Weird error on the Bob exercise

Programming Common Lisp
1

I’m getting a very weird error when testing my solution for the Bob exercise. This is the error message:

 PROLONGED-SILENCE in BOB-SUITE []: 
      Unexpected Error: #<SB-INT:INVALID-ARRAY-INDEX-ERROR expected-type:
                                                     (INTEGER 0 (0))
                                                     datum: -1>
Invalid index -1 for (SIMPLE-ARRAY CHARACTER (0))..

This is my code:

(defun question-p (s)
  (eq (aref s (1- (length s))) #\?))

(defun has-letter-p (s &optional (i 0))
  (if (equal i (length s))
      nil
      (let ((char (aref s i)))
        (if (alpha-char-p char)
            t
            (has-letter-p s (1+ i))))))

(defun yell-p (s &optional (i 0))
  (if (equal i (length s))
      t
      (let ((char (aref s i)))
        (if (and (alpha-char-p char) (lower-case-p char))
            nil
            (yell-p s (1+ i))))))

(defun silence-p (s &optional (i 0))
  (labels ((whitespace-p (c) (member c '(#\Space #\Tab #\Newline))))
    (if (equal i (length s))
        t
        (let ((c (aref s 0)))
          (if (not (whitespace-p c))
              nil
              (silence-p s (1+ i)))))))

(defun response (hey-bob)
  (let ((trimmed (string-trim " " hey-bob)))
    (cond ((and (has-letter-p trimmed)
                (yell-p trimmed)
                (question-p trimmed)) "Calm down, I know what I'm doing!")
          ((question-p trimmed) "Sure.")
          ((and (has-letter-p trimmed) (yell-p trimmed)) "Whoa, chill out!")
          ((silence-p trimmed) "Fine. Be that way!")
          (t "Whatever."))))

I can’t figure this one out. I can’t see any place in my code that even subtracts from the index I’m using. Any ideas?

2

I don’t know Lisp but how would the question-p function handle an empty string?

1 Like
3

Ah, you’re right. Of course it doesn’t. Thanks for spotting my error!