From Newsgroup: comp.lang.lisp

B. Pym wrote:

Drop every N'th element from a list.
Example:
* (drop '(a b c d e f g h i k) 3)
(A B D E G H K)

I'm guilty responding to a spammer, who doesn't help Lisp or OCaml by posting wrong statements about it, so I'll solve your problem:

(defun drop (list n)
(loop for item in list
for i = 1 then (1+ i)
when (/= (mod i n) 0) collect item))

Of course, in Haskell I can write it like this:

myDrop list n=[list!!x|x<-[0..length list-1], mod (1+x) n/=0]

and I'm sure this is not the shortest solution. But for me it is more difficult to develop pure functional algorithms than using Lisp.

It is not that difficult.

(defun drop (list n)
(labels ((drop-aux (list i)
(cond ((null list) nil)
((= i 1) (drop-aux (rest list) n))
(t (cons (first list)
(drop-aux (rest list) (1- i)))))))
(drop-aux list n)))

Gauche Scheme

(define (drop seq n)
(append-map
(lambda (x i) (if (zero? (mod i n)) () (list x)))
seq
(lrange 1)))

(drop '(#f #f X #f #f Y and so on) 3)
===>
(#f #f #f #f and so)

Gauche Scheme

(use srfi-1) ;; circular-list

(define (remove-nth n lst)
(let1 bools (apply circular-list `(,@(make-list (- n 1) #t) #f))
(append-map
(lambda(b x) (if b (list x) ()))
bools
lst)))

(remove-nth 3 '(a b c #f e f g h i j k))
===>
(a b #f e g h j k)
--
[T]he problem is that lispniks are as cultish as any other devout group and basically fall down frothing at the mouth if they see [heterodoxy].
--- Kenny Tilton
The good news is, it's not Lisp that sucks, but Common Lisp. --- Paul Graham --- Synchronet 3.21a-Linux NewsLink 1.2