Testing input integers for the palindrome property

3.23 Palindromes


A palindrome is a number that reads the same forwards
and backwards, like 12321. Write a
program that tests input integers for
the palindrome property.


Hint: This should not be done with the numbers in
their input (character) format.

For the task above, I have written the following program. Rather than converting the number to a string, I recursively divide the number to create a list of individual digits and then compare the digits from outside to in to determine if it's a palindrome. What do you think of my strategy and of my code in general? I understand that tail recursion is not guaranteed to work in Common Lisp - should I rewrite this code to use loops instead?

(defun collect-digits (candidate &OPTIONAL (digit-list ()))
  ; make a list out of the digits of the number
  ; example: (= (collect-digits 12321) (1 2 3 2 1)) is true
  (if (< candidate 10) 
    (cons candidate digit-list)
    (multiple-value-call 
      (lambda (sub-candidate onesdigit) 
        (collect-digits sub-candidate (cons onesdigit digit-list)))
      (floor candidate 10))))

(defun reflection-p (candidate-list &OPTIONAL (len (length candidate-list)))
  ; if c-l[first] = c-l[last]
  (and (= (car candidate-list) (car (last candidate-list 1))) 
       ; and the list is 3 or smaller ex. (3 1 3), it is a reflection 
       (if (<= len 3) t 
     ; if longer, keep looking
     (reflection-p (subseq candidate-list 1 (1- len)) (- len 2)))))

(defun palindrome-p (candidate)
    (reflection-p (collect-digits candidate)))

(format t "12321: ~a ~%3456789 ~a ~%" (palindrome-p 12321) (palindrome-p 3456789))

This one is easy! Remember the reverse-digits function? If a number is the same as its reverse, then it's a palindrome.

StackExchange Code Review Q#1339, answer score: 4