[Rd] Unexpected argument-matching when some are missing

[Emil Bode]

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) {

  print(match.call())

  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")

 

gives:

 

# 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