How to get the mean value of each column, group by sth?

Question

I am using R. I have a dataset.

group attribute1 attribute2 attribute3
A     1          2          3
A     2          3          4
B     2          4          2
         


        

Answer 1

> mydf <- data.frame(group=c('a','a','b','b'), a1=c(1,1,2,2), a2=c(2,3,4,2), a3=c(3,4,2,1))
> aggregate(. ~ group, mydf, mean)
    group a1  a2  a3
1       a  1 2.5 3.5
2       b  2 3.0 1.5

or using plyr:

ddply(mydf, "group", colwise(mean))

Answer 2

Using data.table:

library(data.table)
mydf <- data.frame(group=c('a','a','b','b'), a1=c(1,1,2,2), a2=c(2,3,4,2), a3=c(3,4,2,1))
my.dt <- data.table(mydf)
my.dt[, lapply(.SD, mean), by = group]

   group a1  a2  a3
1:     a  1 2.5 3.5
2:     b  2 3.0 1.5