r - Apply multiple functions to multiple columns in data.table

Question

I am trying to apply multiple functions to multiple columns of a data.table. Example:

DT <- data.table("a"=1:5,
                 "b"=2:6,
                 "c"=3:7)

Let's say I want to get the mean and the median of columns a and b. This works:

stats <- DT[,.(mean_a=mean(a),
               median_a=median(a),
               mean_b=mean(b),
               median_b=median(b))]

But it is way too repetitive. Is there a nice way to achieve a similar result using .SDcols and lapply?

Answer 1

I'd normally do this:

my.summary = function(x) list(mean = mean(x), median = median(x))

DT[, unlist(lapply(.SD, my.summary)), .SDcols = c('a', 'b')]
#a.mean a.median   b.mean b.median 
#     3        3        4        4