What causes an invalid file identifier in MATLAB?

Question

I have a MATLAB script that I could have sworn worked fine the last time I used it (a year ago). Now, I get this error:

Invalid file identifier.  Use fopen         


        

Answer 1

The path with a forward slash at the beginning can cause the same error.

filename = '/data/myfile.txt';

throws this error, while

filename = 'data/myfile.txt';

does not produce an error.