消费清单球拍

Question

(define-struct restaurant (name address lod)) 
(define-struct dish (name main-ingredient price)) 

(define lod 
    (list 
    (make-dish "BNC" "pepper" 15.00) 
    (make-dish "BBQ" "sauce" 20.00) 
    (make-dish "BAW" "vanilla" 18.00) 
    (make-dish "ACA" "tomato" 15.00) 
    (make-dish "NAM" "onions" 25.00) 
    (make-dish "PLA" "milk" 34.00))) 

(define lod1 
    (list 
    (make-dish "AHSH" "carrots" 23.00) 
    (make-dish "PROA" "garlic" 21.00) 
    (make-dish "MENE" "eggs" 16.00) 
    (make-dish "SAYB" "water" 11.00) 
    (make-dish "WAFA" "chicken" 22.00) 
    (make-dish "LDAFK" "chocolate" 10.00))) 


(define Sky-Dragon (make-restaurant "Sky Dragon" "23 Cutoiut, NY" lod)) 
(define Tomas (make-restaurant "Tomas" "34 Rossi, CA" lod1)) 


(define restaurant1 Sky-Dragon) 
(define restaurant2 Tomas) 

我想知道你将如何编写一个函数，消耗一个餐厅，并产生餐厅的总数。就像我带餐馆1一样，我得到6道菜。希望我这样做是正确的。

Answer 1

很简单，只需使用存取程序检索的餐厅菜品的列表，然后内置的程序来决定大小（或“长”），其中列出的：

(define (number-of-dishes restaurant) 
    (<???>     ; how do we determine the size of a list? 
    (<???> restaurant))) ; how do we access a restaurant's list of dishes? 

我会让你弄清楚细节，但请记住 - 无论何时使用define-struct定义一个新结构，都会为其每个字段自动创建一个访问器过程。例如，dish的price是通过在dish：(dish-price dish)上调用dish-price获得的。