r - 抑制 data.table j 函数中的输出

Question

我想使用 R< 在 data.table 中针对 id 变量的不同值绘制一些 ccf 图。问题在于 ccf 返回的值与 data.table 不一致。我不关心返回值，只是想看看实际的情节。一些代码:

require(data.table)
x <- data.table(id=rep(1:10, each=10), a=rep(1:10,10), b=rep(10:1,10))
x[,ccf(a,b),by=id]
Error in `[.data.table`(x, , ccf(a, b), by = id) : 
All items in j=list(...) should be atomic vectors or lists. If you are trying something like j=list(.SD,newcol=mean(colA)) then use := by group instead (much quicker), or cbind or merge afterwards.

Answer 1

鉴于问题中的这一点:

I don't care about the return value but just want to see the actual plot.

然后:

x[, {ccf(a,b);NULL}, by=id]