dim() on 0-column data.table produced in non-data.table-aware package is wrong

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
45/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
r
Domain
data

Research direction

Run the reproducible example with data.table and compare [.data.frame, dim.data.frame, and dim on the resulting zero-column object. Trace how data.table dispatch handles this case; done means the example reports the original 10 rows without breaking ordinary zero-column data.frame behavior.

Written by the indexing model from the issue text.

Description

cedta consistency

If a data.table, which is passed to a function of a non-data.table-aware package, is subsetted there, such that a 0-column data.table/data.frame is produced, dim() on that data.table/data.frame falsely reports 0 rows.

library(data.table)
X <- data.table(a = 1:10)

# imitate subsetting in function of non-data.table-aware package
Y <- `[.data.frame`(X, , character(), drop = FALSE)

dim.data.frame(Y)  # returns c(10, 0)
dim(Y)  # returns c(0, 0), also in function of non-data.table-aware package

> sessionInfo()
R version 3.4.2 (2017-09-28)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Debian GNU/Linux 8 (jessie)

Matrix products: default
BLAS: /usr/lib/atlas-base/atlas/libblas.so.3.0
LAPACK: /usr/lib/atlas-base/atlas/liblapack.so.3.0

locale:
 [1] LC_CTYPE=de_DE.UTF-8       LC_NUMERIC=C               LC_TIME=de_DE.UTF-8        LC_COLLATE=de_DE.UTF-8    
 [5] LC_MONETARY=de_DE.UTF-8    LC_MESSAGES=de_DE.UTF-8    LC_PAPER=de_DE.UTF-8       LC_NAME=C                 
 [9] LC_ADDRESS=C               LC_TELEPHONE=C             LC_MEASUREMENT=de_DE.UTF-8 LC_IDENTIFICATION=C       

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] data.table_1.10.4-2

loaded via a namespace (and not attached):
[1] tools_3.4.2 yaml_2.1.14
Dominant language
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