From: Ramakrishnan Muthukrishnan Date: Wed, 15 Sep 2010 18:22:13 +0000 (+0530) Subject: solution to 2.68 X-Git-Url: https://git.rkrishnan.org/%5B/%5D%20/uri/%22file:?a=commitdiff_plain;h=99989d2c82f2b0aeb5482817b792e35aa7baab3c;p=sicp.git solution to 2.68 --- diff --git a/src/sicp/ex2_68.rkt b/src/sicp/ex2_68.rkt new file mode 100644 index 0000000..0606774 --- /dev/null +++ b/src/sicp/ex2_68.rkt @@ -0,0 +1,43 @@ +#lang racket + +(require "ch2_3.rkt" + rackunit) + +(define sample-tree + (make-code-tree (make-leaf 'A 4) + (make-code-tree + (make-leaf 'B 2) + (make-code-tree (make-leaf 'D 1) + (make-leaf 'C 1))))) + +(define sample-bits '(0 1 1 0 0 1 0 1 0 1 1 1 0)) + +(define sample-message (decode sample-bits sample-tree)) + +(define (encode message tree) + (if (null? message) + '() + (append (encode-symbol (car message) tree) + (encode (cdr message) tree)))) + +(define (encode-symbol symbol tree) + (cond + [(and (leaf? tree) + (eqv? (symbol-leaf tree) symbol)) '()] + [(member? symbol (symbols (left-branch tree))) (cons 0 (encode-symbol symbol (left-branch tree)))] + [(member? symbol (symbols (right-branch tree))) (cons 1 (encode-symbol symbol (right-branch tree)))] + [(not (member? symbol (symbols tree))) (error "Symbol not in the tree -- ENCODE-SYMBOL " symbol)])) + +(define (member? x set) + (cond + [(null? set) #f] + [(eqv? x (car set)) #t] + [else (member? x (cdr set))])) + +;;; a simple test. encode-decode should give me back the same symbols. +(check equal? + (decode (encode sample-message + sample-tree) + sample-tree) + sample-message) + \ No newline at end of file