Question

I have the following filter function that filters out a list, x, that doesn’t satisfy the function f.

For example, I call (filter 'evenp '(0 1 2 3)) and get back (NIL 1 NIL 3). But this is exactly my problem. How do I make it so that I just get back (1 3) ?

(defun filter (f x)
    (setq h (mapcar #'(lambda (x1)
                        (funcall f x1))
                    x))
    (mapcar #'(lambda (a b)
                (cond ((null a) b)))
            h x))

i.e. the problem is right here: (lambda (a b) (cond ( (null a) b) ) ) In my cond I don’t have a t , or else statement, so shouldn’t it just stop right there and not return nil ? How do I make it “return” nothing, not even nil, if the (cond ( (null a) b) ) isn’t satisfied?

Much appreciated. 🙂

Answer 1

Based on this question it would be:

(remove-if #'evenp '(0 1 2 3))