# (quick) Matlab fprintf question

• MATLAB

## Main Question or Discussion Point

I have output (stored in variable "x") that I want saved into a simple text file. If I use the command:

fprintf('%d\n',x);

it prints the data in a nice column. This is what I want. Then to save it to a file I try this:

fileID = fopen('x.txt');
fprintf('%d\n',x);
fclose(fileID);

however Matlab does not like this. When I searched the help file, it gave a similar example, so what am I doing wrong here? Thanks in advance!

## Answers and Replies

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uart
Try specifying the file ID as the first parameter.

fileID = fopen('x.txt');
fprintf(fileID,'%d\n',x);

Try specifying the file ID as the first parameter.

fileID = fopen('x.txt');
fprintf(fileID,'%d\n',x);
Thanks for the help, but unfortunately it did not work. I still get this error from Matlab:

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

when I try my code:

fileID = fopen('x.txt');
fprintf('%d\n',x);
fclose(fileID);

the error I get is:

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

I still can't figure out why?

Mark44
Mentor
When you open the file, you need to specify the permission, which is related to what you want to do with the file (read it, write to it, and so on).

Here's a link to some documentation for fopen: http://amath.colorado.edu/computing/Matlab/OldTechDocs/ref/fopen.html [Broken].

Try this:
Code:
fileID = fopen('x.txt', 'wt');
fprintf('%d\n',x);
fclose(fileID);
I don't have MATLAB, so can't verify that this will work, but I think it will.

Last edited by a moderator:
uart
Thanks for the help, but unfortunately it did not work. I still get this error from Matlab:

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

when I try my code:

fileID = fopen('x.txt');
fprintf('%d\n',x);
fclose(fileID);

the error I get is:

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

I still can't figure out why?
Ok, well in that case it appears that your problem is with the fopen statement (BTW. Is there any particular reason why you didn't post the actual error message in the first place!)

Try letting it know that the file doesn't currently exit and that you wish to create it.

fileID = fopen('x.txt','w');

This tells it that you want to open the file for writing.

Ok, well in that case it appears that your problem is with the fopen statement (BTW. Is there any particular reason why you didn't post the actual error message in the first place!)

Try letting it know that the file doesn't currently exit and that you wish to create it.

fileID = fopen('x.txt','w');

This tells it that you want to open the file for writing.
Thanks! I really can't think of why I did not post the error in the first place, except that I've not had much sleep :)

Edit: OK, got it! Thanks everyone for the help!!