2016 Update

Task/Knuths-algorithm-S/Racket/knuths-algorithm-s.rkt

@@ -1,29 +1,21 @@
 #lang racket/base
 
 (define (s-of-n-creator n)
-  (let ([count 0] ; 'i' in the description
-         [vec (make-vector n)]) ; store the elts we've seen so far
-    (lambda (item)
-      (if (< count n)
-          ; we're not full, so, kind of boring
-          (begin
-            (vector-set! vec count item)
-            (set! count (+ count 1)))
-          ; we've already seen n elts; fun starts
-          (begin
-            (set! count (+ count 1))
-            (when (< (random count) n)
-              (vector-set! vec (random n) item))))
-      vec)))
+  (define i 0)
+  (define sample (make-vector n)) ; the sample of n items
+  (lambda (item)
+    (set! i (add1 i))
+    (cond [(<= i n)               ; we're not full, so kind of boring
+           (vector-set! sample (sub1 i) item)]
+          [(< (random i) n)       ; we've already seen n items; swap one?
+           (vector-set! sample (random n) item)])
+    sample))
 
-(define counts (make-vector 10))
+(define counts (make-vector 10 0))
 
-(for ([iter (in-range 0 100000)]) ; trials
-  (let ([s-of-n (s-of-n-creator 3)]) ; set up the chooser
-    (for ([d (in-vector ; iterate over the chosen digits
-              (for/last ([digit (in-range 0 10)]) ; loop through the digits
-                        (s-of-n digit)))]) ; feed them in
-      (vector-set! counts d (add1 (vector-ref counts d)))))) ; update counts
+(for ([i 100000])
+  (define s-of-n (s-of-n-creator 3))
+  (define sample (for/last ([digit 10]) (s-of-n digit)))
+  (for ([d sample]) (vector-set! counts d (add1 (vector-ref counts d)))))
 
-(for ([d (in-range 0 10)])
-  (printf "~a ~a~n" d (vector-ref counts d)))
+(for ([d 10]) (printf "~a ~a\n" d (vector-ref counts d)))