setDT and [,:=] should share code for re-assigning data.tables

[MichaelChirico]

These two regions are close to identical:

data.table/R/data.table.R

Lines 1224 to 1236 in 70c64ac

	} else if (name %iscall% c('$', '[[') && is.name(name[[2L]])) {
	k = eval(name[[2L]], parent.frame(), parent.frame())
	if (is.list(k)) {
	origj = j = if (name[[1L]] == "$") as.character(name[[3L]]) else eval(name[[3L]], parent.frame(), parent.frame())
	if (is.character(j)) {
	if (length(j)!=1L) stopf("Cannot assign to an under-allocated recursively indexed list -- L[[i]][,:=] syntax is only valid when i is length 1, but its length is %d", length(j))
	j = match(j, names(k))
	if (is.na(j)) internal_error("item '%s' not found in names of list", origj) # nocov
	}
	.Call(Csetlistelt,k,as.integer(j), x)
	} else if (is.environment(k) && exists(as.character(name[[3L]]), k)) {
	assign(as.character(name[[3L]]), x, k, inherits=FALSE)
	}

data.table/R/data.table.R

Lines 2970 to 2985 in 70c64ac

	} else if (name %iscall% c('$', '[[') && is.name(name[[2L]])) {
	# common case is call from 'lapply()'
	k = eval(name[[2L]], parent.frame(), parent.frame())
	if (is.list(k)) {
	origj = j = if (name[[1L]] == "$") as.character(name[[3L]]) else eval(name[[3L]], parent.frame(), parent.frame())
	if (length(j) == 1L) {
	if (is.character(j)) {
	j = match(j, names(k))
	if (is.na(j))
	stopf("Item '%s' not found in names of input list", origj)
	}
	}
	.Call(Csetlistelt,k,as.integer(j), x)
	} else if (is.environment(k) && exists(as.character(name[[3L]]), k)) {
	assign(as.character(name[[3L]]), x, k, inherits=FALSE)
	}

To keep them in sync, the logic should be extracted to an appropriate helper.

[nipun-gupta-3108]

Hi @maintainers,

I’m interested in contributing to this issue. I’ve reviewed the functionality of setDT and [:=] and would like to confirm the following before proceeding:

Could you point me to the primary files where setDT and [:=] handle re-assignments?
Are there specific parts of the logic you’d like to see refactored or reused between these two functions?
Should the shared logic be implemented in R, or would a C utility function be more appropriate?
Additionally, do you have specific test cases or benchmarks you’d like to see included in this enhancement?

Thanks for your guidance!

Best regards,
Nipun

[MichaelChirico]

Hi Nipun, if you click through on the code blocks above, it will take you to the place in the source code where you'll find the relevant code for this issue.