将数据保存在data.table列表中 [英] Saving plots in a data.table list column
问题描述
library(data.table)
DT = CJ(id = 1 ,id2 = 1:3)[,。(
d = as.IDate(2017-01-01)+ 1:10,v = id * 10 + id2 * 1:10
), by =。(id,id2)]
plotDT = DT [,{
par(mfrow = c(uniqueN(id2),1),mar = c(2,2 ,. 5,2))
.SD [,{
plot(d,v,type =l)
},by = id2]
。(p =。( recordPlot()))
},by = id]
x11()
plotDT [id == 1L,replayPlot(p [[1]])]
来自 DT [...]
的图是正确的:
虽然重播是错误的:
x 和的最终值显示在$ p
神奇地应用于所有三个图表,可能是因为data.table处理指向
中。 y
我不是真实的但我确定我明白发生了什么,因为我可以在表面上改变这个例子,编写 v = id2 * 1:10
而不是 v = id * 10 + id2 * 1:10
和 - poof - 问题消失。
我想知道这是怎么回事?(ii)我可以做些什么简单的调整来解决它? 解决方案
j like
with(copy(.SD),{yada yada} )
或类似的,这可以确保引用到 .SD
,不要混淆。
When I run...
library(data.table)
DT = CJ(id = 1, id2 = 1:3)[, .(
d = as.IDate("2017-01-01")+1:10, v = id*10 + id2*1:10
), by=.(id, id2)]
plotDT = DT[, {
par(mfrow = c(uniqueN(id2), 1), mar = c(2,2,.5,2))
.SD[,{
plot(d, v, type="l")
}, by=id2]
.(p = .(recordPlot()))
}, by=id]
x11()
plotDT[id == 1L, replayPlot(p[[1]])]
The plot from DT[...]
is correct:
While the replay is wrong:
In the recordedplot, the final values of x
and y
are magically applied to all three graphs, probably because of how data.table handles pointers to columns defined in by=
groups.
I'm not really sure I understand what's going on, though, because I can change the example superficially, writing v = id2*1:10
instead of v = id*10 + id2*1:10
and -- poof -- the problem disappears.
I'd like to know (i) why is this happening and (ii) what simple tweak can I make to get around it?
One working hack is to write j
like
with(copy(.SD), {yada yada})
or similar, which ensures that references are to the per-group copy of .SD
and don't get conflated.
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