recursive function lisp return list

问题:

I know this is a newbie question I apologize in advance. I'm writing a recursive function which returns the number of 'o in a given list

(defun garde-o (liste)
    (cond
        ((not liste) 0) 
        ((equal (car liste) 'o)  (+ 1 (garde-o(cdr liste)))   )
        ((garde-o(cdr liste))  )
    )
)

Instead of returning the number of occurence I would like to return the given list with only the 'o.

Like that:

(garde-o '(a o x & w o o))

should return => (o o o)

I don't want to use pop,push,set... just I can't find of to return this.

回答1:

Notice that given the number of occurrences, for example 10, you can simply do

(make-list 10 :initial-element 'o)

or equivalently

(loop repeat 10 collect 'o)

To count the 'o in your list, you can do

(count 'o '(a b c o p o a z))

Thus, a simple solution for your function would be

(defun garde-o (a)
    (make-list (count 'o a) :initial-element 'o))

---

However, you can do this recursively too

(defun garde-o (a)
    (cond ((null a) nil)
          ((eq (car a) 'o) (cons 'o (garde-o (cdr a))))
          (t (garde-o (cdr a)))))

and non-recursively

(defun garde-o (a)
    (loop for x in a when (eq x 'o) collect x))