有没有办法在没有任何j列的情况下返回data.table？
我经常想要一把钥匙或一组钥匙的独特组合。
library(data.table)
## Warpbreaks data
wb <- as.data.table(warpbreaks)
## Say I just want the summary of wool / tension combos
## I would like to do one of these, but neither works
wb[ , j = NULL, keyby = list(wool, tension)] ## <--- Seems legit
wb[keyby = list(wool, tension)] ## <--- Even better!
## This works, but is two steps:
subset(wb[ , .N, keyby=list(wool, tension)], select=-N)
## This almost works, but there is no key
unique(wb[ , list(wool, tension)])
由于