In R, how do I add a max by group? [duplicate]

Answer 1

We can use data.table assignment (:=) to create a column in place without copying

library(data.table)
setDT(x)[, max.per.group := max(replicate), by = group]
x
#     group replicate max.per.group
#1:     A         1             5
#2:     A         2             5
#3:     A         3             5
#4:     A         4             5
#5:     A         5             5
#6:     B         1             2
#7:     B         2             2
#8:     C         1             3
#9:     C         2             3
#10:    C         3             3

data

x <- data.frame(group,replicate)