SICP学习笔记 第二章 （2.4）

部分习题：

exercise 2.73

a)对求导过程来说，数字和变量的表示形式对任何“类型标志”（例如+）都是一致的，没有必要实现不同的表示方式。

b)

(define (install-deriv-sum-package)
  ;internal procedure
  (define (addend s) (cadr s))
  (define (augend s) (caddr s))
  (define (make-sum a1 a2)
    (cond ((=number? a1 0) a2)
      ((=number? a2 0) a1)
      ((and (number? a1) (number a2)) (+ a1 a2))
      (else (list '+ a1 a2))))
  ;interface
  (put 'addend '+ addend)
  (put 'augend '+ augend)
  (put 'make-sum '+ make-sum)
  (put 'deriv '+ (lambda (exp var)
           (make-sum (deriv (addend exp) var)
                 (deriv (augend exp) var))))
  'done)

(define (install-deriv-product-package)
  ;internal pricedure
  (define (multiplier p) (cadr p))
  (define (multiplicand p) (caddr p))
  (define (make-product p1 p2)
    (cond ((or (=number? p1 0) (=number? p2 0)) 0)
      ((=number? p1 1) p2)
      ((=number? p2 1) p1)
      ((and (number? p1) (number? p2)) (* m1 m2))
      (else (list '* p1 p2))))
  ;interface
  (put 'multiplier '* multiplier)
  (put 'multiplicand '* multiplicand)
  (put 'make-product '* make-product)
  (put 'deriv '* (lambda (exp var)
           (make-sum (make-product (multiplier exp)
                       (deriv (multiplicand exp) var))
                 (make-product (deriv (multiplier exp) var)
                       (multiplicand exp)))))
  'done)

exercise 2.74

a)人事文件应当有一个tag信息，用于识别该文件属于哪一个分支机构。

exercise 2.75

(define (make-from-mag-ang x y)
  (define (dispatch x y)
    (cond ((eq? op 'real-part) (* x (cos y)))
          ((eq? op 'imag-part) (* x (sin y)))
          ((eq? op 'magnitude) x)
          ((eq? op 'angle) y)
          (else (error "Unkown op -- MAKE-FROM-MAG-ANG" op))))
  dispatch)