data.table删除键行和汇总

Question

我正在寻找一种优雅的方法来迭代data.table的键，删除具有该键的行，然后对剩余的行进行汇总。例如：

mydt <- data.table(cat=c("a","a","b","b","c","c","c"), vals = 1:7)
setkey(mydt,cat)
tmp1 <- mydt[!"a"][,mean(vals)]
tmp2 <- mydt[!"b"][,mean(vals)]
tmp3 <- mydt[!"c"][,mean(vals)]
outdt <- data.table(cat=c("a","b","c"),means=c(tmp1,tmp2,tmp3))

有没有办法循环键并优雅地执行此操作？感谢。

Answer 1

我认为这样做，使用更传统的data.table代码：

setkey(mydt,cat)
mydt[, list(means=mean(mydt[!.BY,vals])), by=cat]

# or without needing to key first
mydt[, list(means=mean(mydt[cat != .BY,vals])), by=cat]

#   cat means
#1:   a   5.0
#2:   b   4.2
#3:   c   2.5