solutions to 3.21 and 3.22

diff --git a/src/sicp/ex3_21.rkt b/src/sicp/ex3_21.rkt
new file mode 100644 (file)
index 0000000..9bff4b6
--- /dev/null
+++ b/src/sicp/ex3_21.rkt
@@ -0,0 +1,49 @@
+#lang r5rs
+
+(define error display)
+
+(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 on an empty queue" queue))
+    (else
+     (set-front-ptr! queue (cdr (front-ptr queue)))
+     queue)))
+
+(define (print-queue queue)
+  (define (copy-queue q1)
+    (let ((q2 (make-queue)))
+      (set-front-ptr! q2 (front-ptr q1))
+      (set-rear-ptr! q2 (rear-ptr q1))
+      q2))
+  (let ((q (copy-queue queue)))
+    (if (not (empty-queue? q))
+        (begin
+          (display (front-queue q))
+          (display " ")
+          (print-queue (delete-queue! q))))))
+        
+      
+     
\ No newline at end of file

diff --git a/src/sicp/ex3_22.rkt b/src/sicp/ex3_22.rkt
new file mode 100644 (file)
index 0000000..d922e4b
--- /dev/null
+++ b/src/sicp/ex3_22.rkt
@@ -0,0 +1,59 @@
+#lang r5rs
+
+(define error display)
+(define (make-queue)
+  (let ((front-ptr '())
+        (rear-ptr '()))
+    (define (set-front-ptr! queue item)
+      (set! front-ptr item))
+    (define (set-rear-ptr! queue item)
+      (set! rear-ptr item))
+    (define (empty-queue? queue)
+      (null? front-ptr))
+    (define (front-queue queue)
+      (if (empty-queue? queue)
+          (error "FRONT called with an empty queue" queue)
+          (car front-ptr)))
+    (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)
+               front-ptr)
+              (else
+               (set-cdr! rear-ptr new-pair)
+               (set-rear-ptr! queue new-pair)
+               front-ptr))))
+    (define (delete-queue! queue)
+      (cond ((empty-queue? queue)
+             (error "DELETE! called with an empty queue" queue))
+            (else
+             (set-front-ptr! queue (cdr front-ptr))
+             front-ptr)))
+    (define (dispatch m)
+      (cond 
+        ((eq? m 'insert-queue!) 
+         (lambda (queue item)
+           (insert-queue! queue item)))
+        ((eq? m 'delete-queue!)
+         (lambda (queue)
+           (delete-queue! queue)))
+        ((eq? m 'empty-queue?)
+         (lambda (queue)
+           (empty-queue? queue)))
+        ((eq? m 'front-queue)
+         (lambda (queue)
+           (front-queue queue)))))
+    dispatch))
+
+(define (empty-queue? queue)
+  ((queue 'empty-queue?) queue))
+
+(define (front-queue queue)
+  ((queue 'front-queue) queue))
+
+(define (insert-queue! queue item)
+  ((queue 'insert-queue!) queue item))
+
+(define (delete-queue! queue)
+  ((queue 'delete-queue!) queue))