增加练习2-88.

Author: ramsayleung

chapter2/exercise2-87.rkt

@@ -1,4 +1,5 @@
 #lang racket
+(require "generic_operation.scm")
 (require "poly.rkt")
 ;;; 练习2-87
 (define (=zero-poly? poly)
@@ -9,3 +10,5 @@
              (all-coeff-zero? (rest-terms term-list)))))
   (all-coeff-zero? (term-list poly)))
 
+(put '=zero? '(polynomial) (lambda (p) (=zero-poly? p)))
+

chapter2/exercise2-88.rkt

@@ -0,0 +1,38 @@
+#lang racket
+(require "generic_operation.scm")
+(require "poly.rkt")
+;;; 先定义一个求负操作 neg
+;;; 加法的表示：a + b.
+;;; 减法的表示: a - b. 或者时a + (neg b). 所以只需要定义一个求负操作，就可以复用
+;;; 已有的加法过程
+
+(define (install-scheme-number-neg)
+  (define (tag x)
+    (attach-tag 'scheme-number x))
+
+  (put 'neg '(scheme-number) (lambda (a) (tag (- a))))
+  )
+
+(define (neg x) (apply-generic 'neg x))
+
+(define (install-polynomial-neg)
+  (define (tag x)
+    (attach-tag 'polynomial x))
+
+  (define (neg-term terms)
+    (if (empty-termlist? terms)
+        terms
+        (let ((first (first-term terms)))
+          (adjoin-term (make-term (order first) (neg (coeff first)))
+                       (neg-term (rest-terms terms))))))
+  (define (neg-poly p)
+    (make-poly (variable p)
+               (neg-term (term-list p))))
+
+  (put 'neg '(polynomial) (lambda (x) (tag (neg-poly p))))
+
+  (define (sub-poly p1 p2)
+    (add-poly p1 (neg-poly p2)))
+
+  (put 'sub '(polynomial polynomial) (lambda (x) (tag (sub-poly p1 p2))))
+  )

chapter2/poly.rkt

@@ -102,4 +102,4 @@
                     (mul (coeff t1) (coeff t2)))
          (mul-term-by-all-terms t1 (rest-terms L))))))
 
-(provide empty-termlist? =zero? first-term rest-terms coeff term-list)
+(provide empty-termlist? =zero? first-term rest-terms coeff term-list adjoin-term variable make-term)