data.table을 통한 column별 group에 따른 mean값 구하기

library(data.table)

# creates data table
dfo <- data.frame(bananas = 1:5, 
             melonas = 6:10,
             yeah = 11:15,
             its = c(1,1,1,2,2)
             )
dto <- data.table(dfo)
library(data.table)
d <- dto[, lapply(.SD, mean), by=its]

foreach나 aggregate를 쓸 때보다 훨씬 빠름.