SICP_2.21-2.23

 1 #lang racket
 2 
 3 ;;;;;;;;;;;;;;;;;;;2.21
 4 (define (square-list items)
 5   (if (null? items)
 6       null
 7       (cons (square (car items))
 8             (square-list (cdr items)))))
 9 
10 (define (square x)
11   (* x x))
12 ;;
13 (define (square-list2 items)
14   (map square items))
15 
16 ;;;;test
17 (square-list (list 1 2 3 4))
18 (square-list2 (list 1 2 3 4))
19 
20 ;;;;;;;;;;;;;;;;;;;;2.22
21 (define (square-list3 items)
22   (define (iter things answer)
23     (if (null? things)
24         answer
25         (iter (cdr things)
26               (append answer
27                       (list (square (car things)))))))
28   (iter items ‘()))
29 
30 ;;test
31 (square-list3 (list 1 2 3 4))
32 
33 ;;;;;;;;;;;;;;;;;;;;2.23
34 (define (for-each proc items)
35   (cond ((not (null? items))
36          (proc (car items))
37          (for-each proc (cdr items)))))
38 
39 ;;test
40 (for-each (lambda (x) (newline) (display x)) (list 57 321 88))