Programming Languages B(Coursera / University of Washington) Assignment4

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本次作业需要racket标准库文档：https://docs.racket-lang.org/

Scheme 大法好
天灭过程式退C保平安
人在做天在看赋值语句留祸患
指针乱用天地灭赶紧重写保平安
诚心诚念SICP好 Scheme大法平安保
众生都为函数来现世险恶忘前缘
Scheme弟子说真相教你编程莫拒绝
早日摆脱面向过程早日获得新生
上网搜“九评丹尼斯·里奇” 有真相

文章目录

- 小结
- Assignment

小结

因为这个课的第二部分换成了Racket(看起来和scheme完全一致)，所以内容越来越接近sicp了。
所以有很多sicp的经典内容（或者说lisp的优秀特性，但是这部分我觉得这个课讲的更好），比如：

赋值语句的side-effect
Lisp原生强大的macro
典中典之lazy evaluation
特殊的procedure衍生的很有用的streams
set-cdr，set-car
cons即procedure

但是同时Racket是一个更加现代并且不断演进的语言，所以有很多sicp中没有提到的feature和技巧。
比如：

let，let*，letrec三种赋值表达式
thunk – 形实转换的procedure
使用[] 进行local bindings
一些库函数
Delay and Force 记忆化
用表进行记忆化递归求解斐波那契数列
Tokenization

Assignment


#lang racket(provide (all-defined-out)) ;; so we can put tests in a second file;; put your code below
;; 1
(define (sequence low high stride)(if (> low high)null(cons low (sequence (+ low stride) high stride))));; 2
(define (string-append-map xs suffix)(map (lambda(x)(string-append x suffix)) xs))
;; 3
(define (list-nth-mod xs n)(cond [(< n 0) error "list-nth-mod: negative number"][(= (length xs) 0) error "list-nth-mod: empty list"][#t (car (list-tail xs (remainder n (length xs))))]))
;; 4
(define (stream-for-n-steps s n)(if (= n 0)null(let ([next (s)])(cons (car next) (stream-for-n-steps (cdr next) (- n 1))))));; 5
(define funny-number-stream(letrec ([helper (lambda (x) (cons (cond [(= 0 remainder x 5) (* -1 x)][#t x]) (lambda () (helper (+ 1 x)))))])(lambda () (helper 1))));; 6
(define dan-then-dog(letrec ([helper (lambda(x) ((let ([tmp (cond [(= x "dog.jpg") "dan.jpg"][#t "dog.jpg"])])cons (tmp) (lambda() (helper(tmp))))))])(lambda () (helper "dog.jpg"))));; 7
(define (stream-add-zero s)(lambda ()(let ([pr (s)])(cons (cons 0 (car pr)) (stream-add-zero (cdr pr))))));;8
(define (cycle-lists xs ys)(letrec ([lenx (length xs)][leny (length ys)])(letrec ([helper (lambda(x) ((let ([tmp1 (remainder lenx x)][tmp2 (remainder leny x)])(cons (cons (car (list-tail xs tmp1)) (car (list-tail ys tmp2)))(lambda() (helper(+ 1 x)))))))])(lambda () (helper 1)))));;9
(define (vector-assoc v vec)(letrec ([f (lambda (x)(if (< x (vector-length vec))(let ([val (vector-ref vec x)])(if (and (cons? val) (equal? v (car val)))val(f (+ x 1))))#f))])(f 0)));;10
(define (cached-assoc xs n)(letrec ([memo (make-vector n #f)][index 0])(lambda (x);;如果能在缓存序列中得到(let ([ans (vector-assoc x memo)])(if ansans (let ([new-ans (assoc x xs)])(begin(vector-set! memo index new-ans)(set! index (remainder (+ index 1) (vector-length memo)))new-ans)))))))