How to save several times without overwriting the file - matlab

Question

I have a problem with my code in matlab.
I have a matrix C (make reshape as a vector) and I want to save several C vectors to one file. This is my code
wynik = reshape(C',1,[]);
fileID = fopen('Desktop\test.txt','r');
fileID_out = fopen('Desktop\test_out.txt','r');
fprintf(fileID, '%d \r', wynik);
fprintf(fileID, '\n');
fprintf(fileID_out, ' %d \r\n', 2);
end
I made a loop at the begin so in a console I have for example 2 different matrix, but using this code it overwrite my file and I save only the last one vector. I would like to have sth like this (shorter example)
A = [ 1 2 3; 4 5 6 ] ( first loop)
A = [7 8 9; 1 2 3 ] ( second loop)
In my file(with spaces between values and with \n on the end of line) :
1 2 3 4 5 6
7 8 9 1 2 3

Answer 1

The example in your question is quite unclear because you ask about saving data but all your file opening instructions use only reading permissions.
I'll give you an example which works for your 2nd (shorter) example because it is clearer what you are trying to achieve.
I strongly recommend to read the documentation for:
fopen, and particularly the usage of the parameter permission.
fprintf the the parameter formatSpec will be useful.
Armed with that documentation, you will realise that to write to an existing file already containing data is called append to the file. So for your usage: The first time you create the file, open it with the permission 'w'. For all the other time you want to add (=append) something to the file, open it with the permission 'a', then write normally to it.
Your second example in code:
%% Initial data
A = [1,2,3;4,5,6];
%% prepare format specifier for a complete line
nElem = numel(A) ;
baseformat = '%d ' ; % base number format
writeFormat = repmat( baseformat , 1 , nElem ) ; % replicated "nElem" times
writeFormat = [writeFormat(1:end-1) '\n'] ; % remove last trailing space and replace by newline
% => now writeFormat = "%d %d %d %d %d %d\n"
%% Open the file the first time to write the first line
% permission 'w' => Open or create new file for writing. Discard existing contents, if any.
fidout = fopen('myfileout.txt', 'w') ;
fprintf( fidout , writeFormat , A(:) ) ;
fclose(fidout) ;
%% Now let's write 5 additional lines
for iLine=1:5
% simulate a different matrix [A]
A = A + nElem ; % A will continue counting
% permission 'a' => Open or create new file for writing. Append data to the end of the file.
fidout = fopen('myfileout.txt', 'a') ;
fprintf( fidout , writeFormat , A(:) ) ;
fclose(fidout) ;
end
Which should give you the file myfileout.txt, containing:
1 2 3 4 5 6
7 8 9 10 11 12
13 14 15 16 17 18
19 20 21 22 23 24
25 26 27 28 29 30
31 32 33 34 35 36