select rows with largest value of variable within a group in r

a.2<-sample(1:10,100,replace=T)
b.2<-sample(1:100,100,replace=T)
a.3<-data.frame(a.2,b.2)

r<-sapply(split(a.3,a.2),function(x) which.max(x$b.2))

a.3[r,]

returns the list index, not the index for the entire data.frame

Im trying to return the largest value of b.2 for each subgroup of a.2. How can I do this efficiently?