1 12687dd9 2023-08-04 jrmu ;; The first three lines of this file were inserted by DrScheme. They record metadata
2 12687dd9 2023-08-04 jrmu ;; about the language level of this file in a form that our tools can easily process.
3 12687dd9 2023-08-04 jrmu #reader(lib "htdp-intermediate-reader.ss" "lang")((modname 18.1.11-2) (read-case-sensitive #t) (teachpacks ((lib "draw.ss" "teachpack" "htdp"))) (htdp-settings #(#t constructor repeating-decimal #f #t none #f ((lib "draw.ss" "teachpack" "htdp")))))
4 12687dd9 2023-08-04 jrmu (define-struct star (name instrument))
6 12687dd9 2023-08-04 jrmu ;A star is a structure
7 12687dd9 2023-08-04 jrmu ;(make-star name instrument)
8 12687dd9 2023-08-04 jrmu ;where both name and instrument are symbols.
10 12687dd9 2023-08-04 jrmu (define alos
11 12687dd9 2023-08-04 jrmu (list (make-star 'Chris 'saxophone)
12 12687dd9 2023-08-04 jrmu (make-star 'Robby 'trumpet)
13 12687dd9 2023-08-04 jrmu (make-star 'Matt 'violin)
14 12687dd9 2023-08-04 jrmu (make-star 'Wen 'guitar)
15 12687dd9 2023-08-04 jrmu (make-star 'Matt 'radio)))
17 12687dd9 2023-08-04 jrmu ;A list-of-stars is either
18 12687dd9 2023-08-04 jrmu ;1. empty or
19 12687dd9 2023-08-04 jrmu ;2. (cons s los)
20 12687dd9 2023-08-04 jrmu ;where s is a star structure and los is a list-of-stars.
22 12687dd9 2023-08-04 jrmu ;last-occurrence : symbol list-of-star -> star/false
23 12687dd9 2023-08-04 jrmu ;Given a-name and a-los, find the last occurrence of a star with a-name in the name field and return this star structure. Return false if there is no star with a-name.
25 12687dd9 2023-08-04 jrmu (define (last-occurrence a-name a-los)
27 12687dd9 2023-08-04 jrmu [(empty? a-los) false]
28 12687dd9 2023-08-04 jrmu [else (local ((define r (last-occurrence a-name (rest a-los))))
30 12687dd9 2023-08-04 jrmu [(star? r) r]
31 12687dd9 2023-08-04 jrmu [(symbol=? a-name (star-name (first a-los))) (first a-los)]
32 12687dd9 2023-08-04 jrmu [else false]))]))
34 12687dd9 2023-08-04 jrmu (last-occurrence 'Matt
35 12687dd9 2023-08-04 jrmu (list (make-star 'Matt 'violin)
36 12687dd9 2023-08-04 jrmu (make-star 'Matt 'radio)))
39 12687dd9 2023-08-04 jrmu [false false]
40 12687dd9 2023-08-04 jrmu [else (local ((define r (last-occurrence 'Matt (list (make-star 'Matt 'radio)))))
42 12687dd9 2023-08-04 jrmu [(star? r) r]
43 12687dd9 2023-08-04 jrmu [(symbol=? 'Matt (star-name (make-star 'Matt 'violin))) (make-star 'Matt 'violin)]
44 12687dd9 2023-08-04 jrmu [else false]))])
46 12687dd9 2023-08-04 jrmu (define r-1 (last-occurrence 'Matt (list (make-star 'Matt 'radio))))
49 12687dd9 2023-08-04 jrmu [false false]
51 12687dd9 2023-08-04 jrmu [(star? r-1) r-1]
52 12687dd9 2023-08-04 jrmu [(symbol=? 'Matt (star-name (make-star 'Matt 'violin))) (make-star 'Matt 'violin)]
53 12687dd9 2023-08-04 jrmu [else false])])
55 12687dd9 2023-08-04 jrmu Now, we know that r-1 is
56 12687dd9 2023-08-04 jrmu (last-occurrence 'Matt (list (make-star 'Matt 'radio)))
58 12687dd9 2023-08-04 jrmu (last-occurrence 'Matt (list (make-star 'Matt 'radio)))
61 12687dd9 2023-08-04 jrmu [false false]
62 12687dd9 2023-08-04 jrmu [else (local ((define r-2 (last-occurrence 'Matt empty)))
64 12687dd9 2023-08-04 jrmu [(star? r-2) r-2]
65 12687dd9 2023-08-04 jrmu [(symbol=? 'Matt (star-name (make-star 'Matt 'radio))) (make-star 'Matt 'radio)]
66 12687dd9 2023-08-04 jrmu [else false]))])
68 12687dd9 2023-08-04 jrmu (define r-2 (last-occurrence 'Matt empty))
71 12687dd9 2023-08-04 jrmu [false false]
73 12687dd9 2023-08-04 jrmu [(star? r-2) r-2]
74 12687dd9 2023-08-04 jrmu [(symbol=? 'Matt (star-name (make-star 'Matt 'radio))) (make-star 'Matt 'radio)]
75 12687dd9 2023-08-04 jrmu [else false])])
77 12687dd9 2023-08-04 jrmu Now, we know that r-2 is
78 12687dd9 2023-08-04 jrmu (last-occurrence 'Matt empty)
79 12687dd9 2023-08-04 jrmu which is false, so r-2 is
83 12687dd9 2023-08-04 jrmu [false false]
86 12687dd9 2023-08-04 jrmu [(symbol=? 'Matt 'Matt) (make-star 'Matt 'radio)]
87 12687dd9 2023-08-04 jrmu [else false])])
90 12687dd9 2023-08-04 jrmu [false false]
93 12687dd9 2023-08-04 jrmu [true (make-star 'Matt 'radio)]
94 12687dd9 2023-08-04 jrmu [else false])])
96 12687dd9 2023-08-04 jrmu Hence, r-1 is
97 12687dd9 2023-08-04 jrmu (make-star 'Matt 'radio)
100 12687dd9 2023-08-04 jrmu (define r-1 (make-star 'Matt 'radio))
103 12687dd9 2023-08-04 jrmu [false false]
104 12687dd9 2023-08-04 jrmu [else (cond
105 12687dd9 2023-08-04 jrmu [true (make-star 'Matt 'radio)]
106 12687dd9 2023-08-04 jrmu [(symbol=? 'Matt (star-name (make-star 'Matt 'violin))) (make-star 'Matt 'violin)]
107 12687dd9 2023-08-04 jrmu [else false])])
109 12687dd9 2023-08-04 jrmu (make-star 'Matt 'radio)
