R help - Aug 2025 - Extracting a data frame element using a missing argument as index results in no error

NONCONFIDENTIAL // EXTERNAL

The following function foo indexes the mtcars data frame by x, and I supply no
default value to the argument x. When I call the function without any argument
values, I surprisingly get no error and the entire mtcars data frame is
returned. It seems that x is being treated as an empty index and thus mtcars[x]
is being treated as mtcars[]. But is using x as the index for `[` not enough to
get the promise to be resolved and hit the expected 'argument is
missing' error? If I use x by itself beforehand in foo2 below, I get the
expected error.


foo <- function(x) mtcars[x]
foo()
# mtcars is printed in full

foo2 <- function(x) {x; mtcars[x]}
foo2()
Error in foo2() : argument "x" is missing, with no default


Session info:

R version 4.4.1 (2024-06-14)
Platform: x86_64-ubuntu18-linux-gnu
Running under: Ubuntu 18.04.6 LTS

Matrix products: default
BLAS/LAPACK:
/apps/intel/2019.1/compilers_and_libraries_2019.1.144/linux/mkl/lib/intel64_lin/libmkl_gf_lp64.so;
LAPACK version 3.7.0

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

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

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

other attached packages:
[1] MKLthreads_0.1

loaded via a namespace (and not attached):
[1] compiler_4.4.1

William Barcelona
Data & Technology Analyst
Global Financial Institutions | Division of International Finance
Federal Reserve Board
william.l.barcelona at frb.gov<mailto:william.l.barcelona at frb.gov>



	[[alternative HTML version deleted]]

On 2025-08-15 1:31 p.m., William Barcelona via R-help
wrote:
> NONCONFIDENTIAL // EXTERNAL
> 
> The following function foo indexes the mtcars data frame by x, and I supply
no default value to the argument x. When I call the function without any
argument values, I surprisingly get no error and the entire mtcars data frame is
returned. It seems that x is being treated as an empty index and thus mtcars[x]
is being treated as mtcars[]. But is using x as the index for `[` not enough to
get the promise to be resolved and hit the expected 'argument is
missing' error? If I use x by itself beforehand in foo2 below, I get the
expected error.
Since [ can handle a missing value for the first index, passing it a 
missing value is fine. This isn't unusual, it happens with most 
functions, e.g.

   f <- function(x) if (missing(x)) print("x is missing")
   g <- function(y) f(y)
   g()
   # [1] "x is missing"

Duncan Murdoch
> 
> 
> foo <- function(x) mtcars[x]
> foo()
> # mtcars is printed in full
> 
> foo2 <- function(x) {x; mtcars[x]}
> foo2()
> Error in foo2() : argument "x" is missing, with no default
> 
> 
> Session info:
> 
> R version 4.4.1 (2024-06-14)
> Platform: x86_64-ubuntu18-linux-gnu
> Running under: Ubuntu 18.04.6 LTS
> 
> Matrix products: default
> BLAS/LAPACK:
/apps/intel/2019.1/compilers_and_libraries_2019.1.144/linux/mkl/lib/intel64_lin/libmkl_gf_lp64.so;
LAPACK version 3.7.0
> 
> locale:
> [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C
>   [3] LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8
>   [5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8
>   [7] LC_PAPER=en_US.UTF-8       LC_NAME=C
>   [9] LC_ADDRESS=C               LC_TELEPHONE=C
> [11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C
> 
> time zone: America/New_York
> tzcode source: system (glibc)
> 
> attached base packages:
> [1] stats     graphics  grDevices utils     datasets  methods   base
> 
> other attached packages:
> [1] MKLthreads_0.1
> 
> loaded via a namespace (and not attached):
> [1] compiler_4.4.1
> 
> William Barcelona
> Data & Technology Analyst
> Global Financial Institutions | Division of International Finance
> Federal Reserve Board
> william.l.barcelona at frb.gov<mailto:william.l.barcelona at frb.gov>
> 
> 
> 
> 	[[alternative HTML version deleted]]
> 
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