Question

Hi everyone I have implemented a solution to find the average for a nested list. So I was wondering if you could think of a better solution or any possible errors with mine cheers.

; takes a nested list and return a flattened list
(defun flatten-list (list)
  (cond 
     ((null list) list)  
     ((atom list) (list list))  
     ((list (first list))
            (append (flatten-list (first list))
                    (flatten-list (rest  list)))) 

   (t      
            (append (list (first list))
                          (flatten-list (rest  list))))
   ))

;takes a flattened list and return the sum of the numbers
(defun sum-list (list)
  (cond ((null list)
         0)    
        (t
         (+ (first list) (sum-list(rest list))))
        ))

;uses the flatten-list and nested-average to find the average
(defun nested-average (list)  
  (sum-list  (flatten-list list))
  (defvar flat-lis)
  (setf flat-list (flatten-list list))
  (/ (sum-list  flat-list) (length flat-list)
  ))

Answer 1

I think this is the better solution…

(defun tree-average (tree)
  (labels ((%tree-average
            (branch sum visited)
            (cond
             ((consp branch)
              (multiple-value-call
               #'%tree-average (cdr branch)
               (%tree-average (car branch) sum visited)))
             ((numberp branch)
              (values (+ sum branch) (1+ visited)))
             (t (values sum visited)))))
    (multiple-value-call #'/ (%tree-average tree 0 0))))

Well, if you ask.