August 2018
Intermediate to advanced
227 pages
4h 16m
Chinese
Content preview from 高效R语言编程
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72
｜
第
3
章
mpg = mpg[-row_to_remove, ]
plot(mpg$cty, mpg$hwy)
lines(lowess(mpg$cty, mpg$hwy), col = 2)
}
我们使用
memoise
通过缓存结果来提升速度。一个快速的基准测试如下：
m_plot_mpg = memoise
（
plot_mpg
）
microbenchmark
（
times = 10, unit = "ms", m_plot_mpg
（
10
）
, plot_mpg
（
10
））
#> Unit: milliseconds
#> expr min lq mean median uq max neval cld
#> m_plot_mpg(10
）
0.04 4e-02 0.07 8e-02 8e-02 0.1 10 a
#> plot_mpg(10
）
40.20 1e+02 95.52 1e+02 1e+02 107.1 10 b
意味着我们能获得
100
倍的速度提升。
练习
1.
为标准绘图函数和
memoise
版绘图函数构建时间箱线图。
函数闭包
本节将以应用案例形式介绍闭包。更多信息参见
Hadley Wickham
的
《
Advanced R
》（
CRC Press
）一书。
使用函数闭包可以提供更高级的缓存。
R
中的闭包是包含函数以及函数所依
赖的环境的对象。从技术上说，
R
中所有函数都有该属性，但我们使用函数
闭包来表明其环境不在
.GlobalEnv_
中的函数。与函数相关的环境称为包围环 ...
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