sample

sample.lisp

(defmethod sample ((input list) k &aux (length (length input)))
  (assert (<= 1 k length))
  (let ((indices (loop with result = '()
                       with i = k
                       for j = (random length)
                       unless (member j result)
                         do (push j result)
                            (decf i)
                       while (< 0 i)
                       finally (return result))))
    (setq indices (sort indices #'<))
    (loop with idx = (pop indices)
          for i upfrom 0
          for x in input
          while idx
          when (= i idx)
            collect x
            and do (setq idx (pop indices)))))