Add solution for exercise 3-21

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: 20/82
+  - 章节三: 21/82
   - 章节四: TODO
   - 章节五: TODO
 * 运行

chapter3/exercise3-21.org

@@ -0,0 +1,15 @@
+#+LATEX_CLASS: ramsay-org-article
+#+LATEX_CLASS_OPTIONS: [oneside,A4paper,12pt]
+#+AUTHOR: Ramsay Leung
+#+EMAIL: [email protected]
+#+DATE: 2025-06-25 Wed 23:57
+* 打印输出的问题
+  - Scheme 的标准打印不知道队列的特殊结构，仍然是按照序列进行打印
+  - 它简单打印整个序对，因此 =((a b)b)= 中的 =b= 看起来重复，实际上只是 =rear-ptr= 指向最后一个元素
+* 删除后的状态
+  - 当删除所有元素后，=front-ptr= 指向一个空序对 =()=
+  - 但 =rear-ptr= 仍然指向最后一个被删除的元素 =b=, 因为我们只更新了 =front-ptr=
+  - 打印为 =(() b)= 看起来像是非空队列，实际上队列是空的.
+
+[[file:../img/chapter3/exercise-3-21.png]]
+

chapter3/exercise3-21.rkt

@@ -0,0 +1,28 @@
+#lang racket
+(require "queue.rkt")
+
+(define (print-queue queue)
+  (displayln (front-ptr queue)))
+
+(module+ test
+  (require rackunit)
+
+  (test-case "Test for empty queue"
+             (define q (make-queue))
+             (check-equal? (with-output-to-string (lambda () (print-queue q))) "()\n"))
+
+  (test-case "Test for queue with 2 elements"
+             (define q (make-queue))
+             (insert-queue! q 'a)
+             (insert-queue! q 'b)
+             (check-equal? (with-output-to-string (lambda () (print-queue q))) "{a b}\n"))
+
+  (test-case "Test for queue after deleting elements"
+             (define q (make-queue))
+             (insert-queue! q 'a)
+             (insert-queue! q 'b)
+             (check-equal? (with-output-to-string (lambda () (print-queue q))) "{a b}\n")
+             (delete-queue! q)
+             (check-equal? (with-output-to-string (lambda () (print-queue q))) "{b}\n")
+             )
+  )

chapter3/queue.rkt

@@ -0,0 +1,37 @@
+#lang racket
+(define (front-ptr queue) (mcar queue))
+
+(define (rear-ptr queue) (mcdr queue))
+
+(define (set-front-ptr! queue item) (set-mcar! queue item))
+
+(define (set-rear-ptr! queue item) (set-mcdr! queue item))
+
+(define (empty-queue? queue) (null? (front-ptr queue)))
+
+(define (make-queue) (mcons '() '()))
+
+(define (front-queue queue)
+  (if (empty-queue? queue)
+      (error "FRONT called with an empty queue" queue)
+      (mcar (front-ptr queue))))
+
+(define (insert-queue! queue item)
+  (let ((new-pair (mcons item '())))
+    (cond ((empty-queue? queue)
+           (set-front-ptr! queue new-pair)
+           (set-rear-ptr! queue new-pair)
+           queue)
+          (else
+           (set-mcdr! (rear-ptr queue) new-pair)
+           (set-rear-ptr! queue new-pair)
+           queue))))
+
+(define (delete-queue! queue)
+  (cond ((empty-queue? queue)
+         (error "DELETE! called with an empty queue" queue))
+        (else
+         (set-front-ptr! queue (mcdr (front-ptr queue)))
+         queue)))
+
+(provide front-ptr rear-ptr set-front-ptr! set-rear-ptr! empty-queue? make-queue front-queue insert-queue! delete-queue!)