Just Wrote the Game Twenty Questions in Common Lisp

20 Questions in Common Lisp

(defun input-question ()
  (print "Please enter a question which has a yes answer for your word. This question should result in a no for the incorrect word given earlier:")
  (read-line))

(defun input-chosen-word ()
  (print "Please enter the word that you chose at the start of the game:")
  (read-line))

(defun input-question-answer (question)
  (print question)
  (print "The only valid answers are yes and no.")
  (if (equalp "yes"
	      (let ((response (read-line)))
		(if (or (equalp response "yes") (equalp response "no"))
		    response
		    (input-question-answer question))))
      T
      NIL))

(defun get-question  (tree) (first tree))
(defun get-yes-child (tree) (second tree))
(defun get-no-child  (tree) (third tree))

(defun is-branch (node) (consp node))

(defun twenty-questions (tree)
  (if (input-question-answer (get-question tree))
      (let ((current-node (get-yes-child tree)))
	(if current-node
	    (if (is-branch current-node)
		(twenty-questions current-node)
		(unless (input-question-answer current-node)
		  (setf (second tree)
			(let ((question (input-question))
			      (answer (input-chosen-word))
			      (old (second tree)))
			  (list question answer old)))))
	    (setf (second tree) (input-chosen-word))))
      (let ((current-node (get-no-child tree)))
	(if current-node
	    (if (is-branch current-node)
		(twenty-questions current-node)
		(unless (input-question-answer current-node)
		  (setf (third tree)
			(let ((question (input-question))
			      (answer (input-chosen-word))
			      (old (third tree)))
			  (list question answer old)))))
	    (setf (third tree) (input-chosen-word))))))      

(defparameter q-tree '("Is it bigger than a breadbox?" "Airplane" nil))
(defun main ()
  (loop
     while (input-question-answer "Do you want to play another round?")
     do
       (print "Please choose a word and I'll try to guess it.")
       (twenty-questions q-tree)))