data.table: Sum by all existing combinations in table

Question

I have a data.table out like this (in reality it is much larger):

out <-      code weights group
        1:    2   0.387      1
        2:            


        

Answer 1

I had a similar problem, and CJ did not work for some reason. A relatively simple solution I ended up using is first calling dcast and then melt (similar to the xtable solution above)- this also conveniently lets you specify the fill value for the missing combinations.

sum.dt <- dcast(out, code ~ group, value.var = 'weights', 
                fun.aggregate = sum, fill = 0)
sum.dt <- melt(sum.dt, id.vars = 'code', variable.name = 'group')

This gives

> sum.dt
   code group value
1:    1     1 0.399
2:    2     1 1.997
3:    3     1 0.000
4:    1     2 0.212
5:    2     2 0.373
6:    3     2 1.322
7:    1     3 0.474
8:    2     3 0.569
9:    3     3 0.316