How to extract last value in each group

HI Steve,

Thanks for testing.

When I run a slightly bigger dataset:
set.seed(1254)
name<- sample(letters,1e7,replace=TRUE)
number<- sample(1:10,1e7,replace=TRUE)

datTest<- data.frame(name,number,stringsAsFactors=FALSE)
library(data.table)

dtTest<- data.table(datTest)

system.time(res3<- dtTest[,list(Sum_Number=sum(number)),by=name])
 #user  system elapsed
 # 0.592   0.028   0.623

#Then I tried this:

dtTest1<- data.table(datTest,key=name)
#Error: C stack usage is too close to the limit

Cstack_info()
#      size    current  direction eval_depth
 #  8388608       7320          1          2