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Inconsistent logical indexing behaviour #758
Description
Activity
Your
data.frameequivalent is adding columns, not rows, or rather a new column with identical number of rows. They're not equivalent operations. You can usetransformto get the approx. equivalent operation, which is more or less identical in behaviour:transform(data.frame(x=1:2)[c(F,T), , drop=FALSE], y=3:4) # x y # 1 2 3 # 2 2 4 # Warning message: # In data.frame(list(x = 2L), y = 3:4) : # row names were found from a short variable and have been discarded transform(data.frame(x=1:2)[c(F,F), ], y=3:4) # Error in data.frame(list(X_data = integer(0)), y = 3:4) : # arguments imply differing number of rows: 0, 2
There are two things here:
- The order of operations
DT[i, j]. It first evaluatesi, and thenj. Not the other way around. So, in the first case, after the row subset usingc(FALSE,TRUE), it's left with:
# x # 1: 2
And then, use use
list(x, y=3:4), where, the shorter column is automatically recycled to fit the longest column's length.- For the same reason, in the second case, after the subset,
xis of length 0 =integer(0), and therefore could be recycled to fit the length of 2, with the valueNA. But this resulted in an error because of an invalid condition check. I'll fix this (after checking in with Matt).
data.tablealways tries to recycle columns automatically to fit the longest column, and warns if the recycling leaves a reminder.- The order of operations
To fix - only the case:
data.table(x = 1:2)[c(F, F), list(x, y = 3:4)]
which should result in
## Empty data.table (0 rows) of 2 cols: x,y- added a commit that references this issue
on Oct 2, 2014
When the
iindex contains a mix ofTandF, surprisingly there are no less rows then the originaldata.table:I would have expected
This is at odds with
data.frameintuition:Edge-cases
Moreover, the case where
i = Fdoes not even yield a valid result, which makes this an annoying edge-case to deal with:I would have expected
The other edge case where all
i = Tdoes work as expected:Is there any explanation behind this behaviour?