Apply several summary functions on several variables by group in one call

Question

I have the following data frame

x <- read.table(text = \"  id1 id2 val1 val2
1   a   x    1    9
2   a   x    2    4
3   a   y    3    5
4   a   y    4            


        

Answer 1

You can also use the plyr::each() to introduce multiple functions:

aggregate(cbind(val1, val2) ~ id1 + id2, data = x, FUN = plyr::each(avg = mean, n = length))