Data.table: .N returned as 1 using the subsetting format DT["a", .N, on="x"] when it should be 0

[dbrookeUAB]

Is this a bug?

library(data.table)
DT <- as.data.table(mtcars)

# example of something with any zeros
DT[vs==0,length(vs)]
#> [1] 18
DT[.(0),.N,on="vs"]
#> [1] 18

# example of something without any zeros
DT[mpg==0,length(mpg)]
#> [1] 0
DT[.(0),length(vs),on="mpg"]
#> [1] 1
DT[.(0),.N,on="mpg"]
#> [1] 1
DT[mpg==0,.N]
#> [1] 0

# Output of sessionInfo()

sessionInfo()
#> R version 3.6.0 (2019-04-26)
#> Platform: x86_64-apple-darwin15.6.0 (64-bit)
#> Running under: macOS Mojave 10.14.5
#> 
#> Matrix products: default
#> BLAS:   /Library/Frameworks/R.framework/Versions/3.6/Resources/lib/libRblas.0.dylib
#> LAPACK: /Library/Frameworks/R.framework/Versions/3.6/Resources/lib/libRlapack.dylib
#> 
#> locale:
#> [1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8
#> 
#> attached base packages:
#> [1] stats     graphics  grDevices utils     datasets  methods   base     
#> 
#> loaded via a namespace (and not attached):
#>  [1] compiler_3.6.0  magrittr_1.5    tools_3.6.0     htmltools_0.3.6
#>  [5] yaml_2.2.0      Rcpp_1.0.1      stringi_1.4.3   rmarkdown_1.13 
#>  [9] highr_0.8       knitr_1.23      stringr_1.4.0   xfun_0.8       
#> [13] digest_0.6.19   evaluate_0.14

[renkun-ken]

By default, DT[.(0), on = "mpg"] returns a one-row data table with mpg = 0 and all others NA if there's no mpg == 0 in DT, which right-join by definition.

If you want to get 0, you may try DT[.(0),.N,on="mpg",nomatch=0].

[jangorecki]

already answered by @renkun-ken, closing.
also note that nomatch=NULL instead of nomatch=0 should be preferred.