Check the success of fopen in every case, under all circumstances, ever, nerver omit this test:
filename = 'myfile.txt';
filepath = cd;
file = fullfile(filepath, filename);
fid = fopen(file, 'wt');
if fid==-1
error('Cannot open file for writing: %s', file);
end
I guess you do not have privileges to write to the current folder.
