为什么** 0.5似乎比sqrt()更有效 [英] Why does **0.5 appear to be more efficient than sqrt()

问题描述

我尝试测量这两种求平方根的速度:

I have tried measuring the speed of these two ways for taking square root:

> system.time(expr = replicate(10000, 1:10000 ** (1/2)))
##   user  system elapsed 
##  0.027   0.001   0.028 
> system.time(expr = replicate(10000, sqrt(1:10000)))
##   user  system elapsed 
##  3.722   0.665   4.494 

如果sqrt()功能不能与** 0.5竞争，为什么我们需要这样的功能?

If the sqrt() function cannot compete with ** 0.5, why do we need such a function?

(系统为OS X Yusemite，R版本为3.1.2)

(system is OS X Yusemite, and R version is 3.1.2)

推荐答案

您忘记了重要的括号.以下是纠正该问题后的时间安排:

You forgot important parentheses. Here are the timings after correcting that:

system.time(expr = replicate(10000, (1:10000) ** (1/2)))
#user  system elapsed 
#4.76    0.32    5.12 
system.time(expr = replicate(10000, sqrt(1:10000)))
#user  system elapsed 
#2.67    0.57    3.31

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