August 2018
Intermediate to advanced
227 pages
4h 16m
Chinese
Content preview from 高效R语言编程
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56
｜
第
3
章
4.
通过缓存变量避免不必要的计算。
5.
字节编译包可使性能轻而易举大幅提升。
一般性建议
低级程序语言如
C
语言和
Fortran
对程序员要求更多一些。它们强制你必须声
明所使用的每个变量的类型，让你负担内存管理的繁重职责，并且必须编译。
相比
R
，这类语言的长处是运行速度更快。缺点是它们需要很长时间来学习
并且不能交互式运行代码。
维基的编译器优化页面很好地介绍了标准优化技术。
R
用户无需担心数据类型。这在创建简洁代码方面是有优势的，但容易导
致
R
代码效率低下。虽然类似并行化之类的优化可大幅提升速度，但糟糕的
代码很容易让速度变慢，所以理解导致代码运行慢的原因很重要。
Patrick
Burns
的《
The R Inferno
》（
Lulu.com
）讲述该主题，所有上进的
R
程序员都
应该阅读一下。
最终，调用的
R
函数都是在调用底层的
C/Fortran
代码后才结束的。例如，基
本的
R
函数 runif() 仅仅包含一条代码，调用了 C_runif()。
function (n, min = 0, max = 1)
.Call(C_runif, n, min, max)
R
编程的黄金法则是尽可能快地访问底层的
C/Fortran
函数，实现一个函数所
需的函数调用越少越好。例如，
x
是长度为
n
的标准向量，那么
:
x = x + 1
仅有一次
+
函数的调用，但
for
循环：
for(i in seq_len(n))
x[i] = x[i] + 1
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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.