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 17.6.6) (read-case-sensitive #t) (teachpacks ((lib "draw.ss" "teachpack" "htdp") (lib "arrow.ss" "teachpack" "htdp") (lib "dir.ss" "teachpack" "htdp") (lib "hangman.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") (lib "hangman.ss" "teachpack" "htdp")))))
4 ;A list-of-symbols is either
5 ;1. empty or
6 ;2. (cons s los)
7 ;where s is a one-letter symbol and los is a list-of-symbols.
8 ;
9 ;DNAprefix : list-of-symbols list-of-symbols -> boolean
10 ;Given pattern and search-string, determine if pattern is a prefix in search-string.
11 
12 ;(define (DNAprefix pattern search-string)
13 ;  (cond
14 ;    [(> (length pattern) (length search-string)) false]
15 ;    [(empty? pattern) true]
16 ;    [else (and (symbol=? (first pattern)
17 ;                         (first search-string))
18 ;               (DNAprefix (rest pattern) (rest search-string)))]))
19 
20 ;(define (DNAprefix pattern search-string)
21 ;  (cond
22 ;    [(and (empty? pattern)
23 ;          (empty? search-string)) true]
24 ;    [(and (empty? pattern)
25 ;          (cons? search-string)) true]
26 ;    [(and (cons? pattern)
27 ;          (empty? search-string)) false]
28 ;    [(and (cons? pattern)
29 ;          (cons? search-string)) (and (symbol=? (first pattern) (first search-string))
30 ;                                      (DNAprefix (rest pattern) (rest search-string)))]))
31 
32 (define (DNAprefix pattern search-string)
33   (cond
34     [(empty? pattern) true]
35     [(empty? search-string) false]
36     [else (and (symbol=? (first pattern) (first search-string))
37                (DNAprefix (rest pattern) (rest search-string)))]))
38 
39 ;properDNAprefix : list-of-symbols list-of-symbols -> symbol/boolean
40 ;Given pattern and search-string, return the first symbol in search-string right after pattern if pattern is a proper prefix of search-string.  However, if pattern and search-string are identical lists, then simply return true.  If pattern is not a prefix, return false.
41 
42 ;(define (properDNAprefix pattern search-string)
43 ;  (cond
44 ;    [(> (length pattern) (length search-string)) false]
45 ;    [(empty? pattern)
46 ;     (cond
47 ;       [(empty? search-string) true]
48 ;       [else (first search-string)])]
49 ;    [else 
50 ;     (cond
51 ;       [(symbol=? (first pattern)
52 ;                  (first search-string))
53 ;        (properDNAprefix (rest pattern) (rest search-string))]
54 ;       [else false])]))
55 
56 (define (properDNAprefix pattern search-string)
57   (cond
58     [(and (empty? pattern)
59           (empty? search-string)) true]
60     [(and (empty? pattern)
61           (cons? search-string)) (first search-string)]
62     [(and (cons? search-string)
63           (symbol=? (first pattern) (first search-string)))
64      (properDNAprefix (rest pattern) (rest search-string))]
65     [else false]))
66 
67 (properDNAprefix empty empty)
68 (properDNAprefix empty '(a))
69 (properDNAprefix '(a) empty)
70 (properDNAprefix '(a) '(a))
71 (properDNAprefix '(a) '(a b))