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 loop 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))
Good point about reverse-digits. In that case, this program simplifies to:
(defun reverse-digits (n) (labels ((next (n v) (if (zerop n) v (multiple-value-bind (q r) (truncate n 10) (next q (+ (* v 10) r)))))) (next n 0))) (defun palindrome-p (candidate) (= candidate (reverse-digits candidate))) (format t "12321: ~a ~%3456789 ~a ~%" (palindrome-p 12321) (palindrome-p 3456789))
(credit to Chris Jester-Young for the reverse-digits implementation) Do you have any suggestions regarding my coding style (referring to the first solution?)