在数据中保存图表。表名单列

When I run...

当我运行……

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:

情节从DT[…是正确的:

While the replay is wrong:

而重播是错误的:

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.

在recordedplot中，x和y的最终值会神奇地应用到这三个图上，这可能是因为数据的缘故。表处理由=组定义的列的指针。

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.

我不太确定我是否理解发生了什么，因为我可以简单地改变例子，写v = id2*1:10而不是v = id*10 + id2*1:10，然后——噗——问题就消失了。

I'd like to know (i) why is this happening and (ii) what simple tweak can I make to get around it?

我想知道(I)为什么会这样，(ii)我能做些什么简单的调整来绕过它?

1 个解决方案

#1

One working hack is to write j like

一个工作骇客就是写j。

with(copy(.SD), {yada yada})

or similar, which ensures that references are to the per-group copy of .SD and don't get conflated.

或者类似的，这样可以确保每个组的引用都是. sd的副本，并且不会被合并。