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

Here is another option using base functions aggregate and merge:

merge(x, aggregate(value ~ group, data = x, mean), 
     by = "group", suffixes = c("", "mean"))

  group id value.x value.y
1     a  1      10      15
2     a  2      20      15
3     b  3     100     150
4     b  4     200     150

You can get "better" column names with suffixes:

merge(x, aggregate(value ~ group, data = x, mean), 
     by = "group", suffixes = c("", ".mean"))


  group id value value.mean
1     a  1    10         15
2     a  2    20         15
3     b  3   100        150
4     b  4   200        150