括号前的点，.（ ），在 data.table 中

问：

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

它似乎正在实现聚合。根据此符号在以下代码中的位置，我希望它来自不是来自，但我尝试检查两者都无济于事。h2o

该示例来自 Kaggle 竞赛，代码有效（要重现它，请转到此处）：

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")

我希望文档和/或对此有很好的解释。