acl-ch2.lisp

;; 7. define a function that takes a list and checks if one of its elements is a list
(defun elt-list? (x)
  (if (null x)
      nil
      (if (listp (car x))
	  t
	  (elt-list? (cdr x)))))

;; 8. Give iterative and recursive definitions of a function that
;; a) prints n dots
(defun print-dots-iter (n)
  (do ((i 0 (+ i 1)))
      ((= i n) 'done)
    (format t ".")))

(defun print-dots-r (n)
  (if (= n 0)
      'done
      (progn
	(format t ".")
	(print-dots-r (- n 1)))))

;; b) counts the number of times that a symbol appears in a list
(defun count-member-iter (sym lst)
  (let ((n 0))
    (dolist (x lst)
      (if (eql sym x)
	  (incf n 1)))
    n))

(defun count-member-r (sym lst)
  (if (null lst)
      0
      (if (eql sym (car lst))
	  (+ 1 (count-member-r sym (cdr lst)))
	  (count-member-r sym (cdr lst)))))

;; 9.
;; a) remove is not destructive, so the first call does nothing to lst
;; need to nest the call to remove inside the call to apply
(defun summit-1 (lst)
  (apply #'+ (remove nil lst)))

;; b) there is no check for the base case
;; needs to check for when lst is null or it will loop forever
(defun summit-2 (lst)
  (if (null lst)
      0
      (let ((x (car lst)))
	(if (null x)
	    (summit-2 (cdr lst))
	    (+ x (summit-2 (cdr lst)))))))