[Rd] Unexpected argument-matching when some are missing

Emil Bode emil@bode @ending from d@n@@kn@w@nl
Thu Nov 29 11:09:23 CET 2018

When trying out some variations with `[.data.frame` I noticed some (to me) odd behaviour, which I found out has nothing to do with `[.data.frame`, but rather with the way arguments are matched, when mixing named/unnamed and missing/non-missing arguments. Consider the following example:


myfun <- function(x,y,z) {


  cat('x=',if(missing(x)) 'missing' else x, '\n')

  cat('y=',if(missing(y)) 'missing' else y, '\n')

  cat('z=',if(missing(z)) 'missing' else z, '\n')


myfun(x=, y=, "z's value")




# myfun(x = "z's value")

# x= z's value 

# y= missing 

# z= missing


This seems very counterintuitive to me, I expect the arguments x and y to be missing, and z to get “z’s value”. 

When I call myfun(,y=,"z's value"), x is missing, and y gets “z’s value”.

Are my expectations wrong or is this a bug? And if my expectations are wrong, where can I find more information on argument-matching?

My gut-feeling says to call this a bug, but then I’m surprised no-one else has encountered it before.


And I don’t have multiple installations to work from, so could somebody else confirm this (if it’s not my expectations that are wrong) for R-devel/other R-versions/other platforms?

My setup: R 3.5.1, MacOS 10.13.6, both Rstudio 1.1.453 and R --vanilla from Bash


Best regards, 

Emil Bode 

More information about the R-devel mailing list