1 ;; The first three lines of this file were inserted by DrScheme. They record metadata
2 ;; about the language level of this file in a form that our tools can easily process.
3 #reader(lib "htdp-intermediate-reader.ss" "lang")((modname |21.2|) (read-case-sensitive #t) (teachpacks ()) (htdp-settings #(#t constructor repeating-decimal #f #t none #f ())))
4 ;move-all : (listof posn) -> (listof posn)
5 ;Given alop, translate every posn to the right by 3.
6 
7 ;move-x-3 : posn -> posn
8 ;Given aposn, translate to the right by 3.
9 
10 (define (move-all alop)
11   (local ((define (move-all alop)
12             (map move-x-3 alop))
13           (define (move-x-3 aposn)
14             (make-posn (+ 3 (posn-x aposn))
15                        (posn-y aposn))))
16     (move-all alop)))
17 
18 (define-struct toy (name price))
19 
20 ;A toy is a structure
21 ;(make-toy n p)
22 ;where n is a symbol and p is a number.
23 ;
24 
25 
26 ;eliminate-exp : (listof toy) number -> (listof toy)
27 ;Given alot, return all those (listof toy) whose price is below ua.
28 
29 ;local
30 ;eliminate-exp : (listof toy) -> (listof toy)
31 ;Given alot, return all those (listof toy) whose price is below ua.
32 ;
33 ;expensive : toy -> boolean
34 ;Given ua, return true if the atoy has a price below ua.
35 
36 (define (eliminate-exp alot ua)
37   (local ((define (eliminate-exp alot)
38             (filter expensive? alot))
39           (define (expensive? atoy)
40             (< (toy-price atoy) ua)))
41     (eliminate-exp alot)))
42 
43 ;recall : symbol (listof symbol) -> (listof symbol)
44 ;Consumes the name of a toy, called ty, and a list of names, called lon, and produces a list of names that contains all components of lon with the exception of ty
45 
46 (define (recall ty lon) 
47   (local ((define (recall lon)
48             (filter not-equal lon))
49           (define (not-equal name)
50             (not (symbol=? ty name))))
51     (recall lon)))
52 
53 selection : (listof symbol) (listof symbol) -> (listof symbol)
54 
55 (define (selection list1 list2)
56   (cond
57     [(empty? list2) empty]
58     [(filter in-other-list? (first list2))
59      (cons (first list2)
60            (selection list1 (rest list2)))]
61     [else (selection list1 (rest list2))]))
62 
63 in-other-list? :  symbol -> boolean
64 Given name, determine if it occurs in the other list.
65 
66 (filter listequal list1)
67 listequal
68 = (first list2) (first list1)
69 = (first list2) (second list1)
70 ...
71 = (first list2) (first list1)
72 
73 
74 ;selection : (listof symbol) (listof symbol) -> (listof symbol)
75 ;selection, which consumes two lists of names and selects all those from the second one that are also on the first.
76 
77 (define (selection list1 list2)
78   (cond
79     [(empty? list2) empty]
80     [(in-other-list? (first list2) list1) (cons (first list2)
81                                                 (selection list1 (rest list2)))]
82     [else (selection list1 (rest list2))]))
83 
84 (define (in-other-list? an-item a-list)
85   (cond
86     [(empty? a-list) false]
87     [(equal? (first a-list) an-item) true]
88     [else (in-other-list? an-item (rest a-list))]))
89 
90 in-other-list? symbol -> boolean
91 Given name and list2, determine if name occurs anywhere in list2.
92 
93 
94 
95 #|
96 
97 Exercise 21.2.3.   Here is the version of filter that DrScheme provides:
98 
99 ;; filter : (X  ->  boolean) (listof X)  ->  (listof X)
100 ;; to construct a list of X from all those items on alon
101 ;; for which predicate? holds 
102 (define (filter predicate? alon)
103   (cond
104     [(empty? alon) empty]
105     [else (cond
106             [(predicate? (first alon)) 
107              (cons (first alon) (filter predicate? (rest alon)))]
108             [else (filter predicate? (rest alon))])]))
109 1. eliminate-exp, which consumes a number, ua, and a list of toy structures (containing name and price) and produces a list of all those descriptions whose price is below ua;
110 2. recall, which consumes the name of a toy, called ty, and a list of names, called lon, and produces a list of names that contains all components of lon with the exception of ty;
111 3. selection, which consumes two lists of names and selects all those from the second one that are also on the first.
112 
113 |#