'fopen' function error when placed inside of user defined function

I am currently doing a question which requires me to write a function that has a file as the function input. The functions task is simply to open any file required containing just numbers and display it as a matrix. As I understand it, I can do this using the fopen and fscanf functions. This works fine, however when I try implement it as a function, I run into an error. This is what I have, first without it being inside my user function, which works, and second with it being inside my user created function which does not:

%#1 
fid = fopen('1.txt');
x = fscanf(fid, '%f\n', [13, inf]);

%#2
function x = fun(file_name)
fid = fopen(file_name);
x = fscanf(fid, '%d\n', [13, inf]);

#2 Returns the error:

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

Error in fun (line 3)
x = fscanf(fid, '%f\n', [13, inf]);

I'm really not sure how to fix this and any help would be much appreciated. (also I have included and if statement for if the file cannot be found but with or without this I still get the above error)

[Image Analyst]

Can you attach the text file, or at least the first few lines of it as a file with the paperclip icon?

[Harry Carr]

Star Strider answered my question perfectly, but thank you anyway :)

Answer 1

[Star Strider]

You don’t say how you call your function in your main program, and that would be useful to know. Your ‘file_name’ variable you send as a function argument has to be a string.

For instance:

file_name = '1.txt';
x = fun(file_name);

should work, assuming text file 1.txt exists in your MATLAB search path.

EDIT — Also consider adding the 'r+' permission:

fid = fopen(file_name, 'r+');

[Harry Carr]

I see my mistake. When I was implementing my function I was unaware that you needed to include " ' " on each side of your file to make it a string:

x = fun('1.txt')

Thankyou so much for your help, can't believe I didn't realise this :D

[Star Strider]

My pleasure!

Believe me, we’ve likely all done the same (I know I have), so you’re in quite good company!

[Image Analyst]

Don't quit with just that fix. It would still be a good idea to put in the more robust coding practices like I suggested, especially if you're going to write code that others will run without you being present.

Answer 2

[Image Analyst]

Try this more robust code for your function:

if exist(file_name)
    fid = fopen(file_name, 'rt');
    if fid ~= -1
        x = fscanf(fid, '%d\n', [13, inf]);
        fclose(fid);
    else
        message = sprintf('Unable to open file for reading:\n%s\nPerhaps it is locked by another process.', file_Name);
        uiwait(warndlg(message));
    end
else
    message = sprintf('File not found:\n%s', file_Name);
    uiwait(warndlg(message));
end