Hola.
Esta es una manera:
require(data.table)
dt <-
data.table(v1=letters[1:30],v2=round(runif(30,max=20)),v3=rep(c("x","y","z"),10))
dt[unlist(dt[,.I[which.max(v2)],by=v3,drop=TRUE][,2])]
Saludos.
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