r - data.table 中圆括号前的点，.()

Question

我不熟悉这个df[, .(...), Col]符号。如果我遗漏了一些明显的东西，我深表歉意，但我找不到这种符号样式的引用，尽管它看起来非常有用。

它似乎正在实现聚合。基于此符号在下面代码中的位置，我希望它来自 R 而不是来自 h2o但我试过检查两者都无济于事。

该示例来自 Kaggle 比赛并且代码有效(重现它 go here):

trainHex<-as.h2o(train[,.(
  dist   = mean(radardist_km, na.rm = T),
  refArea5   = mean(Ref_5x5_50th, na.rm = T),
  refArea9  = mean(Ref_5x5_90th, na.rm = T),
  meanRefcomp = mean(RefComposite,na.rm=T),
  meanRefcomp5 = mean(RefComposite_5x5_50th,na.rm=T),
  meanRefcomp9 = mean(RefComposite_5x5_90th,na.rm=T),
  zdr   = mean(Zdr, na.rm = T),
  zdr5   = mean(Zdr_5x5_50th, na.rm = T),
  zdr9   = mean(Zdr_5x5_90th, na.rm = T),
  target = log1p(mean(Expected)),
  meanRef = mean(Ref,na.rm=T),
  sumRef = sum(Ref,na.rm=T),
  records = .N,
  naCounts = sum(is.na(Ref))
),Id][records>naCounts,],destination_frame="train.hex")

我会喜欢文档和/或对此的一个很好的解释。

Answer 1

.()是 数据表便利函数，作为 list() 的简洁别名.让事情稍微复杂一点(主要是对于像你这样的人，试图弄清楚 . 到底做了什么!)是这样一个事实，即它仅在对 [.data.table() 的调用范围内被解释为这样的事实。 .
在这里，来自 ?data.table :

 DT = data.table(x=rep(c("a","b","c"),each=3), y=c(1,3,6), v=1:9)
 setkey(DT,x,y)             # 2-column key

 DT["a"]                    # join to 1st column of key
 DT[.("a")]                 # same, .() is an alias for list()
 DT[list("a")]              # same

 ## But note that *this* doesn't work (my addition --- not in ?data.table)
 .("a")

另见小插图 Introduction to data.table :

data.table also allows wrapping columns with .() instead of list(). It is an alias to list(); they both mean the same. Feel free to use whichever you prefer