Looping over lines in a string in Common Lisp

Question

I'm trying to understand why this small piece of code does not work as expected. I would expect it to print out "foo", but in fact what I get is

CL-USER> (stringloop)
null output T
 line output NIL

NIL

I expect I am using do wrong, but I've not be able to figure out what.

(defun stringloop ()
(with-input-from-string (s "foo" :index j )
  (do ((line (read-line s nil) ;; var init-form
         (read-line s nil))) ;; step=form
      ((null line) (progn (format t "null output ~a~% "(null line)) (format t "line output ~a~% " line))))))

Answer 1

You didn't put anything in the loop body. Your function reads a line ("foo"), does nothing with it, then reads another line (nil), your termination condition becomes true, and you print the null line.

Run this modified version to see what's happening:

     (defun stringloop ()
       (with-input-from-string (s "foo")
         (do ((line (read-line s nil) ;; var init-form
                    (read-line s nil))) ;; step=form
             ((null line) (format t "termination condition - line: ~s~% " line))
           (format t "in loop - line: ~s~%" line))))