compute means of a group by factor

Question

Is there a way that this can be improved, or done more simply?

means.by<-function(data,INDEX){
  b<-by(data,INDEX,function(d)apply(d,2,mean))
  return(         


        

Answer 1

With plyr

library(plyr)
df <- ddply(x, .(id),function(x) data.frame(
mean=mean(x$var)
))
print(df)

Update:

data<-data.frame(I=as.factor(rep(letters[1:10],each=3)),x=rnorm(30),y=rbinom(30,5,.5))
ddply(data,.(I), function(x) data.frame(x=mean(x$x), y=mean(x$y)))

See, plyr is smart :)

Update 2:

In response to your comment, I believe cast and melt from the reshape package are much simpler for your purpose.

cast(melt(data),I ~ variable, mean)