updating data.table by reference has side effect of altering other non-data.table objects
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Assessment
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Newbie friendliness
- 35/100
Research direction
Start by running the minimal R example in the issue and comparing the behavior of := and setcolorder after assigning names(x). Trace the relevant data.table entry points for these operations; done means the separately assigned xnames remains unchanged when the table is modified by reference.
Written by the indexing model from the issue text.
Description
If we assign a data.table's names to an R variable, then update the data.table columns by reference (with :=) the R object that stored the original names also (surprisingly) gets updated by reference. This does not seem to be desired behaviour to me. Especially as the use case where I discovered this was when trying to save the column names before alteration for later use.
Minimal example:
x = as.data.table(matrix(nrow = 10, ncol = 10))
xnames = names(x)
xnames
# [1] "V1" "V2" "V3" "V4" "V5" "V6" "V7" "V8" "V9" "V10"
x[, (paste0('V', 3:7)) := NULL]
xnames
# [1] "V1" "V2" "V8" "V9" "V10"
x[, newcol := 1]
xnames
# [1] "V1" "V2" "V8" "V9" "V10" "newcol"
setcolorder(x, 'newcol')
xnames
# [1] "newcol" "V1" "V2" "V8" "V9" "V10"
We can work around this issue by using xnames = copy(names(x)), however the fact that updating a data.table by reference has this side effect is probably best avoided in the first place.
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