Adding a column of means by group to original data [duplicate]

by

Two alternative ways of doing this:

1) with the dplyr package:

library(dplyr)
df1 <- df1 %>% 
  group_by(X) %>% 
  mutate(Y.new = mean(Y))

2) with the data.table package:

library(data.table)
setDT(df1)[, Y.new := mean(Y), by = X]

both give the following result:

> df1
   X Y Y.new
1: A 1     2
2: A 2     2
3: A 3     2
4: B 4     5
5: B 5     5
6: B 6     5

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