代码

#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)      '()      (cons low (sequence (+ stride low) high stride))));2(define (string-append-map xs suffix)  (map (lambda (el) (string-append el suffix)) xs));3(define (list-nth-mod xs n)  (let ([xl (length xs)])      (cond        [(< n 0) (error "list-nth-mod: negative number")]        [(= 0 xl) (error  "list-nth-mod: empty list")]        [else (car (list-tail xs (remainder n xl)))])));4(define (stream-for-n-steps s n)  (if (= n 0)      '()      (cons (car (s)) (stream-for-n-steps (cdr (s)) (- n 1)))));5(define funny-number-stream  (letrec ([f (lambda (x) (cons                           (if (= 0 (remainder x 5))                               (- x)                               x)                           (lambda () (f (+ x 1)))))])    (lambda () (f 1))));6(define dan-then-dog  (letrec ([f (lambda (x) (if (string=? x "dan.jpg")                              (cons "dan.jpg" (lambda () (f "dog.jpg")))                              (cons "dog.jpg" (lambda () (f "dan.jpg")))))])    (lambda () (f "dan.jpg"))));7(define (stream-add-zero s)  (letrec ([f (lambda (s) (cons (cons 0 (car (s))) (lambda () (f (cdr (s))))))])    (lambda () (f s))));8(define (cycle-lists xs ys)  (letrec ([f (lambda (n) (cons (cons (list-nth-mod xs n) (list-nth-mod ys n))                                (lambda () (f (+ n 1)))))])    (lambda () (f 0))));9(define (vector-assoc a vec)  (letrec ([f (lambda (n) (cond                            [(>= n (vector-length vec)) #f]                            [(not (pair? (vector-ref vec n))) (f (+ n 1))]                            [(equal? (car (vector-ref vec n)) a) (vector-ref vec n)]                            [else (f (+ n 1))]))])    (f 0)));10(define (cached-assoc xs n)  (letrec ([cache (make-vector n #f)]           [index 0]           [f (lambda (v) (let ([vres (vector-assoc v cache)])                           (cond                            [vres vres]                            [(not vres) (let ([xres (assoc v xs)])                                          (cond                                            [xres (vector-set! cache index xres)                                                  (set! index (remainder (+ 1 index) n))                                                  xres]                                            [(not xres) #f]))])))])    f))