primes*

a little prime-function-extension

(defun primes* (n &key (offset 0) (every-x 1) (reverse nil))
  (let ((n (* n every-x)) (seq)) 
    (progn 
      (setf seq (find-everyother every-x (subseq (primes (+ n offset)) offset (+ n offset))))
      (if (equal reverse nil)
        seq
        (reverse seq)))))



(primes* 4 :offset 0)
=> (2 3 5 7)

(primes* 4 :offset 1)
=> (3 5 7 11)

(primes* 6 :offset 8)
=> (23 29 31 37 41 43)

(primes* 5 :offset 5 :every-x 2)
=> (13 19 29 37 43)

(primes* 5 :offset 3 :every-x 4)
=> (7 19 37 53 71)

(primes* 5 :offset 5 :every-x 3 :reverse t)
=> (61 47 37 23 13)


;;;; in combination with "reading-list-by-steps"

(defun reading-list-by-steps (&key steps values (start (car values)))
  (let ((pos (car (position-item start values))))
    (append (list (nth pos values))
            (loop for i in steps
              do (setf pos (+ pos i))
              when (> pos (length values))
              do (setf pos (+ 0 i))
              collect (nth pos values)))))

(list-plot
 (reading-list-by-steps :steps '(1 2 -1 3 4 -1)
                        :values (primes* 10 :offset 4 :reverse t))
  :join-points t)