[Rd] Unexpected argument-matching when some are missing

[S Ellison]

> When trying out some variations with `[.data.frame` I noticed some (to me)
> odd behaviour, 

Not just in 'myfun' ...

plot(x=1:10, y=)
plot(x=1:10, y=, 10:1)

In both cases, 'y=' is ignored. In the first, the plot is for y=NULL (so not 'missing' y)
In the second case, 10:1 is positionally matched to y despite the intervening 'missing' 'y='

So it isn't just 'missing'; it's 'not there at all'

Steve E

> -----Original Message-----
> From: R-devel [mailto:r-devel-bounces using r-project.org] On Behalf Of Emil
> Bode
> Sent: 29 November 2018 10:09
> To: r-devel using r-project.org
> Subject: [Rd] Unexpected argument-matching when some are missing
> 
> 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



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