在球拍中使用停止时 [英] using stop-when in racket

问题描述

我一直在搞这个程序.它需要一个数字并在其上加 1.我想知道你怎么能在这里使用 stop-when ?例如，让它停在 5 点?我想这里需要一个条件声明.谢谢.

I've been messing around with this program. It takes a number and adds 1 to it. I am wondering how exactly could you use stop-when here? For example, to make it stop at 5? I suppose a cond statement is necessary here. Thanks.

(require 2htdp/image)
(require 2htdp/universe)

(define (my-tick n)
(add1 n))

(define (my-render n)
  (text (number->string n) 36 "silver"))


(big-bang 1 (on-tick my-tick 2) (to-draw my-render))

推荐答案

给 stop-when 一个谓词，它消耗一个世界并返回 true 或 false.

Give stop-when a predicate that consumes a world and returns true or false.

有关更多信息，请参阅文档 这里.

For more info, see the docs here.

这是在 5 点停止的示例版本:

Here's a version of your example that stops at 5:

(require 2htdp/image)
(require 2htdp/universe)

(define (my-tick n) (add1 n))

(define (my-render n)
  (text (number->string n) 36 "silver"))

(define (=5 n) (= n 5))

(big-bang 1 (on-tick my-tick 2) (to-draw my-render) (stop-when =5))

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