LISP filter function

Question

I have to define a function filter that has as arguments a predicate and a list and returns as value the initial list with THE ONLY ATOMS -for every level of depth- which satisfy the initial predicate (keeping attention at NIL value in order to mantain the list structure).

Example:

(filter 'evenp '(24 5 (7) d (((4))) 3 ()))

(24 () (((4))) ())

The code i was thinking is like this:

(defun filter (pred list)
  (cond ((null list) nil)
        ((funcall pred (car list))
         (cons (car list)
               (filter pred (cdr list))))
        (T (filter pred (cdr list)))))

How can i implement the depth fact, keeping so the round bracket as displayed in the example?

thank you all

You need to check if the first element is a list and then do something. — Rainer Joswig, Jan 05 '18 at 16:21
You need to first check if the car of your list is an atom or not. Then decide to do something — Dan Robertson, Jan 05 '18 at 16:45
[Since you are operating on the _tree_, not _list_ (you say "for every level of depth"), you should treat `car` and `cdr` identically](https://stackoverflow.com/a/47759265/850781) — sds, Jan 05 '18 at 17:27

Renzo · Accepted Answer · 2018-01-06T19:52:50.963

Here is a possible solution:

(defun filter (predicate x)
   (if (consp x)  ; if x is a cons, that is a tree:
       (let ((ca (car x))
             (cd (filter predicate (cdr x)))) ; filter always the cdr
         (if (listp ca)                       ; if the car of x is a list (nil or cons)
             (cons (filter predicate ca) cd)  ; then filter also the car
             (if (funcall predicate ca) (cons ca cd) cd))) ; car is a non-nil atom!
       x))        ; if x is a atom (nil or the last cdr of an improper list), return x

CL-USER> (filter 'evenp '(24 5 (7) 5 (((4))) 3 ()))
(24 NIL (((4))) NIL)

Thank you, seems to be close enough to the solution i was looking at — Fred_2, Jan 06 '18 at 17:31

LISP filter function

1 Answers1