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