Data commit

Task/Additive-primes/Common-Lisp/additive-primes.lisp

@@ -0,0 +1,19 @@
+(defun sum-of-digits (n)
+ "Return the sum of the digits of a number"
+  (do* ((sum 0 (+ sum rem))
+        rem )
+       ((zerop n) sum)
+    (multiple-value-setq (n rem) (floor n 10)) ))
+
+(defun additive-primep (n)
+  (and (primep n) (primep (sum-of-digits n))) )
+
+
+; To test if a number is prime we can use a number of different methods. Here I use Wilson's Theorem (see Primality by Wilson's theorem):
+
+(defun primep (n)
+  (unless (zerop n)
+    (zerop (mod (1+ (factorial (1- n))) n)) ))
+
+(defun factorial (n)
+  (if (< n 2) 1 (* n (factorial (1- n)))) )