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joshhwuu
reviewed
Jun 8, 2026
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| ### d) Side effects and testing | ||
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| Because `:=` modifies by reference, changes are cumulative. If the same *data.table* is reused—for example, in a loop or when using `test()` with multiple `optimization` levels—subsequent runs will start with the modified table from the previous run. Use `copy()` to ensure each run starts with the same data. |
jangorecki
reviewed
Jun 9, 2026
| \code{DT[a > 4, b := c]} is different from \code{DT[a > 4][, b := c]}. The first expression updates (or adds) column \code{b} with the value \code{c} on those rows where \code{a > 4} evaluates to \code{TRUE}. \code{X} is updated \emph{by reference}, therefore no assignment needed. Note that this does not apply when \code{i} is missing, i.e. \code{DT[]}. | ||
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| The second expression on the other hand updates a \emph{new} \code{data.table} that's returned by the subset operation. Since the subsetted data.table is ephemeral (it is not assigned to a symbol), the result would be lost; unless the result is assigned, for example, as follows: \code{ans <- DT[a > 4][, b := c]}. | ||
| Note that \samp{:=} modifications are cumulative. When reusing a \code{data.table} in loops or multi-level tests, use \code{\link{copy}} to ensure a fresh state. |
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I don't think cumulative is good word here. In-place changing it's input seems to be more precise.
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closes #7409
Documented
:=side effects in repeated execution acrossdatatable-reference-semantics.Rmd,assign.Rd, andtest.Rd.hi @ben-schwen, whenever you have a chance, could you please take a look at this PR.
thanks