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feat : added condition to check the RHS of := #7161

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cfcd238
enhanced error message
venom1204 Jul 11, 2025
9fbd711
test correction
venom1204 Jul 11, 2025
c8cec80
better as a BUG FIX
MichaelChirico Jul 11, 2025
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2 changes: 2 additions & 0 deletions NEWS.md
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Expand Up @@ -72,6 +72,8 @@

12. `print(..., col.names = 'none')` now correctly adapts column widths to the data content, ignoring the original column names and producing a more compact output, [#6882](https://github.com/Rdatatable/data.table/issues/6882). Thanks to @brooksambrose for the report and @venom1204 for the PR.

13. Assigning a raw `function` to a new column, e.g. `DT[, new_col := mean]`, now throws an informative error. This prevents a common mistake and guides the user to wrap the `function` in a `list()`, e.g. `DT[, new_col := list(mean)]`, if the intent is to create a list-column of functions. Thanks to @tdhock for the report and @venom1204 for the fix.

### NOTES

1. The following in-progress deprecations have proceeded:
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8 changes: 7 additions & 1 deletion R/data.table.R
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Expand Up @@ -1411,7 +1411,13 @@ replace_dot_alias = function(e) {
}

if (!is.null(lhs)) {
# TODO?: use set() here now that it can add new columns. Then remove newnames and alloc logic above.
newnames = setdiff(lhs, names(x))
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What's the difference between this assignment and the one above?

data.table/R/data.table.R

Line 1218 in 2f0d12f

newnames=setdiff(lhs, names_x)

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both compute setdiff(lhs, names(x)), but the one above uses the cached names_x. I’ll update the later one to use names_x as well for consistency and to avoid unnecessary recomputation.

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sorry i don't understand, why recalculate newnames at all if it's just the same value as above?

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Got it, thanks. The second newnames assignment was redundant. I will take care of it.

if (length(newnames) > 0) {
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does this error not happen for the case of overwriting an existing column?

if (is.function(jval)) {
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@MichaelChirico MichaelChirico Jul 11, 2025 •
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I don't think this will catch the following case (which you should add as a new regression test):

DT=data.table(a=1:10)
DT[,c('a', 'b'):=.(1:10, mean)]

stopf("RHS of `:=` is a function. To create a new column of functions, it must be a list column (e.g., wrap the function in `list()`).")
}
}
# TODO?: use set() here now that it can add new columns.Then remove newnames and alloc logic above.
.Call(Cassign,x,irows,cols,newnames,jval)
return(suppPrint(x))
}
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5 changes: 5 additions & 0 deletions inst/tests/tests.Rraw
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Expand Up @@ -21426,3 +21426,8 @@ test(2330.7, as.data.table(list(z), keep.rownames=TRUE), data.table(rn=rep("", 3

M <- matrix(1:6, nrow=3, dimnames=list(rep("", 3), c("V1", "V2"))) # test of list(M) for empty-rowname'd matrix input
test(2330.8, as.data.table(list(M), keep.rownames=TRUE), data.table(rn=rep("", 3), V1=1:3, V2=4:6))

#5829 checking the RHS of :=
DT = data.table(x = 1:3)
test(2331.1, DT[, y := mean], error = "RHS of `:=` is a function")
test(2331.2, DT[, y := function(x) x], error = "RHS of `:=` is a function")
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