quick/elegant way to construct mean/variance summary table

Question

I can achieve this task, but I feel like there must be a \"best\" (slickest, most compact, clearest-code, fastest?) way of doing it and have not figured it out so far ...

Answer 1

@joran is spot-on with the ddply answer. Here's how I would do it with aggregate. Note that I avoid the formula interface (it is slower).

aggregate(d$y, d[,c("f1","f2","f3")], FUN=function(x) c(mean=mean(x),var=var(x)))