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.SD ignores locally updated vectors in j #7431
Description
I would expect both expressions give the same result.
Filtering in i is irrelevant, I have kept it to reflect my use case.
data.table(a = 1:3, b = 1:3, d = 1:3
)[a %in% 1:2, {
b[d == 2L] <- 0L
.SD
}]
# a b d
#1: 1 1 1
#2: 2 2 2
data.table(a = 1:3, b = 1:3, d = 1:3
)[a %in% 1:2, {
b[d == 2L] <- 0L
.(a, b, d)
}]
# a b d
#1: 1 1 1
#2: 2 0 21.17.0
workaround for now:
data.table::data.table(a = 1:3, b = 1:3, d = 1:3
)[a %in% 1:2, {
b[d == 2L] <- 0L
.SD
}, env = list(.SD = as.list(c("a","b","d")))]
Activity
MichaelChirico commentedon Nov 18, 2025
This is a documentation issue IMO. I don't think there's any way to make this work as you'd hoped.
.SDis populated up-front, not dynamically/lazily.venom1204 commentedon Nov 28, 2025
Hi @MichaelChirico @tdhock ,
My proposed fix focuses entirely on clarifying the static snapshot nature of .SD to prevent future confusion.
Here are the specific documentation files I propose to modify:
Does this targeted documentation plan sound good to you? If so, I am happy to proceed with a Pull Request.
Thanks!
tdhock commentedon Nov 28, 2025
is that true? if so an error would be better.
jangorecki commentedon Nov 28, 2025
To not break existing workflows probably warning would be better. Question is to what degree we are able to "scan" j for assignments to columns that exist in .SD with a minimal overhead. There are many corner cases but even catching simple ones like those may cover majority of use cases.