setDT does not preserve column names of nested data.tables
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Assessment
- Difficulty
- 3/5
- Estimated time
- 1-2 days
- Newbie friendliness
- 35/100
Research direction
Start with the setDT(DT) reproduction in this issue and inspect test 2074.07, along with the surrounding as.matrix.data.table behavior and PR #4144 context. Done means nested data.tables retain their column names after setDT, with a regression test covering the shown structure.
Written by the indexing model from the issue text.
Description
Obscure unintended behaviour of setDT that I picked up while debugging PR #4144 in relation to test 2074.07.
Given the erroneously constructed data.table in test 2074.07:
## as.matrix.data.table when a column has columns (only possible when constructed incorrectly)
DT = structure(list(a=1:5, d=data.table(b=6:10, c=11:15), m=matrix(16:25, ncol=2L)), class = c('data.table', 'data.frame'))
Running setDT(DT), e.g. to detect these erroneous wide columns, leads to a loss of the column names of the data.table stored in DT[,d]:
setDT(DT)
DT[,d]
# <int> <int>
# 1: 6 11
# 2: 7 12
# 3: 8 13
# 4: 9 14
# 5: 10 15
The scenario in which you would run into this problem is extremely contrived, but since its likely unintended behaviour I'm making a note of it here for future reference.
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