Retaining i value in do loop

test.el

(setq input '(a b c d))
(setq target (number-sequence 0 (- (length a) 1)))

(defun loop-test-while(source target i)
  (while (< i (length source))
    (let* ((pos (random (length source)))
          (element (nth pos source)))
      (if (not (member element target))
          (progn
            (setcar (nthcdr i target) element)
            (incf i)
            ))))
  (print target))

(loop-test-while input target 0)