Data Table

Data table is a very useful package in R which allows to facilitate and to improve the efficiency of certain operations in R. Data tables are just like data frames. You can even create them from data frames.

Introduction to Data table (PDF)

install.packages('data.table')
library(data.table)

mydf=data.frame(a=rep(LETTERS,each=1e6),b=rnorm(26*1e6))
mydt=data.table(mydf)
setkey(mydt,a)
mydt[,sum(b),by=a]