(define (quickselect alist k)
 ;; Divide the list into two:
 ;; elements greater and lesser than
 ;; the median.
(let* ((x (pick-mid alist))
       (lt (lesser x alist))
       (gt (greater x alist))
       (gt-len (length gt)))
 (cond ((<= k gt-len)
           ;; kth largest must be in gt
           (quickselect gt k))
          ((> k (+ gt-len 1))
           (quickselect lt (- k gt-len 1)))
         (else x))))

(define a '(10 3 5 8 20 90))
(printf "~a~n" (quickselect a 5)) ;; => 5
(printf "~a~n" (quickselect a 1)) ;; => 90
(printf "~a~n" (quickselect a 4)) ;; => 8
(printf "~a~n" (quickselect a 3)) ;; => 10
(printf "~a~n" (quickselect a 2)) ;; => 20

(define (lesser x alist)
  (filter (lambda (a) (< a x)) alist))

(define (greater x alist)
  (filter (lambda (a) (> a x)) alist))

(define (pick-mid alist)
  (list-ref alist (round (/ (length alist) 2))))