Complete 3.21

3_21.rkt

@@ -0,0 +1,50 @@
+#lang sicp
+
+(define (front-ptr queue) (car queue))
+(define (rear-ptr queue) (cdr queue))
+(define (set-front-ptr! queue item) (set-car! queue item))
+(define (set-rear-ptr! queue item) (set-cdr! queue item))
+
+(define (empty-queue? queue) (null? (front-ptr queue)))
+(define (make-queue) (cons '() '()))
+
+(define (front-queue queue)
+  (if (empty-queue? queue)
+      (error "FRONT called with an empty queue" queue)
+      (car (front-ptr queue))))
+
+(define (insert-queue! queue item)
+  (let ((new-pair (cons item '())))
+    (cond ((empty-queue? queue)
+           (set-front-ptr! queue new-pair)
+           (set-rear-ptr! queue new-pair)
+           queue)
+          (else
+           (set-cdr! (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 (cdr (front-ptr queue)))
+         queue))) 
+
+
+(define (print-queue queue)
+  (display (front-ptr queue))
+  (newline))
+
+;; EXERCISE 3.21
+(define q1 (make-queue))
+(print-queue q1)
+(insert-queue! q1 'a)
+(print-queue q1)
+(insert-queue! q1 'b)
+(print-queue q1)
+(delete-queue! q1)
+(print-queue q1)
+(delete-queue! q1)
+(print-queue q1)
+