[R] aggregating variables (sum within groups)

[David Studer]

Hello everybody!

I have a (probabely very easy) problem. Even though I was looking in
several r-books
I could not find a suitable function to this problem, that's why I hope
that someone here
could help me:

# Sample data:
group<-c("A","A","A","B","B","C","C","C")
var1<-c(1,0,0,1,1,0,NA,1)
var2<-c(0,1,NA,0,1,1,0,0)
testdata<-data.frame(group, var1, var2)

Now, I'd like to generate two aggregated variables:

testdata$x<- ???   should count the sum of var1 within each group (=4)
testdata$y<- ???   should count the sum of var2 within each group (=3)

Therefore I am looking for a function like ave() which does not calculate
the mean value but a sum.

Thank you for any hints!

David

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