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

You can use dplyrfor this:

library(dplyr)
modes_by_age <- summarise(group_by(DT, age), group_mode = g(number))
inner_join(DT, modes_by_age)

This gives your desired output:

Source: local data table [5 x 4]

  age         v number group_mode
1   3 0.5524352      5          5
2   3 0.2869912      5          5
3  12 0.8987475    122        122
4  12 0.9740715    125        122
5  12 2.5058450    122        122