R data.table“ j”对“ by”变量的引用非常不直观吗？

Question

I'm just doing the data.table datacamp excercises and there is something which really disturbes my sense for logic. Somehow columns which are refered to by the "by" operator are treated different to other columns?

The used data table is the following:

         DT
      x  y  z
   1: 2  1  2
   2: 1  3  4
   3: 2  5  6
   4: 1  7  8
   5: 2  9 10
   6: 2 11 12
   7: 1 13 14

When I enter DT[,sum(x),x] I would expect:

   x V1
1: 2  8
2: 1  3

but I get:

   x V1
1: 2  2
2: 1  1

for other columns I get the group sum as I would expect it:

> DT[,sum(y),x]
      x V1
   1: 2 26
   2: 1 23

Answer 1

One way to fix this would be to name the grouping variable with a different name

setnames(DT[, sum(x), .(xN=x)], "xN", "x")[]
#   x V1
#1: 2  8
#2: 1  3