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

[Charles C. Berry]

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 http://stackoverflow.com/questions/3892580/).
> 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
> http://had.co.nz/
>
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>

Charles C. Berry                            (858) 534-2098
                                             Dept of Family/Preventive Medicine
E mailto:cberry at tajo.ucsd.edu	            UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/  La Jolla, San Diego 92093-0901