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 17.3.0) (read-case-sensitive #t) (teachpacks ((lib "draw.ss" "teachpack" "htdp") (lib "arrow.ss" "teachpack" "htdp") (lib "dir.ss" "teachpack" "htdp"))) (htdp-settings #(#t constructor repeating-decimal #f #t none #f ((lib "draw.ss" "teachpack" "htdp") (lib "arrow.ss" "teachpack" "htdp") (lib "dir.ss" "teachpack" "htdp")))))
4 ;A list-of-symbols is either
5 ;1. empty or
6 ;2. (cons s los)
7 ;where s is a symbol and los is a list-of-symbols.
8 ;
9 ;A natural-number [>=1] (N[>=1]) is either
10 ;1. 1 or
11 ;2. (add1 n) where n is [N(>=1)]
12 
13 ;list-pick : list-of-symbols N[>=1] -> symbol
14 ;Given a-los and n, find the nth symbol in a-los.  The first symbol in a-los has an index of 1.
15 
16 (define (list-pick a-los n)
17   (cond
18     [(and (= n 1)
19           (empty? a-los)) (error 'list-pick "list too short")]
20     [(and (= n 1)
21           (cons? a-los)) (first a-los)]
22     [(and (> n 1)
23           (empty? a-los)) (error 'list-pick "list too short")]
24     [(and (> n 1)
25           (cons? a-los)) (list-pick (rest a-los) (sub1 n))]))
26 
27 (define los '(Fred Ted Bill Bob Joe))
28 
29 ;A natural-number [>=0] (N[>=0]) is either
30 ;1. 0 or 
31 ;2. (add1 n) where n is [N(>=0)].
32 
33 ;list-pick : list-of-symbols N[>=1] -> symbol
34 ;Given a-los and n, find the nth symbol in a-los.  The first symbol in a-los has an index of 0.
35 ;(define (list-pick0 a-los n)
36 ;  (cond
37 ;    [(and (= n 0)
38 ;          (empty? a-los)) (error 'list-pick0 "list too short")]
39 ;    [(and (= n 0)
40 ;          (cons? a-los)) (first a-los)]
41 ;    [(and (> n 0)
42 ;          (empty? a-los)) (error 'list-pick0 "list too short")]
43 ;    [(and (> n 0)
44 ;          (cons? a-los)) (list-pick0 (rest a-los) (sub1 n))]))
45 (define (list-pick0 a-los n)
46   (cond
47     [(empty? a-los) (error 'list-pick0 "list too short")]
48     [(= n 0) (first a-los)]
49     [(> n 0) (list-pick0 (rest a-los) (sub1 n))]))
50 
51 
52 ;
53 ;Data Definition
54 ;A list-of-data is either
55 ;1. empty or
56 ;2. (cons d lod)
57 ;where d is a Scheme datum and lod is a list-of-data.
58 ;
59 ;replace-eol-with : list-of-data list-of-data -> list-of-data
60 ;(define (replace-eol-with lod1 lod2)
61 ;  (cond
62 ;    [(empty? lod1)
63 ;     (cond
64 ;       [(empty? lod2) empty]
65 ;       [(cons? lod2) lod2])]
66 ;    [(cons? lod1)
67 ;     (cond
68 ;       [(empty? lod2) lod1]
69 ;       [(cons? lod2) (append (list (first lod1))
70 ;                             (replace-eol-with (rest lod1) lod2))])]))
71 
72 (define (replace-eol-with lod1 lod2)
73   (cond
74     [(and (empty? lod1)
75           (empty? lod2)) empty]
76     [(and (empty? lod1)
77           (cons? lod2)) lod2]
78     [(empty? lod2) lod1]
79     [(cons? lod2) (append (list (first lod1))
80                                 (replace-eol-with (rest lod1) lod2))]))
81