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 |21.2|) (read-case-sensitive #t) (teachpacks ()) (htdp-settings #(#t constructor repeating-decimal #f #t none #f ())))
4 ;selection : (listof symbol) (listof symbol) -> (listof symbol)
5 ;selection, which consumes two lists of names and selects all those from the second one that are also on the first.
6 
7 (define (selection list1 list2)
8   (cond
9     [(empty? list2) empty]
10     [(in-other-list? (first list2) list1) (cons (first list2)
11                                                 (selection list1 (rest list2)))]
12     [else (selection list1 (rest list2))]))
13 
14 (define (in-other-list? an-item a-list)
15   (cond
16     [(empty? a-list) false]
17     [(equal? (first a-list) an-item) true]
18     [else (in-other-list? an-item (rest a-list))]))
19 
20 (define (selection list1 list2)
21   (cond
22     [(empty? list1) empty]
23     [else (local ((define (equal-to-first-of-list1 a-symbol)
24                     (equal? (first list1) a-symbol)))
25             (cons (filter equal-to-first-of-list1? list2) (selection list2 (rest list1))))]))
26 
27 
28 equal-to-first-of-list1 : symbol -> boolean