why fopen can't generate a valid file identifier (when create new file with permission of 'writing')

Hi all, I firstly generate a data file name 'outfilename', and next step, I will write something into this new empty file called 'outfilename' with the code below:

datafile = fopen(outfilename,'w');
fprintf(datafile,('sub_num\tsub_ini\tage\tgender\n'));

however, MATLAB returned:

Error using fprintf
Invalid file identifier. Use fopen to generate a valid file identifier.

I ran my codes step by step, the 'outfilename' was successfully created, but 'datafile' returned value of '-1', which means the 'fopen' code failed to create a new file called 'outfilename'.

I'm wondering why 'fopen' can't generate a valid file identifier. I use these codes before without such problem.

Many thanks if anyone could help!

Answer 1

[Stephen23]

[fid,msg] = fopen(outfilename,'w');
assert(fid>=3,msg)
fprintf(fid,'sub_num\tsub_ini\tage\tgender\n');
fclose(fid);

If the file cannot be opened then the message will give more information why. I recommend that this assert statement is used every time fopen is used.

[bokkie]

Thanks a lot! with assert command, I got further information as below:

No such file or directory

What I successfully created was a file name ('outfilename'), but what I want to do is to create a new file (called 'outfilename') by using 'fopen' command.

MATLAB webpage shows that: 'If you open a file with write or append access and the file is not in the current folder, then fopen creates a file in the current directory.'

So 'fopen' is expected to create a new file, isn't it? I use this way to create new file before, no such error...

[Stephen23]

"So 'fopen' is expected to create a new file, isn't it?"

If the path that you give fopen includes folders that do not exist then this will be an error, because fopen can create files, but it can't create folders. To create folders use the command mkdir.

[bokkie]

yes, exactly! thanks!

Answer 2

[bokkie]

oh, I create a folder for this new empty file, and it works now. Thanks anyway, I learned something new, the 'assert' command!