grouping table by multiple factors and spreading it from long format to wide - the data.table way in R

Question

As an example i will be using the mtcars data available in R:

data(mtcars)
setDT(mtcars)

Lets day I want to group the data by three variables, namely: carb , cyl , and gear . I have done this as follow. However, i am sure there is a better way, as this is quite repetitive.

newDTcars <- mtcars [, mtcars[, mtcars[, .N , by = carb], by = cyl], by= gear]

Secondly, I would like to have the data in a wide format, where there is a separate column for every gear level. For illustration purpose I have done this using tidyr , however i would like to have this done the "data.table" way.

newDTcars %>% tidyr::spread(gear, N)

The emphasis of this question is to keep to solution to the data.table world, as i would like too learn more about data.table .

Answer 1

In data.table , we can group by multiple columns and to reshape we can use dcast .

library(data.table)
dcast(mtcars[, .N, .(carb, cyl, gear)], carb+cyl~gear, value.var = "N")

#   carb cyl  3  4  5
#1:    1   4  1  4 NA
#2:    1   6  2 NA NA
#3:    2   4 NA  4  2
#4:    2   8  4 NA NA
#5:    3   8  3 NA NA
#6:    4   6 NA  4 NA
#7:    4   8  5 NA  1
#8:    6   6 NA NA  1
#9:    8   8 NA NA  1

You may use fill argument in dcast to replace NA s with 0 or any other number.