(defun greet-user (user-name) (format t "~%Ave, O ~A !~2%" user-name)) (defun ask (prompt type) (loop :for input := (progn (format *query-io* "~%~A " prompt) (finish-output *query-io*) (if (subtypep type 'string) (read-line *query-io*) (read *query-io*))) :until (typep input type) :do (format *error-output* "~&Input ~S is not of expected type ~S ; try again.~%" input type) :finally (return input))) (defun ask-user-name () (ask "What is your name? " 'string)) (defun ask-number () (ask "Please give a small integer: " '(integer 0 999))) (defun decimal-representation-length (n) "Return the number of decimal digit used in the usual decimal representation of the non-negative integer N." (check-type n (integer 0)) (if (zerop n) 1 (ceiling (log (1+ n) 10)))) (defun connect (a b) (let ((s (decimal-representation-length b))) (+ (* a (expt 10 s)) b))) (defun reverse-number (n) (loop :repeat (decimal-representation-length) :for c := n :then (truncate c 10) :for r := (mod c 10) :then (+ (* r 10) (mod c 10)) :finally (return r))) (defun program () (greet-user (ask-user-name)) (let ((n (connect (ask-number) (ask-number)))) (* n (reverse-number n))))