Task

Write a function/procedure/subroutine that is called with an integer value and returns the middle three digits of the integer if possible or a clear indication of an error if this is not possible.

Note: The order of the middle digits should be preserved.

Your function should be tested with the following values; the first line should return valid answers, those of the second line should return clear indications of an error:

123, 12345, 1234567, 987654321, 10001, -10001, -123, -100, 100, -12345
1, 2, -1, -10, 2002, -2002, 0

Show your output on this page.




 
(defun mid3 (n)
  (let ((a (abs n))
        (hmd)) ; how many digits
    (labels ((give (fmt &optional x y) (return-from mid3 (format nil fmt x y)))
             (need (x) (give "Need ~a digits, not ~d." x hmd))
             (nbr (n) (give "~3,'0d" n)))
      (when (zerop n) (give "Zero is 1 digit"))
      (setq hmd (truncate (1+ (log a 10))))
      (cond ((< hmd 3) (need "3+"))
            ((= hmd 3) (nbr a))
            ((evenp hmd) (need "odd number of"))
            (t (nbr (mod (truncate a (expt 10 (/ (- hmd 3) 2))) 1000)))))))
 