似乎dcast（）返回data.frame，尽管输入参数是data.table：
d <- data.table(id = c(1,2,3), group = c('a','a','b'), x = c(10,20,30))
dc <- dcast(d, id ~ group, value.var = 'x')
> class(d)
[1] "data.table" "data.frame"
> class(dc)
[1] "data.frame"
有没有办法让dcast（）返回data.table？
> sessionInfo('data.table')
R version 3.2.2 (2015-08-14)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows 7 x64 (build 7601) Service Pack 1
attached base packages:
character(0)
other attached packages:
[1] data.table_1.9.6