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 18.1.11) (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 (define-struct star (name instrument))
5 
6 ;A star is a structure
7 ;(make-star name instrument)
8 ;where both name and instrument are symbols.
9 
10 (define alos
11   (list (make-star 'Chris 'saxophone)
12         (make-star 'Robby 'trumpet)
13         (make-star 'Matt 'violin)
14         (make-star 'Wen 'guitar)
15         (make-star 'Matt 'radio)))
16 
17 ;A list-of-stars is either
18 ;1. empty or
19 ;2. (cons s los)
20 ;where s is a star structure and los is a list-of-stars.
21 ;
22 ;last-occurrence : symbol list-of-star -> star/false
23 ;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.
24 
25 (define (last-occurrence a-name a-los)
26   (cond
27     [(empty? a-los) false]
28     [else (local ((define r (last-occurrence a-name (rest a-los)))) 
29             (cond
30               [(star? r) r]
31               [(symbol=? a-name (star-name (first a-los))) (first a-los)]
32               [else false]))]))
33 
34 (last-occurrence 'Matt
35                  (list (make-star 'Matt 'violin)
36                        (make-star 'Matt 'radio)))
37 
38 (cond
39   [false false]
40   [else (local ((define r (last-occurrence 'Matt (list (make-star 'Matt 'radio))))) 
41           (cond
42             [(star? r) r]
43             [(symbol=? 'Matt (star-name (make-star 'Matt 'violin))) (make-star 'Matt 'violin)]
44             [else false]))])
45 
46 (define r-1 (last-occurrence 'Matt (list (make-star 'Matt 'radio))))
47 
48 (cond
49   [false false]
50   [else (cond
51           [(star? r-1) r-1]
52           [(symbol=? 'Matt (star-name (make-star 'Matt 'violin))) (make-star 'Matt 'violin)]
53           [else false])])
54 #|
55 Now, we know that r-1 is
56 (last-occurrence 'Matt (list (make-star 'Matt 'radio)))
57 
58 (last-occurrence 'Matt (list (make-star 'Matt 'radio)))
59 
60 (cond
61   [false false]
62   [else (local ((define r-2 (last-occurrence 'Matt empty))) 
63           (cond
64             [(star? r-2) r-2]
65             [(symbol=? 'Matt (star-name (make-star 'Matt 'radio))) (make-star 'Matt 'radio)]
66             [else false]))])
67 
68 (define r-2 (last-occurrence 'Matt empty))
69 
70 (cond
71   [false false]
72   [else (cond
73           [(star? r-2) r-2]
74           [(symbol=? 'Matt (star-name (make-star 'Matt 'radio))) (make-star 'Matt 'radio)]
75           [else false])])
76 
77 Now, we know that r-2 is
78 (last-occurrence 'Matt empty)
79 which is false, so r-2 is
80 false
81 
82 (cond
83   [false false]
84   [else (cond
85           [false r-2]
86           [(symbol=? 'Matt 'Matt) (make-star 'Matt 'radio)]
87           [else false])])
88 
89 (cond
90   [false false]
91   [else (cond
92           [false r-2]
93           [true (make-star 'Matt 'radio)]
94           [else false])])
95 
96 Hence, r-1 is
97 (make-star 'Matt 'radio)
98 |#
99 
100 (define r-1 (make-star 'Matt 'radio))
101 
102 (cond
103   [false false]
104   [else (cond
105           [true (make-star 'Matt 'radio)]
106           [(symbol=? 'Matt (star-name (make-star 'Matt 'violin))) (make-star 'Matt 'violin)]
107           [else false])])
108 
109 (make-star 'Matt 'radio)