cpsc 110 compound(中)

javaIsTheBest · Aug '19

还是那句话，经典不妨一看再看
compound这章的视频可以在edx上看，也可以点开youtube的公开课链接看
这里的总结是假设你都看过视频了(￣< ￣)>

我们目标是一头牛跑来跑去，而且按空格会掉头...

跑题了，老规矩，先画domain analysis

如果只是跑来跑去，不掉头的话，那就跟htdw那只喵是一样的，可以用simple atomic data 来表示。
但这里牛会掉头，只是x轴的pixel坐标还不够用，还需要一个表示表示方向的信息。
到现在为止，atomic,interval, enum和itemaization都是一对一的定义。
(enum和itemaization看起来有几种情况，但是也是one of，其实还是一对一的)

这时候，引入了一个叫compound 的数据类型
compound 并不是一对一的，而是包括了两个或以上数据的组合
可以想成是一个盒子，里面装了两种或以上的东西

;; Data definitions:
(define-struct cow (x dx))

当racket见到(define-struct cow (x dx)), 它自动生成关于cow这个struct 的一些function：
make-cow: 创建一个cow
cow? ; 检查是否是一个cow 的struct
cow-x : 读取cow x的信息
cow-dx: 读取cow dx的信息

有了这些function, 就可以写下compound的htdd了

;; Data definitions:
(define-struct cow (x dx))
;; Cow is (make-cow Naturla[0, WIDTH], Integer)
;; interp. (make-cow x dx) is a cow with x coordinates a and velocity dx
;;     the x is the center of the cow
;;     x is in secreen coordinates (pixels)
;;     dx is in pixels per tick
(define C1 (make-cow 10 3))  ;; at 10, moving left -> right
(define C2 (make-cow 20 -4)) ;; at 20, moving right -> left

(@dd-template-rules compound)  ;2 fields

(define (fn-for-cow c)
  (... (cow-x c)    ;; Naturla[0, WIDTH]
       (cow-dx c))) ;; Integer

完整的code，点击这里javaIsTheBest
关于compound 的 function，可以想成一个人收到一个盒子以后，先打开，
然后根据里面的内容做一些事情(例如，创建一个新的盒子，一个image。。。)
以next-cow 的一个check-expect为例，

(check-expect (next-cow (make-cow 20 -3)) (make-cow (- 20 3) -3))
....
(define (next-cow c)
  (cond [(> (+ (cow-x c) (cow-dx c)) WIDTH) 
         (make-cow WIDTH (- (cow-dx c)))]
        [(< (+ (cow-x c) (cow-dx c)) 0) 
         (make-cow 0 (- (cow-dx c)))]
        [else
         (make-cow (+ (cow-x c) (cow-dx c))
                   (cow-dx c))]))

javaIsTheBest · Aug '19

javaIsTheBest 完整的code，点这里

(require spd/tags)
(require 2htdp/image)
(require 2htdp/universe)
(define WIDTH 400)
(define HEIGHT 200)

(define CTR-Y (/ HEIGHT 2))

(define RCOW  (bitmap/url "https://i.imgur.com/akBZSOA.png"))

(define LCOW  (bitmap/url "https://i.imgur.com/rG9Uf7i.png"))

(define MTS (empty-scene WIDTH HEIGHT))

;; Data definitions:
(@htdd Cow)
(define-struct cow (x dx))
;; Cow is (make-cow Natural[0, WIDTH], Integer)
;; interp. (make-cow x dx) is a cow with x coordinate x and velocity dx
;;         the x is the center of the cow
;;         x is in screen coordinates (pixels)
;;         dx is in pixels per tick
;;
(define C1 (make-cow 10 3))  ; at 10, moving left -> right
(define C2 (make-cow 20 -4)) ; at 20, moving left <- right
#;
(define (fn-for-cow c)
  (... (cow-x c)      ;Natural[0, WIDTH]
       (cow-dx c)))   ;Integer

(@dd-template-rules compound) ;2 fields

;; Functions:

(@signature Cow -> Cow)
;; called to make the cow go for a walk; start with (main (make-cow 0 3))
;; no tests for main function
(define (main c)
  (big-bang c
            (on-tick next-cow)      ; Cow -> Cow
            (to-draw render-cow)    ; Cow -> Image
            (on-key handle-key)))   ; Cow KeyEvent -> Cow 

(@htdf next-cow)
(@signature Cow -> Cow)
;; increase cow x by dx; bounce off edges
(check-expect (next-cow (make-cow 20 3)) (make-cow (+ 20 3) 3))  ;middle
(check-expect (next-cow (make-cow 20 -3)) (make-cow (- 20 3) -3))
              



(check-expect (next-cow (make-cow (- WIDTH 3) 3)) (make-cow WIDTH 3))  ;reaches edge
(check-expect (next-cow (make-cow 3 -3)) (make-cow 0 -3))

(check-expect (next-cow (make-cow (- WIDTH 2) 3)) (make-cow WIDTH -3)) ;tries to pass edge
(check-expect (next-cow (make-cow 2 -3)) (make-cow 0 3))


;(define (next-cow c) c)    ;stub

(@template-origin Cow)
(@template
 (define (next-cow c)
  (... (cow-x c)      ;Natural[0, WIDTH]
       (cow-dx c))))

(define (next-cow c)
  (cond [(> (+ (cow-x c) (cow-dx c)) WIDTH) 
         (make-cow WIDTH (- (cow-dx c)))]
        [(< (+ (cow-x c) (cow-dx c)) 0) 
         (make-cow 0 (- (cow-dx c)))]
        [else
         (make-cow (+ (cow-x c) (cow-dx c))
                   (cow-dx c))]))
               



(@signature Cow -> Image)
;; place appropriate cow image on MTS at (cow-x c) and CTR-Y
(check-expect (render-cow (make-cow 99 3))
              (place-image RCOW 99 CTR-Y MTS))
(check-expect (render-cow (make-cow 33 -3))
              (place-image LCOW 33 CTR-Y MTS))

;(define (render-cow c) MTS)  ;stub

(@template-origin Cow)

(@template
 (define (render-cow c)
  (... (cow-x c)      ;Natural[0, WIDTH]
       (cow-dx c))))

(define (render-cow c)
  (place-image (choose-image c) (cow-x c) CTR-Y MTS))
       




(@signature Cow -> Image)
;; produce RCOW or LCOW depending on direction cow is going; LCOW if dx = 0
(check-expect (choose-image (make-cow 10 3)) RCOW)
(check-expect (choose-image (make-cow 11 -3)) LCOW)
(check-expect (choose-image (make-cow 11 -0)) LCOW)


;(define (choose-image c) RCOW)

(@template-origin Cow)

(@template
 (define (choose-image c)
  (... (cow-x c)      ;Natural[0, WIDTH]
       (cow-dx c))))

(define (choose-image c)
  (if (> (cow-dx c) 0)
      RCOW
      LCOW))
       





(@signature Cow KeyEvent -> Cow)
;; reverse direction of cow travel when space bar is pressed
(check-expect (handle-key C1 " ") (make-cow 10 -3))
(check-expect (handle-key C1 "a") C1)

;(define (handle-key c ke) c) ;stub

(@template-origin KeyEvent)

(@template   

 (define (handle-key c ke)
   (cond [(key=? ke " ") (... c)]
         [else 
          (... c)])))


(define (handle-key c ke)
  (cond [(key=? ke " ") (make-cow (cow-x c) (- 0 (cow-dx c)))]
        [else
         c]))