替换()与“[<-"? [英] replace() vs &quot;[&lt;-&quot;?

问题描述

我最近偶然发现了 replace() 和 "[<-".它们似乎具有类似的功能，例如使用 "[<-" 我可以这样做:

I recently stumbled across replace() and "[<-". They seem to have similar functionality, for example with "[<-" I can do something like this:

        > x.tst <- array(1:6, c(2,3))
        > s.tst <- array(0, c(2,3))
        > s.tst
             [,1] [,2] [,3]
        [1,]    0    0    0
        [2,]    0    0    0
        > s.tst[1:3] <- 1
        > "[<-"(x.tst, s.tst==1, 0)
             [,1] [,2] [,3]
        [1,]    0    0    5
        [2,]    0    4    6
        > x.tst
             [,1] [,2] [,3]
        [1,]    1    3    5
        [2,]    2    4    6

有人可以帮助澄清差异吗?replace 与 "[<-" 的优势是什么?反之亦然?

Can somebody help to clarify the difference? What are the strengths of replace vs "[<-" and vis versa?

推荐答案

它们基本上是一回事.如果你查看replace的源代码，你会看到:

They're basically exactly the same thing. If you look at the source code of replace, you'll see :

function (x, list, values) 
{
    x[list] <- values
    x
}
<environment: namespace:base>

因此，replace 只不过是对 [<- 的包装:

So replace is nothing else but a wrapper around [<- :

> replace(x.tst, s.tst==1, 0)
     [,1] [,2] [,3]
[1,]    0    0    5
[2,]    0    4    6

如果您需要这样做一百万次，使用 [<- 可以为您提供加速，因为您会丢失对包装器函数的额外调用.但它真的很边缘，所以这是一个选择的问题.我会说 replace() 更易读

Using [<- can give you a speedup if you need to do this a million times, as you lose the extra call to the wrapper function. But it's really marginal, so it's a matter of choice. I would say that replace()is a bit more readible

顺便说一句，x.tst[s.tst==1] <- 0 比 "[<-"(x.tst, s.tst==1, 0) .没有理由使用该构造，除非您想将结果保存在新的数据框中.

Btw, x.tst[s.tst==1] <- 0 is quite more readible than "[<-"(x.tst, s.tst==1, 0) . No reason to use that construct, unless you want to save the result in a new dataframe.

为了澄清，正如@Andrie 指出的，replace() 和 "[<-"(x.tst, s.tst==1, 0) 你会得到整个 x.tst 的副本，并更改​​了相关值.所以你可以把它放在一个新对象中.这与 x.tst[s.tst==1] <- 0 不同，您可以在其中更改 x.tst 本身的值.请注意，它不会节省内存，因为 R 会在进行操作之前在内部制作 x.tst 的副本.

To clarify, as @Andrie pointed out, both with replace() and "[<-"(x.tst, s.tst==1, 0) you get a copy of the whole x.tst with the relevant values changed. So you can put that in a new object. This is contrary to x.tst[s.tst==1] <- 0, where you change the values in x.tst itself. Mind you, it doesn't save on memory, as R will make internally a copy of x.tst before doing the manipulation.

计时结果:

> system.time(replicate(1e6, replace(x.tst, s.tst==1, 0)))
   user  system elapsed 
  12.73    0.03   12.78 

> system.time(replicate(1e6, "[<-"(x.tst, s.tst==1, 0)))
   user  system elapsed 
   6.42    0.02    6.44 

> system.time(replicate(1e6, x.tst[s.tst==1] <- 0))
   user  system elapsed 
   5.28    0.02    5.32 

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