如何在 SICP、Scheme、练习 2.78 等中获得 put 和 get 函数 [英] How do I get the functions put and get in SICP, Scheme, Exercise 2.78 and on

问题描述

我正在尝试在 SICP 中做练习 2.78，但是 put 和 get 函数未知.我尝试了多种语言，比如相当大、球拍、r5rs、mit-scheme、mzscheme 等.我什至下载了 SICP 支持(http://www.neilvandyke.org/sicp-plt/)，但无济于事.如何让这些功能发挥作用?

I am trying to do exercise 2.78 in SICP, but the functions put and get are unknown. I have tried multiple languages, like pretty big, racket, r5rs, mit-scheme, mzscheme, etc. I even downloaded the SICP support (http://www.neilvandyke.org/sicp-plt/), to no avail. How can I get these functions to work?

推荐答案

是的，我发现 SICP 有时会因为这样的事情而有点烦人.假定存在但实际上不存在的函数使尝试示例变得更加困难.我像这样写了我自己的 (get) 和 (put)(这是在 GNU guile 中):

Yes, I found SICP a little annoying sometimes because of stuff like this. Functions that are assumed to exist but don't actually exist make it harder to try to the examples. I wrote my own (get) and (put) like so (this was in GNU guile):

(define global-array '())

(define (make-entry k v) (list k v))
(define (key entry) (car entry))
(define (value entry) (cadr entry))

(define (put op type item)
  (define (put-helper k array)
    (cond ((null? array) (list(make-entry k item)))
          ((equal? (key (car array)) k) array)
          (else (cons (car array) (put-helper k (cdr array))))))
  (set! global-array (put-helper (list op type) global-array)))

(define (get op type)
  (define (get-helper k array)
    (cond ((null? array) #f)
          ((equal? (key (car array)) k) (value (car array)))
          (else (get-helper k (cdr array)))))
  (get-helper (list op type) global-array))

从本书后面的角度来看，这可能是一个幼稚的实现，但相当简单且运行良好.

Probably a naive implementation from the perspective of later in the book, but fairly simple and worked fine.

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