Euler Project 35

The number, 197, is called a circular prime because all rotations of the digits: 197, 971, and 719, are themselves prime.

There are thirteen such primes below 100: 2, 3, 5, 7, 11, 13, 17, 31, 37, 71, 73, 79, and 97.

How many circular primes are there below one million?

My Solution

(defun primep (n)

(cond ((= 2 n) t)

((evenp n) 'nil)

((= 1 n) 'nil)

(t (let ((limit (floor (sqrt n))))

(do ((i 3 (incf i 2)))

((> i limit) t)

(if (= 0 (mod n i)) (return-from primep 'nil)))))))

(defun IsCircular(n)

(let* ((numberstr (format nil "~a" n))

(lengthofstr (length numberstr))

(IsCircularnumber T))

(loop for i from 0 to (- lengthofstr 1)

do (let ((tempstr (concatenate 'string

(subseq numberstr i lengthofstr)

(subseq numberstr 0 i))))

(if (null (primep (parse-integer tempstr)))

(progn (setq IsCircularnumber nil)(return IsCircularnumber)))))

IsCircularnumber))

(defun Euler35(n)

(loop for i from 2 to n

when (IsCircular i)

sum 1))