Gary/Hongwei writes:
> I'm wondering how I can aggregate data in R with different functions for
> different columns. For example:
> 
> x<-rep(1:5,3)
> y<-cbind(x,a=1:15,b=21:35)
> y<-data.frame(y)
> 
> I want to aggregate "a" and "b" in y by "x". With "a", I want to use
> function "mean"; with "b", I want to use function "sum". I tried:
> 
> > aggregate(y,x,mean(y$a),sum(y$b))

aggregate() only works with one function at a time, ?aggregate will 
probably help.

This might be close to what you want.
y <- data.frame(x=rep(1:5,3), a=1:15, b=21:35)
answers <- merge(aggregate(list(abar=y$a), list(x=y$x), mean)
                ,aggregate(list(bbar=y$b), list(x=y$x), mean)
                ,by='x'
                )


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