[Rd] rnbinom Returns Error that says optional argument is missing

[Joris Meys]

Hi Thomas,

This seems fully expected behaviour. Obviously unspecified arguments are
evaluated as missing regardless of a default value. So if you set mu as a
default, the function will call C_rnbinom with n, size and prob. As prob is
not specified you get the error one would expect. Specifying a default
value for prob also makes rnbinom call C_rnbinom, but in this case there is
a prob value so it works.

I don't know what you consider "unintentional", but everything works as
expected and imho as intended as well. Changing formals to a function comes
with no guarantees, and setting a default value for an argument that
previously had none, comes with the risk of breaking things (like you
noticed)

If you want to use a default value for mu, you have to change the body of
the function as well, eg:

> formals(rnbinom)[c('size','mu')] <- c(1,1)
> body(rnbinom) <- quote(.Call(C_rnbinom_mu, n, size, mu))
> rnbinom(10)
 [1] 0 4 2 0 3 0 4 0 0 2

That's really hacking away and something I would never suggest to people,
but it works.

Hope this explains
Cheers
Joris


On Tue, Jan 31, 2017 at 5:39 PM, Thomas Roh <thmsroh at gmail.com> wrote:

> I am trying to reset the default arguments in the rnbinom function with the
> following example code:
>
> params <- c("size" = 1, "mu" = 1)
> formals(rnbinom)[names(params)] <- params
> rnbinom(n = 10)
>
> It returns the following:
>
> Error in rnbinom(n = 10) : argument "prob" is missing, with no default
>
> If I set the defaults with this code:
>
> params <- c("size" = 1, "prob" = .5)
> formals(rnbinom)[names(params)] <- params
> rnbinom(n = 10)
>
> The function works correctly. The documentation specifies that you can set
> mu or prob with size. I understand that the problem lies in default
> arguments are evaluated as missing, but it seems unintentional that setting
> "prob" and "size" defaults will actually evaluate.
>
> Here is the function call:
>
> function (n, size, prob, mu)
>
> {
>
>     if (!missing(mu)) {
>
>         if (!missing(prob))
>
>             stop("'prob' and 'mu' both specified")
>
>         .Call(C_rnbinom_mu, n, size, mu)
>
>     }
>
>     else .Call(C_rnbinom, n, size, prob)
>
> }
>
>
>
>
>
> --
> Thomas Roh
> thmsroh at gmail.com
>
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>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>



-- 
Joris Meys
Statistical consultant

Ghent University
Faculty of Bioscience Engineering
Department of Mathematical Modelling, Statistics and Bio-Informatics

tel :  +32 (0)9 264 61 79
Joris.Meys at Ugent.be
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