Multiple functions on multiple columns by group, and create informative column names

Question

How to adjust a data table manipulation so that, besides sum per category of several colums, it would also calculate other functions at the same time such as <

Answer 1

It seems that there's not a straightforward answer using data.table since noone has answered this yet. So I'll propose a dplyr-based answer that should do what you want. I use the built-in iris data set for the example:

library(dplyr)
iris %>% 
   group_by(Species) %>% 
  summarise_at(vars(Sepal.Length, Sepal.Width), .funs = c(sum=sum,mean= mean), na.rm=TRUE)

## A tibble: 3 x 5
#  Species    Sepal.Length_sum Sepal.Width_sum Sepal.Length_mean Sepal.Width_mean
#                                                       
#1 setosa                 245.            171.              5.00             3.43
#2 versicolor             297.            138.              5.94             2.77
#3 virginica              323.            149.              6.60             2.97

or using character vector input for the columns and functions:

Colchoice <- c("Sepal.Length", "Sepal.Width")
FunChoice <- c("mean", "sum")
iris %>% 
  group_by(Species) %>% 
  summarise_at(vars(Colchoice), .funs = setNames(FunChoice, FunChoice), na.rm=TRUE)
## A tibble: 3 x 5
#  Species    Sepal.Length_mean Sepal.Width_mean Sepal.Length_sum Sepal.Width_sum
#                                                       
#1 setosa                  5.00             3.43             245.            171.
#2 versicolor              5.94             2.77             297.            138.
#3 virginica               6.60             2.97             323.            149.