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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)
    ((= 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)))
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1 Answer 1

up vote 2 down vote accepted

In list->num you can count down with something like for i downfrom n.

(defun num->list (n)
  (loop for c across (write-to-string n)
        collect (parse-integer (string c))))

In above function you can just collect (digit-char-p c). The function returns the digit value as a number.

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