Add solution for exercise3-16

Author: ramsayleung

chapter3/exercise3-16.org

@@ -0,0 +1,7 @@
+#+LATEX_CLASS: ramsay-org-article
+#+LATEX_CLASS_OPTIONS: [oneside,A4paper,12pt]
+#+AUTHOR: Ramsay Leung
+#+EMAIL: [email protected]
+#+DATE: 2025-06-14 Sat 23:42
+
+[[file:../img/chapter3/exercise-3-16.png]]

chapter3/exercise3-16.rkt

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+#lang racket
+
+(define (count-pairs x)
+  (if (not (pair? x))
+      0
+      (+ (count-pairs (car x))
+         (count-pairs (cdr x))
+         1)))
+
+;; 无法处理共享或循环结构
+(module+ test
+  (require rackunit)
+
+  (test-case "Test for count-pairs return 3, without shared"
+             (define a (list '1 '2 '3))
+             (check-equal? (count-pairs a) 3))
+
+  (test-case "Test for count-pairs return 4, with shared cons"
+             (define shared (cons 1 2))
+             (define b (cons shared (cons shared '())))
+             (check-equal? (count-pairs b) 4))
+
+  (test-case "Test for count-pairs return 7, with multiple shared cons"
+             (define shared (cons 1 2))
+             (define c (cons shared shared))
+             (define d (cons c c))
+             (check-equal? (count-pairs d) 7)
+             )
+  (test-case "Test for not return at all, with circular reference"
+             ;; (define cycle (cons 1 (cons 2 '())))
+             ;; (set-cdr! (cdr cycle) cycle)
+             ;; (count-pairs cycle) ; 无限循环
+             ;; racket 不支持 set-cdr!, 如果使用 =set-mcdr!= 可变的数
+             ;; 据结构, =cons=, =pair?= 都需要作出相应的修改
+             )
+  )
+