September 2017 Update

Task/Fractran/Scheme/fractran.ss

@@ -0,0 +1,62 @@
+(import (scheme base)
+        (scheme inexact)
+        (scheme read)
+        (scheme write))
+
+(define *string-fractions* ; string input of fractions
+  "17/91 78/85 19/51 23/38 29/33 77/29 95/23 77/19
+  1/17 11/13 13/11 15/14 15/2 55/1")
+
+(define *fractions* ; create vector of fractions from string input
+  (list->vector ; convert result to a vector, for constant access times
+    (read (open-input-string ; read from the string of fractions, as a list
+            (string-append "(" *string-fractions* ")")))))
+
+;; run a fractran interpreter, returning the next number for n
+;; or #f if no next number available
+;; assume fractions: ordered vector of positive fractions
+;;                n: a positive integer
+(define (fractran fractions n)
+  (let ((max-n (vector-length fractions)))
+    (let loop ((i 0))
+      (cond ((= i max-n)
+             #f)
+            ((integer? (* n (vector-ref fractions i)))
+             (* n (vector-ref fractions i)))
+            (else
+              (loop (+ 1 i)))))))
+
+;; Task
+(define (display-result max-n)
+  (do ((i 0 (+ 1 i))
+       (n 2 (fractran *fractions* n)))
+    ((= i max-n) (newline))
+    (display n) (display " ")))
+
+(display "Task: ")
+(display-result 20) ; show first 20 numbers
+
+;; Extra Credit: derive first 20 prime numbers
+(define (generate-primes target-number initial-n)
+  (define (is-power-of-two? n)
+    (and (> n 2)
+         (integer? (log n 2))))
+  (define (extract-prime n)
+    (exact (log n 2)))
+  ;
+  (let loop ((count 0)
+             (n initial-n))
+    (when (< count target-number)
+      (cond ((eq? n #f)
+             (display "-- FAILED TO COMPUTE N --\n"))
+            ((is-power-of-two? n)
+             (display (extract-prime n)) (display " ")
+             (loop (+ 1 count)
+                   (fractran *fractions* n)))
+            (else
+              (loop count
+                    (fractran *fractions* n))))))
+  (newline))
+
+(display "Primes:\n")
+(generate-primes 20 2) ; create first 20 primes