リストから要素を間引きする

真ん中優先。真ん中がないときは右側優先。

(define thin-out
  (lambda (lis n)
    (let* ((r (- (length lis) n))
      (a (if (= (mod r 2) 0) (/ r 2) (+ (div r 2) 1)))
      (b (+ a n)))
      (append (take lis a) (drop lis b)))))

(print (iota 10))
(print (thin-out (iota 10) 1))
(print (thin-out (iota 10) 2))
(print (thin-out (iota 10) 3))

^o^ > gosh thin-out.scm
(0 1 2 3 4 5 6 7 8 9)
(0 1 2 3 4 6 7 8 9)
(0 1 2 3 6 7 8 9)
(0 1 2 3 7 8 9)