Answer 5.20

ex-5_20-22.scm

@@ -0,0 +1,48 @@
+(load "util.scm")
+(load "misc/sicp-regsim.scm")
+
+(display "\nex-5.20 - box-and-pointer\n")
+
+(define x (cons 1 2))
+(define y (list x x))
+
+; | Index | 0 | 1 | 2 | 3 | 4 | 5 | 6 |
+; |-------|---|---|---|---|---|---|---|
+; |the-car|   | n1| p1| p1|   |   |   |
+; |the-cdr|   | n2| p3| e0|   |   |   |
+
+(display "[done]\n")
+
+(display "\nex-5.21 - count-leaves\n")
+
+(define (count-leaves tree)
+  (cond ((null? tree) 0)
+        ((not (pair? tree)) 1)
+        (else (+ (count-leaves (car tree))
+                 (count-leaves (cdr tree))))))
+
+(define (not-pair? x) (not (pair? x)))
+
+(define count-leaves-machine
+  (make-machine
+    '(tree val continue)
+    (list (list 'cons cons) (list 'car car) (list 'cdr cdr)
+          (list 'not-pair? not-pair?) (list 'null? null?))
+    '(controller
+        (assign val (const 0))
+        (assign continue (label count-done))
+      count-leaves
+        (test (op null?) (reg tree))
+        (branch (label null-case))
+        (test (op not-pair?))
+        (branch (label not-pair-case))
+      null-case
+      not-pair-case
+        (goto (reg continue))
+      count-done)))
+
+(set-register-contents! count-leaves-machine 'tree '())
+(start count-leaves-machine)
+(assert (get-register-contents count-leaves-machine 'val) 0)
+
+(display "\nex-5.22\n")

ex-5_23-xx.scm

@@ -1,4 +1,5 @@
 (load "util.scm")
 (load "misc/sicp-regsim.scm")
 
-(display "\nex-5.20\n")
+(display "\nex-5.23\n")
+