Implement till 3.22

ex-3_12-20.scm

@@ -175,3 +175,30 @@
 
 (display "\nex-3.20\n")
 
+(define (cons x y)
+  (define (set-x! v) (set! x v))
+  (define (set-y! v) (set! y v))
+  (define (dispatch m)
+    (cond ((eq? m 'car) x)
+          ((eq? m 'cdr) y)
+          ((eq? m 'set-car!) set-x!)
+          ((eq? m 'set-cdr!) set-y!)
+          (else (error "Undefined operation -- CONS" m))))
+  dispatch)
+
+(define (car z) (z 'car))
+(define (cdr z) (z 'cdr))
+
+(define (set-car! z new-value)
+  ((z 'set-car!) new-value)
+  z)
+
+(define (set-cdr! z new-value)
+  ((z 'set-cdr!) new-value)
+  z)
+
+(define x (cons 1 2))
+(define z (cons x x))
+(set-car! (cdr z) 17)
+(assert (car x) 17)
+

ex-3_21-23.scm

@@ -0,0 +1,110 @@
+(load "util.scm")
+
+(display "\nexample - queue\n")
+
+(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 q (make-queue))
+(insert-queue! q 'a)
+(insert-queue! q 'b)
+(delete-queue! q)
+(insert-queue! q 'c)
+(insert-queue! q 'd)
+(delete-queue! q)
+
+(display "\nex-3.21 - print queue\n")
+
+(define print-queue front-ptr)
+
+(define q1 (make-queue))
+(assert (print-queue (insert-queue! q1 'a)) '(a))
+(assert (print-queue (insert-queue! q1 'b)) '(a b))
+(assert (print-queue (delete-queue! q1)) '(b))
+(assert (print-queue (delete-queue! q1)) '())
+
+
+(display "\nex-3.22 - procedural queue\n")
+
+(define (make-queue)
+  (let ((front-ptr '())
+        (rear-ptr '()))
+    (define (empty-queue?) (null? front-ptr))
+    (define (insert-queue! item)
+      (let ((new-pair (cons item '())))
+        (cond ((empty-queue?)
+               (set! front-ptr new-pair)
+               (set! rear-ptr new-pair)
+               dispatch)
+              (else
+               (set-cdr! rear-ptr new-pair)
+               (set! rear-ptr new-pair)
+               dispatch))))
+    (define (delete-queue!)
+      (cond ((empty-queue?)
+             (error "DELETE! called with an empty queue" queue))
+            (else
+             (set! front-ptr (cdr front-ptr))
+             dispatch)))
+    (define (dispatch m)
+      (cond
+        ((eq? m 'insert) insert-queue!)
+        ((eq? m 'delete) delete-queue!)
+        ((eq? m 'print) (lambda () front-ptr))
+        (else (error "Undefined operation -- DISPATCH-QUEUE" m))))
+    dispatch)))
+
+(define q (make-queue))
+((q 'insert) 'a)
+((q 'insert) 'b)
+((q 'insert) 'c)
+((q 'insert) 'd)
+(assert ((q 'print)) '(a b c d))
+((q 'delete))
+((q 'delete))
+(assert ((q 'print)) '(c d))
+
+(display "\nex-3.23 -  deque\n")
+
+; A deque (``double-ended queue'') is a sequence in which items can be inserted
+; and deleted at either the front or the rear.  Show how to represent deques
+; using pairs, and give implementations of the operations.23 All operations
+; should be accomplished in (1) steps.
+
+; make-deque
+; empty-deque?
+; front-deque
+; rear-deque
+; front-insert-deque!
+; rear-insert-deque!
+; front-delete-deque!
+; rear-delete-deque!
+

ex-3_24-xx.scm

@@ -0,0 +1,4 @@
+(load "util.scm")
+
+(display "\nex-3.24\n")
+