我不明白这里发生了什么。为什么sum在data.table之外工作而不在其中？ data.table版本是1.94并且加载了bit64。
dput(dt)
structure(list(Date = c(20150422L, 20150422L, 20150422L, 20150422L,
20150423L, 20150423L, 20150423L, 20150423L, 20150424L, 20150424L,
20150424L, 20150424L), totcap = structure(c(5.30519039464278e-314,
5.34352625144878e-314, 5.21151503979773e-314, 5.18159473949947e-314,
5.36659973716195e-314, 5.3767197559193e-314, 5.31749562227391e-314,
5.48717086915892e-314, 5.34891674084389e-314, 5.22243170680067e-314,
5.22969347328787e-314, 5.23636617172838e-314), class = "integer64")), .Names = c("Date",
"totcap"), class = c("data.table", "data.frame"), row.names = c(NA,
-12L), .internal.selfref = )
> sum(dt$totcap)
integer64
[1] 128782928014
> dt[,sum(totcap),by=Date]
Date V1
1: 20150422 2.104183e-313
2: 20150423 2.154799e-313
3: 20150424 2.103741e-313