:=(通过引用传递)操作符在data.table包中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
测试我的代码时,我发现了如下:如果我将一个data.table DT1
分配给 DT
然后改变 DT
,随后改变 DT1
。因此, DT
和 DT1
似乎内部链接。这是预期的行为吗?虽然我不是一个编程专家,这看起来不对,我用简单的R变量或一个 data.frame
测试它,我无法重现的行为。这里发生了什么?
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]组件2:平均相对差异:1
库表)
data.table 1.7.1对于帮助类型: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值
[1,] a 2
[2,] b 4
[3,] c 6
[4,] d 8
[5 ,] e 10
all.equal(DT1,DT)
[1] TRUE
? data.table :: copy
不返回任何值。数据表通过引用修改。如果需要复印,请首先复印(使用DT2 =复印(DT))。 copy()在以下情况之前有时也很有用:=用于通过引用对列进行子分配。
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
This piece of documentation in data.table
would help. ? data.table::copy
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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