in R, how to calculate mean of all column, by group?

Question

I need to get the mean of all columns of a large data set using R, grouped by 2 variables.

Lets try it with mtcars:

library(dplyr)
g_mtcars <- gro         


        

Answer 1

For the sake of completeness you could use package plyr and do this:

library(plyr)
ddply(mtcars,c('cyl','gear'), summarize,mean_hp=mean(hp))