将对象传递给函数而不更改时将其复制 [英] Pass an object to a function without copying it on change

问题描述

如果将对象x传递给对其进行修改的函数f，则R将在f的环境内创建x的修改后的本地副本，而不是更改原始对象(由于该副本) -on-on-change原则).但是，我遇到的情况是x很大，一旦传递给f就不需要了，所以我想避免在调用f后存储x的原始副本.有什么聪明的方法可以做到这一点吗?

If an object x is passed to a function f that modifies it R will create a modified local copy of x within f's environment, rather than changing the original object (due to the copy-on-change principle). However, I have a situation where x is very big and not needed once it has been passed to f, so I want to avoid storing the original copy of x once f is called. Is there a clever way to achieve this?

f是一个未知的功能，可能由不太聪明的用户提供.

f is an unknown function to be supplied by a possibly not very clever user.

到目前为止，我最好的解决方法是将x包裹在函数forget中，该函数对称为y的x进行新的本地引用，删除工作空间中的原始引用，然后传递新的参考.问题是我不确定它是否能完成我想要的工作，并且只能在globalenv()中工作，这在我当前的情况下是一个大问题.

The best I have so far is to wrap x in a function forget that makes a new local reference to x called y, removes the original reference in the workspace, and then passes on the new reference. The problem is that I am not certain it accomplish what I want and it only works in globalenv(), which is a deal breaker in my current case.

forget <- function(x){
    y <- x
    # x and y now refers to the same object, which has not yet been copied
    print(tracemem(y))
    rm(list=deparse(substitute(x)), envir=globalenv())
    # The outside reference is now removed so modifying `y`
    # should no longer result in a copy (other than the
    # intermediate copy produced in the assigment)
    y
}

f <- function(x){
    print(tracemem(x))
    x[2] <- 9000.1
    x
}

这里是调用上述函数的示例.

Here is an example of calling the above function.

> a <- 1:3
> tracemem(a)
[1] "<0x2ac1028>"
> b <- f(forget(a))
[1] "<0x2ac1028>"
[1] "<0x2ac1028>"
tracemem[0x2ac1028 -> 0x2ac1e78]: f 
tracemem[0x2ac1e78 -> 0x308f7a0]: f 
> tracemem(b)
[1] "<0x308f7a0>"
> b
[1]    1.0 9000.1    3.0
> a
Error: object 'a' not found

底线

我正在做自己希望做的事情，还有更好的方法吗?

Bottom line

Am I doing what I hope I am doing and is there a better way to do it?

推荐答案

(1)环境您可以为此使用环境:

(1) Environments You can use environments for that:

e <- new.env()
e$x <- 1:3
f <- function(e) with(e, x <- x + 1)
f(e)
e$x

(2)参考类，或者由于参考类会自动使用环境，因此会使用以下类:

(2) Reference Classes or since reference classes automatically use environments use those:

E <- setRefClass("E", fields = "x",
    methods = list(
        f = function() x <<- x + 1
    )
)
e <- E$new(x = 1:3)
e$f()
e$x

(3)个原型对象也使用环境:

library(proto)
p <- proto(x = 1:3, f = function(.) with(., x <- x + 1))
p$f()
p$x

添加:原始解决方案

已更新:为了与问题保持一致，将函数名称更改为f.

UPDATED: Changed function name to f for consistency with question.

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