Calculate group mean (or other summary stats) and assign to original data

Question

I want to calculate mean (or any other summary statistics of length one, e.g. min, max, length, sum) of a nu

Answer 1

You may do this in dplyr using mutate:

library(dplyr)
df %>%
  group_by(group) %>%
  mutate(grp.mean.values = mean(value))

...or use data.table to assign the new column by reference (:=):

library(data.table)
setDT(df)[ , grp.mean.values := mean(value), by = group]