Attach the inherited area using in data.table

Question

I am on data.table 1.9.3, and maybe I'm wrong, but I don’t remember as expected earlier.

I am building 2 data.tables, dta and dtb

> 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") 

In both cases, the idx key.

Following work of course

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

However it is not

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

But it does

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

If we see an intermediate step

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

I would expect

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

Where am I wrong?

Answer 1

Just guess

 library(data.table) dta <- data.frame(idx=c(1,2,3), vala=c(2,4,6), fdx=c('a','a','b')) dta <- data.table(dta) dtb <- data.frame(idx=c(1,4), valb=c(3,6)) dtb <- data.table(dtb) setkey(dta,idx) setkey(dtb,idx) 

Therefore when you call

 dta[dtb, sum(valb)] 

it looks like a challenge

 tmp <- dta[dtb] attach(tmp) sum(valb) detach(tmp) 

However, if you call

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

it’s kind of like a challenge

 tmp <- dta[dtb] # attach(tmp) : attach doesn't happen sum(valb) # detach(tmp) : detach doesn't happen 

The function does not know what to do with the additional arguments. For example, this will also cause an error:

 dta[dtb, class(fdx), sum(valb)] 

However it works

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

which looks like

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

As I said, this is just an assumption why the function may not work as expected.