data.table 包中的 := (pass by reference) 运算符同时修改另一个数据表对象 [英] := (pass by reference) operator in the data.table package modifies another data table object simultaneously

问题描述

在测试我的代码时，我发现了以下内容:如果我将 data.table DT1 分配给 DT 并在之后更改 DT，DT1 随之改变.所以 DT 和 DT1 似乎是内部链接的.这是预期的行为吗?虽然我不是编程专家，但这对我来说似乎是错误的，并且使用简单的 R 变量或 data.frame 对其进行测试，但我无法重现该行为.这里发生了什么?

While testing my code, I found out the following: If I assign a data.table DT1 to DT and change DT afterwards, DT1 changes with it. So DT and DT1 seem to be internally linked. Is this intended behavior? Although I'm not a programming expert, this looks wrong to me, and testing it with simple R variables or a data.frame, I couldn't reproduce the behavior. What's happening here?

DF <- data.frame(ID=letters[1:5],
                  value=1:5)
DF1 <- DF
all.equal(DF1, DF)
[1] TRUE
DF[1, "value"] <- DF[1, "value"]*2
all.equal(DF1, DF)
[1] "Component 2: Mean relative difference: 1"

library(data.table)
data.table 1.7.1  For help type: help("data.table")
DT <- data.table(ID=letters[1:5],
                  value=1:5)
DT1 <- DT
all.equal(DT1, DT)
[1] TRUE
DT[, value:=value*2]
     ID value
[1,]  a     2
[2,]  b     4
[3,]  c     6
[4,]  d     8
[5,]  e    10
all.equal(DT1, DT)
[1] TRUE

推荐答案

data.table 中的这段文档会有所帮助.<代码>?data.table::copy

This piece of documentation in data.table would help. ? data.table::copy

没有返回值.data.table 通过引用进行修改.如果需要副本，请先获取副本(使用 DT2=copy(DT)).在 := 用于通过引用对列进行子分配之前，copy() 有时也可能很有用.

No value is returned. The data.table is modified by reference. If you require a copy, take a copy first (using DT2=copy(DT)). copy() may also sometimes be useful before := is used to subassign to a column by reference.

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