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

One option is to use plyr. ddply expects a data.frame (the first d) and returns a data.frame (the second d). Other XXply functions work in a similar way; i.e. ldply expects a list and returns a data.frame, dlply does the opposite...and so on and so forth. The second argument is the grouping variable(s). The third argument is the function we want to compute for each group.

require(plyr)
ddply(dat, "group", transform, grp.mean.values = mean(value))

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