setDT modifies the class of the origin data.frame
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Assessment
- Difficulty
- 2/5
- Estimated time
- 1-3 hours
- Newbie friendliness
- 35/100
Research direction
Start at the setDT entry point and reproduce the issue with the data.frame example from the report. Inspect the two places where setattr and setalloccol are called, then verify that converting d2 leaves d1 with class data.frame while d2 becomes data.table.
Written by the indexing model from the issue text.
Description
Take this code:
> d1 <- data.frame(a=c(1,2,3,4), b=c(2,3,4,5))
> d2 <- d1
> setDT(d2)
At this point d2 is a shallow copy of d1, meaning they are not the same R object (although they point to the same columns).
d2's class is data.table, as expected, but so is d1
> class(d2)
[1] "data.table" "data.frame"
> class(d1)
[1] "data.table" "data.frame"
Unlike #4783 , I think this is an easy fix. In two places in setDT's source you see the pair of lines -
setattr(x, "class", .resetclass(x, "data.frame"))
setalloccol(x)
setalloccol is what initiates the shallow copy. If the order of the rows were reversed, the class of only the current shallow copy (d2 in the example above) would be modified, as expected.
# Output of sessionInfo()
R version 3.6.1 (2019-07-05)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows 10 x64 (build 19041)
Matrix products: default
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