When using fopen, what's the difference when we include/exclude 'w' in the second half of the parenthesis?

Suppose we want to open a file called exp.txt using fopen. What is the difference between these two code snippets:

fid = fopen('exp.txt', 'w'); 

fid = fopen('exp.txt');

Answer 1

[Harry MacDowel]

The 'w' simply means write.

So if the file 'exp.txt' does not exist, that command will create exactly the file, and allows you to write to it.

If the file is already there, then it will overwrite the original content.

Without the 'w', Matlab just read the file, that's all.