R function which.max with tapply

Question

I am trying to make a data frame with the maximum over records by a factor. I would like a data frame with 4 rows (one for each G) with the max for X in that group and the c

Answer 1

library(dplyr)
Data %>% 
    group_by(G) %>% 
    filter(X==max(X))

If you don't want to include ties, then

Data %>%
    group_by(G) %>%
    arrange(desc(X)) %>%
    slice(1)

Answer 2

  library(data.table)
  set.seed(1)
  Data<-data.frame(X=rnorm(200), Y=rnorm(200), G=rep(c(1,2,3,4), each=50))
  setDT(Data)[,list(X=max(X),Y=Y[which.max(X)]),by=G]
   G        X          Y
1: 1 1.595281 -0.3309078
2: 2 2.401618  0.9510128
3: 3 2.087167  0.9160193
4: 4 2.307978 -0.3887222

Answer 3

You can use by and reference the rownames of the row returned by which.max:

Data[by(Data, Data$G, function(dat) rownames(dat)[which.max(dat$X)] ),]

#           X          Y G
#4   1.595281 -0.3309078 1
#61  2.401618  0.9510128 2
#147 2.087167  0.9160193 3
#171 2.307978 -0.3887222 4

(This assumes set.seed(1) for reproducibility's sake)