Join inherited scope with by in data.table
Just a 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)So when you call
dta[dtb, sum(valb)]it's kind of like calling
tmp <- dta[dtb]attach(tmp)sum(valb)detach(tmp)However, if you call
dta[dtb, sum(valb), by=fdx]then it's kind of like calling
tmp <- dta[dtb]# attach(tmp) : attach doesn't happensum(valb)# detach(tmp) : detach doesn't happenThe function doesn't know what to do with the additional arguments. For instance, this would also throw an error:
dta[dtb, class(fdx), sum(valb)]However, this works
dta[dtb][, sum(valb), by=fdx]which is kind of like
tmp <- dta[dtb]tmp[, sum(valb), by=fdx]Like I said, this is just a guess as to why the function may not be working as expected.