.0 (Build 22000)
Java Version: Java 1.8.0_202-b08 with Oracle Corporation Java HotSpot(TM) 64-Bit Server VM mixed mode
-----------------------------------------------------------------------------------------------------
MATLAB                                                Version 9.12        (R2022a)
MATLAB Coder                                          Version 5.4         (R2022a)
MATLAB Compiler                                       Version 8.4         (R2022a)
Optimization Toolbox                                  Version 9.3         (R2022a)
Parallel Computing Toolbox                            Version 7.6         (R2022a)
Signal Processing Toolbox                             Version 9.0         (R2022a)

>> sprintfc('%f', [1:3]) % yeah close your eyes master

ans =

  1×3 cell array

    {'1.000000'}    {'2.000000'}    {'3.000000'}

>> which sprintfc
built-in
>> 

[endystrike]

@Bruno Luong thanks, I'll try reinstalling Matlab... Maybe something went wrong!

edit: I don't know how and why but now it seems to be working...

>> sprintfc('%04d', magic(3))

ans =

  3×3 cell array

    {'0008'}    {'0001'}    {'0006'}
    {'0003'}    {'0005'}    {'0007'}
    {'0004'}    {'0009'}    {'0002'}