MATLAB: How to add new column in existing text file

Question

I want to add new column in text files as their file names. I have 12000 files and i want to update them with their numbers. kindly help me to write a code for them. The desired text files should be as shown

Answer 1

for ii = 1:12000 
    filename = [num2str(ii), '.txt'];   % your file names as 1, 2.. .txt
    mat = readmatrix(filename);    % get file data
    mat = [repmat(ii, size(mat, 1), 1), mat];   % append filename value to data
    writematrix(mat, filename, 'Delimiter',' ');    % write file with name
 end

Hope this works for you !!