Hi, R users,

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))

But I got the error:

Error in match.fun(FUN) :
  'mean(y$a)' is not a function, character or symbol


Anyone can tell me how to fix this problem? Thanks.

Gary

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