R data.table怪异值/引用语义 [英] R data.table weird value/reference semantics

问题描述

（这是 this 。）

检查此玩具代码：

> x <- data.frame(a = 1:2)
> foo <- function(z) { setDT(z) ; z[, b:=3:4] ; z } 
> y <- foo(x)
> 
> class(x)
[1] "data.table" "data.frame"
> x
   a
1: 1
2: 2

看起来setDT确实发生了变化x的类，但数据的添加不适用于x。

发生了什么事？

It looks like setDT did change x's class, but the addition of data did not apply to x.
What happened here?

推荐答案

在您的函数中， z 是对 x 的引用，直到 setDT

In your function z is a reference to x until setDT.

library(data.table)
foo <- function(z) {print(address(z)); setDT(z); print(address(z))} 
x <- data.frame(a = 1:2)
address(x)
#[1] "0x555ec9a471e8"
foo(x)
#[1] "0x555ec9a471e8"
#[1] "0x555ec9ede300"

在 setDT 中，出现以下行，其中 z 仍指向与 x 类似的地址：

In setDT it comes to the following line where z is still pointing to the same address like x:

setattr(z, "class", data.table:::.resetclass(z, "data.frame"))

setattr 不会复制。因此 x 和 z 仍指向相同的地址，并且现在都属于 data类.frame ：

setattr does not make a copy. So x and z are still pointing to the same address and both are now of class data.frame:

x <- data.frame(a = 1:2)
z <- x
class(x)
#[1] "data.frame"
address(x)
#[1] "0x555ec95de600"
address(z)
#[1] "0x555ec95de600"

setattr(z, "class", data.table:::.resetclass(z, "data.frame"))

class(x)
#[1] "data.table" "data.frame"
address(x)
#[1] "0x555ec95de600"
address(z)
#[1] "0x555ec95de600"

然后调用 setalloccol 在这种情况下调用：

Then setalloccol is called which calls in this case:

assign("z", .Call(data.table:::Calloccolwrapper, z, 1024, FALSE))

现在让 x 和 z

address(x)
#[1] "0x555ecaa09c00"
address(z)
#[1] "0x555ec95de600"

而且都具有类 数据.frame

class(x)
#[1] "data.table" "data.frame"
class(z)
#[1] "data.table" "data.frame"

我认为他们什么时候会用

I think when they would have used

class(z) <- data.table:::.resetclass(z, "data.frame")

而不是

setattr(z, "class", data.table:::.resetclass(z, "data.frame"))

不会发生此问题。

x <- data.frame(a = 1:2)
z <- x
address(x)
#[1] "0x555ec9cd2228"
class(z) <- data.table:::.resetclass(z, "data.frame")
class(x)
#[1] "data.frame"
class(z)
#[1] "data.table" "data.frame"
address(x)
#[1] "0x555ec9cd2228"
address(z)
#[1] "0x555ec9cd65a8"

，但在 class（z）<-value <$之后c $ c> z 不会指向之前指向的同一地址：

but after class(z) <- value z will not point to the same address where it points before:

z <- data.frame(a = 1:2)
address(z)
#[1] "0x5653dbe72b68"
address(z$a)
#[1] "0x5653db82e140"
class(z) <- c("data.table", "data.frame")
address(z)
#[1] "0x5653dbe82d98"
address(z$a)
#[1] "0x5653db82e140"

，但是在 setDT 之后，它也不会指向相同的地址

but after setDT it will also not point to the same address where it points before:

z <- data.frame(a = 1:2)
address(z)
#[1] "0x55b6f04d0db8"
setDT(z)
address(z)
#[1] "0x55b6efe1e0e0"

正如@ Matt-dowle指出的那样，也可以更改 x 超过 z ：

As @Matt-dowle pointed out, it is also possible to change the data in x over z:

x <- data.frame(a = c(1,3))
z <- x
setDT(z)
z[, b:=3:4]
z[2, a:=7]
z
#   a b
#1: 1 3
#2: 7 4
x
#   a
#1: 1
#2: 7

R.version.string
#[1] "R version 4.0.2 (2020-06-22)"
packageVersion("data.table")
#[1] ‘1.12.8’

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