Hi all:
My data:data(sleep)

If I wanna calculate each group's extra,what I can do is:
#method1
attach(sleep)
mean(extra[group==1])
mean(extra[group==1])


#method2
result<-matrix(,0,2)
g<-split(sleep,sleep$group)
for(i in 1:length(g))
{
result<-rbind(result,data.frame(unique(g[[i]]$group),mean(g[[i]]$extra)))
}
colnames(result)<-c("name","mean")

But the above 2 method is a little bit tedious.Is there a "short cut" manner
to get the same result?

Thanks a lot!

My best

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