To start with Common Lisp I am doing Project Euler using this language. Usually I manage to solve problems but I am quite sure that my code is not as efficient as it could be in Common Lisp. That is why I need a review from experienced lispers.
This is my code for problem 35. Please offer any improvements.
(defun prime-p (n) (cond ((= n 1) nil) ((= n 2) t) ((evenp n) nil) (t (loop for i from 3 to (isqrt n) by 2 never (zerop (mod n i)))))) (defun list->num (lst) (loop for i in lst for p = (- (length lst) 1) then (- p 1) sum (* i (expt 10 p)))) (defun num->list (n) (loop for c across (write-to-string n) collect (parse-integer (string c)))) (defun rotate (lst) (append (last lst) (butlast lst))) (defun number-rotations (n) (let* ((digits (num->list n)) (digits-count (length digits))) (loop repeat digits-count for rotated = digits then (rotate rotated) collect (list->num rotated)))) (defun problem-35 (limit) (let ((hash-primes (make-hash-table))) (loop for n from 1 to limit if (prime-p n) do (setf (gethash n hash-primes) t)) (loop for p being the hash-keys in hash-primes if (loop for n in (number-rotations p) always (gethash n hash-primes)) collect p)))