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 20.2.3) (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 #|
5 
6 Exercise 20.2.3.   Use filter1 to develop a function that consumes a list of symbols and extracts all those that are not equal to 'car. Give filter1's corresponding contract.
7 
8 |#
9 
10 ;filter1 : (X Y -> boolean) (listof X) Y -> (listof X)
11 
12 (define (filter1 rel-op alox y)
13   (cond
14     [(empty? alox) empty]
15     [(rel-op (first alox) y) 
16      (cons (first alox)
17            (filter1 rel-op (rest alox) y))]
18     [else
19      (filter1 rel-op (rest alox) y)]))
20 
21 ;not=? : Y Y -> boolean
22 ;Given item1 and item2, return true if the two are not equal, false if they are.
23 
24 (define (not=? item1 item2)
25   (not (equal? item1 item2)))
26 
27 (define list1 '(automobile
28                 vehicle
29                 motorcycle
30                 van
31                 truck
32                 car
33                 sedan
34                 car
35                 pick-up
36                 train))
37 
38 (filter1 not=? list1 'car)
39 (filter1 not=? list1 'automobile)
40