You check if the variable theSimfile exists, and if it doesn't, you create a variable called simfile. Then you use theSimfile (which may not exist) in fopen, and simfile (which also may not exist) is unused.
I imagine that the intent in checking for the existence of a variable is to use a default file if it is not already specified, but probably the variable you create should be the one you just found not to exist (doesn't matter if it's called simfile or theSimfile). Then use that same variable in fopen.
Also, in case the file can't be opened for whatever reason, you can take the second output from fopen and throw it as an error.
(And don't forget to fclose the file.)
if ~exist('simfile','var')
simfile = 'Users/ponce/Desktop/folder 10/windcity.dat';
end
[simfid,simmsg] = fopen( simfile, 'w' );
if simfid == -1
error(simmsg);
end
fprintf( simfid, ['#state1\tstate2\tstate3\tstate4\tstate5\n', ...
'original1\toriginal2\toriginal3\toriginal4\toriginal5\toriginal6\n', ...
'error1\terror2\terror3\terro4\n'] );
% close the file when you are done writing!
fclose( simfid );
