Add implementation of exercise 3-68

Author: ramsayleung

README.org

@@ -25,7 +25,7 @@
   习题完成情况:
   - 章节一: 43/46
   - 章节二: 88/97
-  - 章节三: 62/82
+  - 章节三: 63/82
   - 章节四: TODO
   - 章节五: TODO
 * 运行

chapter3/exercise3-69.rkt

@@ -0,0 +1,37 @@
+#lang racket
+(require "stream.rkt")
+(require "infinite-stream.rkt")
+
+(define (triples s t u)
+  (if (or (stream-null? s)
+          (stream-null? t)
+          (stream-null? u))
+      the-empty-stream
+      (cons-stream
+       (list (stream-car s) (stream-car t) (stream-car u))
+       (interleave
+        (stream-map (lambda (pair) (list (stream-car s) (car pair) (cadr pair)))
+                    (stream-cdr (pairs t u)))
+        (triples (stream-cdr s) (stream-cdr t) (stream-cdr u))))))
+(define (square x)
+  (* x x))
+
+(define (pythagorean)
+  (stream-filter (lambda (triple)
+                   (= (+ (square (car triple))
+                         (square (cadr triple)))
+                      (square (caddr triple))))
+                 (triples integers integers integers)))
+(module+ test
+  (require rackunit)
+
+  (test-case "Test for triples"
+             (define s1 (list-to-stream '(1 2 3)))
+             (define s2 (list-to-stream '(1 2 3)))
+             (define s3 (list-to-stream '(1 2 3)))
+             (check-equal? (stream-to-list (triples s1 s2 s3) 10) '((1 1 1) (1 1 2) (2 2 2) (1 2 2) (2 2 3)))
+             )
+  (test-case "Test for pythagorean"
+             (check-equal? (stream-to-list (pythagorean) 2) '((3 4 5) (6 8 10)))
+             )
+  )