Mode in R by groups

Question

I need to calculate the mode of an identity number for each group of ages. Let\'s suposse the following table:

library(data.table)
DT = data.table(age=c(12,1         


        

Answer 1

Here's a base R solution. You could compute the mode for each group and then merge with your original data:

merge(DT, setNames(aggregate(number~age, data=DT, g), c("age", "moda")), by="age")
#    age          v number moda
# 1:   3  1.7148357      5    5
# 2:   3  0.9504811      5    5
# 3:  12 -0.7648237    122  122
# 4:  12  0.9011115    125  122
# 5:  12 -0.8718779    122  122

There may be a data table-specific approach, but this would work even if DT were a data frame.