(setf gpa 0
     total-credit-hours 0)

(format t "This program will calculate your GPA.~%Enter zero for credit hours to stop entering new courses~%")

(loop while t do
     (format t "~%New Course~%Credit hours: ")
     (setf credit-hours (read))
     (if (zerop credit-hours)
     (progn
       (format t "~%Done entering courses~%")
       (return))
   (progn
     (format t "Grade points: ")
     (setf grade-points (read))))
     (setf gpa (+ gpa (* credit-hours grade-points)))
     (setf total-credit-hours (+ total-credit-hours credit-hours))
     )

(setf gpa (/ gpa total-credit-hours))

(format t "~%Your GPA: ~$~%" gpa)