solution to 3.26

diff --git a/src/sicp/ex3_26.rkt b/src/sicp/ex3_26.rkt
new file mode 100644 (file)
index 0000000..4feb501
--- /dev/null
+++ b/src/sicp/ex3_26.rkt
@@ -0,0 +1,69 @@
+#lang r5rs
+
+(define (make-entry k v) (cons k v))
+(define (key entry) (car entry))
+(define (value entry) (cdr entry))
+(define (set-key! entry k) (set-car! entry k))
+(define (set-value! entry v) (set-cdr! entry v))
+(define (null-entry? e)
+  (if (null? e)
+      #t
+      (or (null? (key e))
+          (null? (value e)))))
+
+;(define (set-entry! k v e)
+;  (set-key! k e)
+;  (set-value! v e))
+
+(define (make-node entry left right)
+  (cons entry (cons left (cons right '()))))
+
+(define (entry-node node) (car node))
+(define (left-node node) (car (cdr node)))
+(define (right-node node) (car (cdr (cdr node))))
+
+(define (make-tree)
+  (make-node (make-entry '() '()) '() '()))
+
+(define (entry-tree tree)
+  (entry-node tree))
+
+(define (left-branch tree)
+  (left-node tree))
+
+(define (right-branch tree)
+  (right-node tree))
+
+(define (set-left-branch! tree val)
+  (set-car! (cdr tree) val))
+
+(define (set-right-branch! tree val)
+  (set-car! (cdr (cdr tree)) val))
+
+(define (set-entry! tree entry)
+  (set-car! tree entry))
+
+(define (make-tree-node k v)
+  (make-node (make-entry k v) '() '()))
+
+(define (lookup k table)
+  (let ((e (entry-tree table)))
+    (cond
+      ((null-entry? e) #f)
+      ((= k (key e)) (value e))
+      ((< k (key e)) (lookup k (left-branch table)))
+      ((> k (key e)) (lookup k (right-branch table))))))
+
+(define (insert! k v table)
+  (cond
+    ((null-entry? (entry-tree table)) (set-entry! table (make-entry k v)))
+    ((= k (key (entry-tree table))) (set-value! (entry-tree table) v))
+    ((< k (key (entry-tree table))) 
+     (if (null? (left-branch table))
+         (set-left-branch! table (make-tree-node k v))
+         (insert! k v (left-branch table))))
+    ((> k (key (entry-tree table))) 
+     (if (null? (right-branch table))
+         (set-right-branch! table (make-tree-node k v))
+         (insert! k v (right-branch table)))))
+'ok)
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