Data commit

Task/Amicable-pairs/Racket/amicable-pairs.rkt

@@ -0,0 +1,31 @@
+#lang racket
+(require "proper-divisors.rkt")
+(define SCOPE 20000)
+
+(define P
+  (let ((P-v (vector)))
+    (λ (n)
+      (set! P-v (fold-divisors P-v n 0 +))
+      (vector-ref P-v n))))
+
+;; returns #f if not an amicable number, amicable pairing otherwise
+(define (amicable? n)
+  (define m (P n))
+  (define m-sod (P m))
+  (and (= m-sod n)
+       (< m n) ; each pair exactly once, also eliminates perfect numbers
+       m))
+
+(void (amicable? SCOPE)) ; prime the memoisation
+
+(for* ((n (in-range 1 (add1 SCOPE)))
+       (m (in-value (amicable? n)))
+       #:when m)
+  (printf #<<EOS
+amicable pair: ~a, ~a
+  ~a: divisors: ~a
+  ~a: divisors: ~a
+
+
+EOS
+          n m n (proper-divisors n)  m (proper-divisors m)))