Re: [Rd] What do you call the value that represents a missing argument?

From: Charles C. Berry <>
Date: Fri, 08 Oct 2010 20:08:10 -0700

On Fri, 8 Oct 2010, Hadley Wickham wrote:

> Hi all,
> What's the official name for the value that represents a missing argument?
> e.g.
> formals(plot)$x

See ?list

It is a 'dotted pair list'

Are you looking for 'alist'?

"alist handles its arguments as if they described function arguments. So the values are not evaluated, and tagged arguments with no value are allowed whereas list simply ignores them. alist is most often used in conjunction with formals."

> alist(x=)$x==formals(plot)$x

[1] TRUE

HTH, Chuck

> str(formals(plot)$x)
> deparse(formals(plot)$x)
> is.symbol(formals(plot)$x)
> What's the correct way to create an object like this? (for example if
> you are manipulating the formals of a function to add an argument with
> no default value, as in
> as.symbol("") returns an error. Both substitute() and bquote() return
> that object, but it's not obvious if this is on purpose.
> Hadley
> --
> Assistant Professor / Dobelman Family Junior Chair
> Department of Statistics / Rice University
> ______________________________________________
> mailing list

Charles C. Berry                            (858) 534-2098
                                             Dept of Family/Preventive Medicine
E	            UC San Diego La Jolla, San Diego 92093-0901 mailing list Received on Sat 09 Oct 2010 - 03:09:49 GMT

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