加载data.table对象后立即使用get（data.table）生成新变量 [英] Generating a new variable with get(data.table) directly after loading data.table object

问题描述

我正在尝试在data.table中生成一个新变量，该变量已再次生成，保存和加载。加载后，我通过get（）间接处理了data.table，这对于生成新变量不起作用，只要我之前不直接为变量创建而处理。
可能是某种环境问题吗？

I am trying to generate a new variable in a data.table which I generated, saved and loaded again. After loading I adress the data.table indirectly through get() and this does not work for generating a new variable as long as I dont adress it directly for variable creation before. Possibly it is some kind of environment issue?

# Generate data.table
t<-data.table(x=c(1,2,3,4))
tStr<-"t"
names(t)

# Generate Variable a -> ok
get(tStr)[, a:=1]
names(t)

# Generate Variable b -> ok
t[, b:=1]
names(t)

# Save
save(t, file="test.Robj")
load("test.Robj", .GlobalEnv)

# Generate Variable c -> fails 
get(tStr)[, c:=1] 
names(t)

# Generate Variable d -> ok
t[, d:=1]
names(t)

# Generate Variable e -> ok again !?
get(tStr)[, e:=1]
names(t)

感谢您的帮助

推荐答案

这是因为重要的元数据无法在存储操作中幸存：

This is because important meta data does not survive the storage action:

> t<-data.table(x=c(1,2,3,4))
> attr(t, ".internal.selfref")
<pointer: 0x0000000000100788>
> save(t, file="test.Robj")
> load("test.Robj", .GlobalEnv)
> attr(t, ".internal.selfref")
<pointer: (nil)>
> t[, d:=1]
> attr(t, ".internal.selfref")
<pointer: 0x0000000000100788>

注意如何丢失内存指针。我不确定这是不是一个错误，因为 data.table 是什么与保存。为了使此方法正常工作，我们需要一个特殊的 load 方法，该方法在加载 data.table 对象。

Notice how you lose the memory pointer. I'm not sure this is so much a bug as an inherent conflict between what a data.table is and what save does. It seems in order for this to work properly we would need a special load method that re-assigns the internal pointer on loading data.table objects.

在这种情况下，使用按引用修改似乎会重置指针。

In this case, using the modify by reference seems to reset the pointer.

编辑：作为使用案例的一种解决方法，您可以尝试：

EDIT: as a workaround in your use case, you can try:

t <- data.table(x=c(1,2,3,4))
save(t, file="test.Robj")
load("test.Robj", .GlobalEnv)
assign("t", get("t")[, c:=3])
t

可按预期工作：

   x c
1: 1 3
2: 2 3
3: 3 3
4: 4 3

也请注意：

get("t")[, c:=3]

将正常工作，就像期望的那样：

will work is a little bit like expecting that:

get("x") <- 5

即可。 data.table 将来可能会添加此功能，但是您正在涉足 data.table 的阴暗区域code>确实开始与R语义发生冲突。

will work. data.table might in the future add this feature, but it you're treading in this murky area where the reference nature of data.table really starts conflicting with the R semantics.

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