unique returns a mutable DT alias when number of rows is <= 1

[dshemetov]

# Minimal reproducible example; please be sure to set verbose=TRUE where possible!

# Alias
library(data.table)
DT <- data.table(a = 1, b = 1)
DT2 = unique(DT)
address(DT)
#> [1] "0x55b61aaf4b40"
address(DT2)
#> [1] "0x55b61aaf4b40"

# No alias
library(data.table)
DT <- data.table(a = c(1, 2), b = c(3, 4))
DT2 = unique(DT)
address(DT)
#> [1] "0x55573fddfba0"
address(DT2)
#> [1] "0x555741ec0f80"

This behavior seems to be pretty clearly indicated here. Is this intended? We're adding logic to our package around unique to ensure safety.

# Output of sessionInfo()

r$> sessionInfo()
R version 4.3.1 (2023-06-16)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Ubuntu 20.04.6 LTS

Matrix products: default
BLAS:   /usr/lib/x86_64-linux-gnu/blas/libblas.so.3.9.0 
LAPACK: /usr/lib/x86_64-linux-gnu/lapack/liblapack.so.3.9.0

locale:
 [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C               LC_TIME=en_US.UTF-8       
 [4] LC_COLLATE=en_US.UTF-8     LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8   
 [7] LC_PAPER=en_US.UTF-8       LC_NAME=C                  LC_ADDRESS=C              
[10] LC_TELEPHONE=C             LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C       

time zone: America/Los_Angeles
tzcode source: system (glibc)

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

other attached packages:
 [1] data.table_1.14.8 lubridate_1.9.3   forcats_1.0.0     stringr_1.5.0    
 [5] dplyr_1.1.4       purrr_1.0.2       readr_2.1.4       tidyr_1.3.0      
 [9] tibble_3.2.1      tidyverse_2.0.0   ggplot2_3.4.3    

loaded via a namespace (and not attached):
 [1] vctrs_0.6.5      rspm_0.4.0       cli_3.6.1        rlang_1.1.1     
 [5] stringi_1.7.12   generics_0.1.3   glue_1.6.2       colorspace_2.1-0
 [9] hms_1.1.3        scales_1.2.1     fansi_1.0.4      grid_4.3.1      
[13] munsell_0.5.0    tzdb_0.4.0       lifecycle_1.0.3  compiler_4.3.1  
[17] timechange_0.2.0 pkgconfig_2.0.3  R6_2.5.1         tidyselect_1.2.0
[21] utf8_1.2.3       pillar_1.9.0     magrittr_2.0.3   tools_4.3.1     
[25] withr_2.5.0      gtable_0.3.4    

[dshemetov]

Found this conversation that seemed in agreement about fixing it for the maxgrpn case, filed and fixed here, but the nrows(x) <= 1 case wasn't touched in that fix.

[MichaelChirico]

LGTM & it looks like you've already found what to fix. Do you want to file a PR?

[dshemetov]

Sure, can do that sometime soon.