Brace indexing is not supported for variables of this type.

How do I call the sort_nat function to sort folders?

clear all
clc
PF = 'D:\Project\DB1\test\' % Parent Folder
SFL = dir(PF) ;% List the sub folders
[mp1, np] = size(SFL); % compute size = number of subfolders & files & . & ..
csf1=0; % counter of JUST subfolders found in PF
t=1;
for i=3:mp1
    %% keep only folders:
    P = strfind(SFL(i).name,'.');
    if isempty(strfind(SFL(i).name,'.'))
       csf1 = csf1 +1; % one sub folder found
    SFN = SFL(i).name ;% extract his name
    %-----
     [SFN,index]= sort_nat(SFN);
     c1 =  [SFN,index];
     %---------------
    tifList = ls(sprintf('%s%s%s%s',PF,c1,'\','*.tif')); % list all jpg files
    [ms1, ns] = size(tifList); % ms = number of image files found
    %% Processing for each tif file:
    for j=1:ms1
        tifFileName = tifList(j,:); % extract name of tif file
    
        Group_Test1(t)=[i-2]
        t=t+1;
 %       PF_SFN_imgName = sprintf('%s%s%s',PF,SFN,'\',tifFileName);
    end
    end
end
save('Group_Test','Group_Test1');

[per isakson]

SFL = dir(PF) ;% List the sub folders

returns folders and files of the folder, PF

[per isakson]

"Brace indexing is not supported for variables of this type" There is no brace indexing in the code of the question(?).

Please show the full error message.

Answer 1

[per isakson]

"How do I call the sort_nat function to sort folders?"

Examples

Find sorted file names

>> sad = dir('d:\m\cssm\*.txt');
>> cell_array_of_file_names = { sad.name };
>> sorted_cell_array_of_file_names = sort_nat( cell_array_of_file_names );

Find sorted folder names

%%
sad = dir('d:\m\cssm\');  % Returns both folders and files 
cell_array_of_folder_names = {sad([sad.isdir]).name};  % Select folder names 
cell_array_of_folder_names( strncmp( cell_array_of_folder_names, ".", 1 ) ) = []; % Remove '.' and '..' 
sorted_cell_array_of_folder_names = sort_nat( cell_array_of_folder_names );

and

%%
list_of_names = {'data_1','data_2','data_3','data_21','data_11','data_12'};
sort_nat( list_of_names )

outputs

ans =
  1×6 cell array
  Columns 1 through 4
    {'data_1'}    {'data_2'}    {'data_3'}    {'data_11'}
  Columns 5 through 6
    {'data_12'}    {'data_21'}

[sun rise]

clear all
clc

sad = dir('D:\Project\DB1\test\');  % Returns both folders and files 
cell_array_of_folder_names = {sad([sad.isdir]).name};  % Select folder names 
cell_array_of_folder_names( strncmp( cell_array_of_folder_names, ".", 1 ) ) = []; % Remove '.' and '..' 
sorted_cell_array_of_folder_names = sort_nat( cell_array_of_folder_names );
%----------------

[mp1, np] = size(sorted_cell_array_of_folder_names); % compute size = number of subfolders & files & . & ..
csf1=0; % counter of JUST subfolders found in PF
t=1;
for i=3:mp1
%% keep only folders:
P = strfind(sorted_cell_array_of_folder_names(i).name,'.');
if isempty(strfind(sorted_cell_array_of_folder_names(i).name,'.'))
csf1 = csf1 +1; % one sub folder found
SFN = sorted_cell_array_of_folder_names(i).name ;% extract his name
tifList = ls(sprintf('%s%s%s%s',PF,SFN,'\','*.tif')); % list all jpg files
[ms1, ns] = size(tifList); % ms = number of image files found

%% Processing for each tif file:

for j=1:ms1
tifFileName = tifList(j,:); % extract name of tif file
IM=imread([PF SFN '\' tifFileName]);
%t=1;
%for i=1:csf1
 % for j=1:ms1
         Group_Test1(t)={i-2};
        t=t+1;
end
end
PF_SFN_imgName = sprintf('%s%s%s',PF,SFN,'\',tifFileName);

end
save('Group_Test','Group_Test1');
%----------------------

Error using save

Variable 'Group_Test1' not found.

Error in Untitled2 (line 37)

save('Group_Test','Group_Test1');

>>

[per isakson]

Matlab has good debugging features, see Debug a MATLAB Program.

The value of sorted_cell_array_of_folder_names is a row vector (the expression {sad([sad.isdir]).name}; returns a row). Thus, the statement

[mp1, np] = size(sorted_cell_array_of_folder_names);

assigns the value 1 to mp1 and consequently the statements of the for-loop (for i=3:mp1) are never executed. That explains why Group_Test1 is not defined at the end of your script.