Question

Trying to learn lisp, want to delete every nth. I only managed to delete the first (nth) element

(defun delete-nth (n list)
(if (zerop n)
(cdr list)
(let ((cons (nthcdr (1- n) list)))
  (if cons
    (setf (cdr cons) (cddr cons))
    cons))))

I’d like to delete the next nth and so on

Also I tried this:

(defun remove-nth (list n)
(remove-if (constantly t) list :start n :end (+ 1 n)))

No idea how to start again

What I was thinking was concatenating, but I have no idea of how to keep track of my position.

Answer 1

An alternative way to do the same thing:

(defun remove-all-nth (list period)
   (remove-if
    (let ((iterator 0))
      (lambda (x)
        (declare (ignore x))
        (= 0 (mod (incf iterator) period)))) list))
(remove-all-nth '(1 2 3 4 5 6 7 8 9 0) 3)
; (1 2 4 5 7 8 0)