桁数を求める for xyzzy

digit

(defun digit (n)
  "return digit for number."
  (setq n (truncate (1+ n)))
  (let ((d 0))
    (while (> n 1)
      (setq n (/ n 10))
      (incf d))
    d))

;; (digit 1000)
;; =>4
;; (digit 999)
;; =>3