blob: 31f486afd64dd40ac70d62f1170986849fb35689 (
plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
|
#lang racket
(require sicp)
(require "../../shared/chapter1.rkt")
;; Here we are iterating forward, while consing in reverse, this would be a good way to reverse a list
;; The difference is that iterating is like going forward and recursing is like going backwards.
;; Recursing works because it is going in reverse and we also need to cons in reverse.
(define (square-list items)
(define (iter things answer)
(if (null? things)
answer
(iter (cdr things)
(cons (square (car things))
answer))))
(iter items nil))
(square-list (list 1 2 3 4))
;; This attempts the reverse the consing direction by chaning the first in the pair with the second
;; The result is that we get pairs that point to other pairs with car, which is not how a list works.
;; (define (square-list2 items)
;; (define (iter things answer)
;; (if (null? things)
;; answer
;; (iter (cdr things)
;; (cons answer (square (car things))))))
;; (iter items nil))
;; ;;(square-list2 (list 1 2 3 4))
;;
;; ;; One thing we could try is also growing the answer list forward somehow
;; (define (square-list3 items)
;; (define (iter things answer)
;; (if (null? things)
;; answer
;; (iter (cdr things) (append answer (list (square (car things)))))))
;; (iter items (list)))
;; (square-list3 (list 1 2 3 4))
|
