CODEWITHSUNDEEP
listrecursioncommon-lisp
Romsca Z.
Romsca Z.

Reputation: 43

how to split a list recursively with using conditions in clisp

I am trying to learn clisp and I have started learning how to manipulate lists recursively. I am unsure wether it is my logic or if I am just too unfamiliar with the constructs of lisp, for some I am able to do it i.e. for procedure (separate '(a 1 b 2 c 3 d 4)) => ((1 2 3 4) (a b c d)) by doing

(defun separate (lst)
  (if (endp lst)
      '(nil nil)
      (let ((x (separate (cdr lst))))
         (if (numberp (car lst))
             (list (cons (car lst)
                         (car x)))
             (list (car x)
                   (cons (car lst) (cadr x)))))))

but when I do the same approach for another procedure

(greater-than-n '(9 1 8 2 7 3 6 4) 5)

I would expect to get a list : ((9 8 7 6) (1 2 3 4))

but instead I get: ((9 8 7 6))

My program thus far is:

(defun greater-than-n (lst n)
    (if (endp lst)
        '(() ())
        (let ((x (greater-than-n (cdr lst) n)))
             (if (> (car lst) n) 
                 (list (cons (car lst)      
                             (car x)))
                 (list (car x)
                       (cons (car lst) 
                             (car x)))))))

I appreciate any help or comments.

Upvotes: 1

Views: 124

Answers (1)

sds
sds

Reputation: 60062

Your bug is in the 

             (list (cons (car lst)      
                         (car x)))

form: you are returning a list of 1 element.

PS. Your functions seem like a textbook case for using multiple values instead of a list of values.

Upvotes: 2

Related Questions