You can't do that. The commented out line is treated as a blank line, which switches over the parsing to vertical concatenation, the same as if you were using { 'test1.txt', ... 'test2.txt' 'test4.txt' }
which would be equivalent to trying to do
{ 'test1.txt', 'test2.txt'; 'test4.txt'}
What I suggest is that you switch over to vertical concatenation anyhow. Transpose the resulting cell array if you need to:
input_file_array = {...
'test1.txt';...
'test2.txt'; ...
%'test3.txt'; ...
'test4.txt'; ...
};
The % will be treated as a blank line with the implied vertical concatenation, but since you are using vertical concatenation anyhow you don't run in to a conflict.
You can simplify this code as:
input_file_array = {
'test1.txt'
'test2.txt'
%'test3.txt'
'test4.txt'
};
taking advantage of the implicit vertical concatenation.
