Extract structures in a loop - matlab

Question

I have a structure ('data'), with 26 fields (A, B, C, D, etc.). Each field contains 1x30 cells (one for each participant), and each cell contains a structure.
I would like to extract all the structures (i.e., one structure per field) corresponding to each participant. That is, I would like to obtain 30 new ‘data’, each with 26 fields, and each field containing 1x1 structure, with the structure corresponding to the participant. I have tried the following code:
data = load('D:\filepath\mydata.mat'); %load file with data. 1x1 struct.
all_fields = fieldnames(data); %store the fields of the structure. 26x1 cell.
forStr = length(all_fields); %26
n_ppts = 30; %total number of participants.
%for each participant, extract the corresponding structure in each field.
for nn = 1:n_ppts
for idx_field = 1:forStr
name_field = all_fields{idx_field};
data2 = data;
data2.(name_field) = data.(name_field){nn};
end
%save the 'data' for each participant. The 'data' should include 26 fields, and 1 structure for each field.
name = ppt_fname(nn); %Generate the new name for saving
savename =string(regexprep(name,'_oldname.set','_newname.mat'));
save(savename, '-struct', 'data');
end
The code doesn’t give any error. However, it doesn’t run as I expected.
‘data2’ still contains 26 fields, but only the last field contains 1 structure corresponding to the participant. The other fields contain 1x30 cell.
I guess it is because every time I run the loop it overwrites the previous fields, leaving only the last field correct. So, I think I might need a temporary variable where to store each iteration of the loop.
I thought to create as the temporary storage for each field
structure = [];
namelist = {‘A’;’B’;’C’;’D’;’E’;’F’;’G’;’H’;’I’;’J’;’K’;’L’;’M’;’N’;’O’;’P’;’Q’;’R’;’S’;’T’;’U’;’V’;’W’;’X’;’Y’;’Z’};
for i = 1:length(namelist)
structure.(namelist{i})={};
end
But cannot figure out how to make it work.

Answer 1

You need to take the line data2 = data; out of the for loops.

Answer 2

Antoine T is right, you always copy the original structure data again to data2 in every loop. That is why it won't get changed (except for the very last step of the loop, where you add a single field name to it.)
Regarding your other problem:
% create empty struct:
S = struct();
% loop
for i = 1:25
% create field name
nm = char( double('A') +i );
% create new field with empty cell.
S.(nm) = {};
end
It is just nice to convert number to chars as fieldnames. Your primary error was that you used the wrong inverted comma to create chars.
A minor flaw though was that you allocated strucutre = [] as an empty matrix rather than as an empty struct