get the nth prime

gistfile1.lisp

(defmethod get-prime ((a integer))
  (let ((vector (make-array 100)))
    (setf (aref vector 0) 2)
    (loop for i from 1 upto a do
          (loop for j from (1+ (aref vector (1- i))) do
                (when (= i (loop for k below i
                                 until (zerop (mod j (aref vector k)))
                                 finally (return k)))
                  (setf (aref vector i) j)
                  (return))))
    (aref vector a)))