r - 在data.table中的圆括号前面

我不熟悉这种df[, .(...), Col]表示法。如果我遗漏了一些明显的东西，我对此表示歉意，但是尽管它看起来非常有用，但我找不到该符号样式的引用。

它似乎正在实现聚合。根据以下代码中此符号的位置，我希望它来自R，而不是h2o，但是我尝试检查两者均无济于事。

该示例来自Kaggle竞赛，其代码有效(将其复制为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")

我很喜欢这个文档和/或一个很好的解释。

最佳答案

.()是 data.table 便利函数，用作list()的简洁别名。稍微复杂一点(对于像您这样的人，主要是想弄清楚.的作用!)是事实，它仅在对[.data.table()的调用范围内被解释为这样。

在这里，从?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")

关于r - 在data.table中的圆括号前面，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/33808705/

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r - 在data.table中的圆括号前面