filename = tempname() + ".txt";
writelines(str, filename);
fid = fopen(filename, 'r');
datacell = textscan(fid, '%f%f', 1, 'Delimiter', ',', 'MultipleDelimsAsOne', 0);
rest = char(fread(fid, [1 inf], '*uchar'));
datacell
datacell =
{[12]} {[453]}
What does this tell us? Well, it tells us that after textscan finishes processing the format, it examines the input stream, consuming Whitespace and up to the first copy of the Delimiter, and then stops.
So in your example when processing 'a,b,' with %q it first processes the a as part of the %q format, and then eats the delimiter that is there, leaving 'b,' in the input stream. No count is specified so it tries processing again, reads the b with the %q format, eats the delimiter. No count is specified so it tries processing again. The input stream is empty, so it stops. Yes, this is the same thing that would happen for the case of 'a,b' with no final delimiter.
A potential alternative would have been if the delimiter was left in the buffer, and it was up to the processing to skip one leading delimiter. But in such a case the input ',b,c' would be treated the same as 'b,c' and it seems unlikely to me that you would want that to be the case.
Yes, textscan could have been designed to set an internal flag indicating that a delimiter had been seen before the current position, but I am not sure that would meet with expectations. For example if the input were 123,abc,<newline>456,def, and the format were %f%q then with the current behaviour that would generate {{123;456}, {'abc'; 'def'}} . If trailing delimiters should invoke the missing-contents behaviour (your interpretation) then that would imply that the 123 should be accepted by the first time %f is processed, then the abc should be accepted by the first time %q is processed, then the empty field at the end of the line should be accepted and turned into nan by the second time %f is processed, then the 456 should be accepted and turned into character vector by the second time %q is processed, then the def should trigger a format mismatch the third time %f is processed, stopping the scanning and leaving def, in the buffer. It might be potentially be a consistent way of processing, but is it what would be expected and useful ?
