August 2018
Intermediate to advanced
227 pages
4h 16m
Chinese
Content preview from 高效R语言编程
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高效编程
｜
73
watch = stop_watch()
watch 对象是包含两个函数的列表。一个函数用于启动计时：
watch$start()
另外一个用于停止定时器：
watch$stop()
如果不使用函数闭包，秒表函数将非常长、复杂，当然也很低效。应用得当，
函数闭包是编写简洁代码的非常有用的工具。
练习
1.
不使用函数闭包编写一个秒表函数。
2.
很多秒表除了可测量你的总时间，还可测量你一圈时间。在前面的 stop_
watch() 中增加 lap() 函数，用来记录个体时间，同时保留记录总时间功能。
一个与函数闭包相关的概念是非标准化评估（
NSE
），或编程语言，
R
中总
是会遇到
NSE
的问题。例如，我们执行 plot(height, weight)，
R
自动地
将
x
轴标为高度，
y
轴标为宽度。这是一个很强大的概念，能帮我们简化代码。
详见
Hadley Wickham
的《
Advanced R
》（
CRC Press
）一书中的”
Nonstandard
evaluation
”小节。
字节编译
由
R
的核心成员
Luke Tierney
编写的
compiler
包自
2.13.0
版后成为
R
的一部
分。
compiler
包可将函数编译成字节代码，从而函数运行更快
注
1
。编译过程
清除了大量解释器必须执行的耗时操作，例如变量查询。
自从
2.14.0
版，所有基础
R
中的标准函数和包均被预编译成字节代码。这可
通过基本函数 mean() 验证：
注
1
：
作者尚未发现字节编译代码运行的非常慢的情况。
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.