How to call a function that returns multiple rows and columns in a data.table?

Question

I want to call a function inside a data.table that calculates a set of summary statistics like the following:

summ.stats <- function(vec) {
    list(
             


        

Answer 1

Alternatively you can modify your function as follows:

summ.stats <- function(vec) {
    list(
        Var = names(vec),
         Min = sapply(vec, min),
         Mean = sapply(vec, mean),
         S.D. = sapply(vec, sd),
         Median = sapply(vec, median),
         Max = sapply(vec, max))
}

DT[, summ.stats(.SD), by=a] # no need for as.list(.SD) as Roger mentions
   a Var Min Mean      S.D. Median Max
1: 1   b   1  1.5 0.7071068    1.5   2
2: 1   c   2  3.5 2.1213203    3.5   5
3: 2   b   1  2.5 2.1213203    2.5   4
4: 2   c   2  2.5 0.7071068    2.5   3
5: 3   b   3  3.5 0.7071068    3.5   4
6: 3   c   1  2.5 2.1213203    2.5   4