What is wrong with my lisp function?

Question

I am writing a function called palindrome that tests if a list is a palindrome. It works 100% for regular lists such as (1 2 1), but if I use (1 (2) 1), I get a bad argument type error.

Here is my function

(defun palindrome (x)                      
   (if (NULL x) t                           
   (let ( ( a (car x)) (b (lastelement x)))  
      (if ( = a b)                           
        (palindrome (cdr (butlast x)))       
      nil))))

Leo · Accepted Answer · 2016-11-05 05:39:48Z

1

The function = is reserved exclusively for numbers. When used with a list or other type of element it returns an error. Use instead eql.

Incidentally, note that your function is very inefficient. Normally (i.e. not for homework) you would create a reversed copy of the list and compare it element by element with the original list.

answered Nov 5, 2016 at 5:39

Leo

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2 Comments

Gary Over a year ago

Thank you very much. You caught me red handed as this is a homework question XD, and my instructor told us to not use reverse.

Gary Over a year ago

I am very new to lisp, and I completely forgot about the difference between the = and eq operator. Thank you!

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