If you select >1 file, the output of uigetfile() is stored in a cell array which your for-loop is designed to handle. If you select 1 file, the output of uigetfile() is stored as a char array which your for-loop is not designed to handle.
What you want to do is change the char array to a cell array.
if ischar(files_to_compare)
files_to_compare = {files_to_compare};
end
Then enter your for-loop.
I'd add one more conditional to handle the event of selecting 0 files.
if isnumeric(files_to_compare) && isequal(files_to_compare,0)
return % quit the function
% error('User did not select any files.') % _or_ throw an error
end
