《how to design programs》第10章表的进一步处理

返回表的函数：

下面是一个求工资的函数：

;; wage : number  ->  number
;; to compute the total wage (at $12 per hour)
;; of someone who worked for h hours
(define (wage h)
  (* 12 h))

显示，只能求单个人的工资，我们相求多个人的工资，于是想传入一个list过去:

;; hours->wages : list-of-numbers  ->  list-of-numbers
;; to create a list of weekly wages from a list of weekly hours (alon)
(define (hours->wages alon) ...)

我们希望函数返回一个list，里面的每个元素对应传入的时间。完整代码：

(define (wage h)
  (* 12 h))
;hours->ages:list-of-numbers->list-of->numbers
(define (hours->wages alon)
  (cond
    [(empty? alon)  empty]
    [else (cons (wage (car alon)) (hours->wages (cdr alon)) )]
    ))
(hours->wages (cons 480 (cons 336 empty)) )
输出：'(5760 4032)

包含结构体的表:

下面定义了一个库存记录inventory-record:

(define-struct ir (name price))

现在我们可以存储清单是下列2者之一：
1.empty
2.(cons ir inv) 其中ir是一条库存记录，inv是一个库存清单。

The simplest example of an inventory is empty. To create a larger inventory, we must create an inventory record and cons it onto another inventory:

(cons (make-ir 'doll 17.95)
  empty)

From here, we can create yet a larger inventory listing:

(cons (make-ir 'robot 22.05)
  (cons (make-ir 'doll 17.95)
    empty))


如何求所有库存的总价格。

(define-struct ir (name price))
(define (sum an-inv)
  (cond
    [(empty? an-inv) 0]
    [else (+ (ir-price (car an-inv))
             (sum (cdr an-inv)))]
    ))

(define x
  (cons (make-ir 'robot 22.05)
        (cons (make-ir 'doll  17.95)
              empty)))
(sum x) ;output 40