问在data.table中将继承作用域与by联接

EN

> dta
   idx vala fdx
1:   1    2   a
2:   2    4   a
3:   3    6   b

> dtb
   idx valb
1:   1    3
2:   4    6

> dput(x = dta)
structure(list(idx = c(1, 2, 3), vala = c(2, 4, 6), fdx = c("a",
"a", "b")), .Names = c("idx", "vala", "fdx"), row.names = c(NA,
-3L), class = c("data.table", "data.frame"), .internal.selfref =
<pointer: 0x0000000000110788>, sorted = "idx")

> dput(x = dtb)
structure(list(idx = c(1, 4), valb = c(3, 6)), .Names = c("idx",
"valb"), row.names = c(NA, -2L), class = c("data.table", "data.frame"
), .internal.selfref = <pointer: 0x0000000000110788>, sorted = "idx")

> dta[dtb, sum(valb)]
[1] 9

> dta[dtb, sum(valb), by = fdx]
Error in `[.data.table`(dta, dtb, sum(valb), by = fdx) :
  object 'valb' not found

> dta[dtb][, sum(valb), by = fdx]
   fdx V1
1:   a  3
2:  NA  6

> dta[dtb]
   idx vala fdx valb
1:   1    2   a    3
2:   4   NA  NA    6

dta[dtb, sum(valb), by = fdx] == dta[dtb][, sum(valb), by = fdx]