CODEWITHSUNDEEP
lispcommon-lisp
Chow Zheng Xuan
Chow Zheng Xuan

Reputation: 1

`NIL' is not of the expected type `NUMBER'

When I was trying to check the number is equal to any element in list, I get the following error: 'NIL' is not of the expected type 'NUMBER'

(defun check-num (li n)
  (cond
    ((= n (car li)) t)
    ((not (= n (car li))) (check-num (cdr li) n)) 
    ((not (= n (car li))) nil)))

(check-num '(1 2 3 4 5) 6)

I found that the problem occur when it try to excute the function 'check-num' with the empty list with the number entered after perform all checking.

The error come out when the compiler try to perform the following function:

(check-num nil 6)

Upvotes: 0

Views: 1286

Answers (1)

Svante
Svante

Reputation: 51521

The immediate problem is that you never check for the end of the list. The car of the empty list is nil again. = compares numbers. Nil is not a number.

Check for the end of the list first. Some minor stylistic issues: when using conses as a list, use first and rest instead of car and cdr, name things as they are (list), use proper formatting.

(defun check-num (list n)
  (cond ((endp list) nil) ; <-
        ((= (first list) n) t)
        (t (check-num (rest list) n))))

However, it should be noted that there are several ways already in the standard to achieve this check, e. g.:

(member n list)

(find n list)

Upvotes: 2

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