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

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Hello, I have a problem, every program I try to run gives me this error. Even with the matlab examples.
This is the code from the mathworks page:
A = magic(4);
fileID = fopen('myfile.txt','w');
nbytes = fprintf(fileID,'%5d %5d %5d %5d\n',A);
  1 Comment
Benjamin Kraus
Benjamin Kraus on 24 Jan 2018
Can you clarify your question?
  • When you say "every program", do you mean every command you try to run in MATLAB?
  • If you run those commands one at a time, do you get any other error messages?
  • Are you running those commands in a directory in which you have write access?
  • What is the value of fileID after calling fopen?

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Accepted Answer

Benjamin Kraus
Benjamin Kraus on 24 Jan 2018
When fopen fails, it does not generate an error message. Instead, it sets the fileID to -1.
Check the value of fileID after calling fopen.
If it is -1, check whether you have write permissions in the directory you are trying to write to. Most likely you are trying to write a file to a directory that is read-only, and you either need to change directories or specify an absolute path instead of a relative path.

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More Answers (2)

Walter Roberson
Walter Roberson on 24 Jan 2018
You do not have write access to the directory you are in. Your current directory is probably a directory that MATLAB is installed in.
fileID = fopen('myfile.txt','w');
[fileID, message] = fopen('myfile.txt','w');
if fileID < 0
error('Failed to open myfile because: %s', message);

Akash kumar
Akash kumar on 4 Jun 2021
if you open the fopen file in his format and you run the again matlab file then it gives the error.
First:- You close the fopen file in your system . In your case it is 'myfile.txt'
second:- Now you run your code.
Note:- YOUR code will be definately run.
Walter Roberson
Walter Roberson on 5 Jun 2021
But it will not be a "permission denied" situation. The message will be different. See for how to get the message.
And when you cannot open a file because it is open for writing in Excel, then fclose() of the file in your MATLAB session does not help: you would need to convince Excel to close the file.

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