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.




#lang racket
(define (middle x)
  (cond
    [(negative? x) (middle (- x))]
    [(< x 100)     "error: number too small"]
    [else 
     (define s (number->string x))
     (define l (string-length s))
     (cond [(even? l) "error: number has even length"]
           [else (define i (quotient l 2)) 
                 (substring s (- i 1) (+ i 2))])]))
 
(map middle (list 123 12345 1234567 987654321 10001 -10001 -123 -100 100 -12345))
(map middle (list 1 2 -1 -10 2002 -2002 0))