Let DT[-integer(0)] return the full table

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
35/100
Issue type
Feature
Clarity
Mostly clear
Activity status
Stale
Tech stack
r
Domain
data

Research direction

Start at the [.data.table entry point referenced by the warning and reproduce the supplied DT[-w] examples with w = integer(0) and an out-of-range negative index. Done means DT[-integer(0)] returns the full table while the existing warning and other negative-indexing behavior remain appropriate.

Written by the indexing model from the issue text.

Description

consistency

One annoying quirk of base R is the fact that x[-w] excludes the positions in w iff length(w) > 0. This happens because -w is computed before being passed to [. Since data.table evaluates the whole DT[...] call together, it can break consistency here, and I think should do so.

library(data.table)
DT = data.table(id = 1:2)
w = integer(0)

# current behavior
DT[-w]
# Empty data.table (0 rows) of 1 col: id

# desired behavior
DT[-w]
#    id
# 1:  1
# 2:  2

The workaround mentioned in the link above is to set w = nrow(DT) + 1L. This works with vectors, x = 1:2; x[-3L], but data.table gives a helpful warning for this hack.

w2 = 3L
DT[-w2]
#    id
# 1:  1
# 2:  2
# Warning message:
# In `[.data.table`(DT, -w2) :
#   Item 1 of i is -3 but there are only 2 rows. Ignoring this and 0 more like it out of 1.

I doubt this would break backward compatibility in a way that matters for folks; and I guess it can be done without much of any efficiency tradeoff (?).

Example of the negative consequences of current behavior from SO: http://stackoverflow.com/a/43682893/

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